LA Course #GCE-1-1902
Sustainability Issues 2 - Diversification, Invasives and Tree Selection
Sections Go directly to the Section by clicking on the title below
Sustainability Issues 2 - Diversification, Invasives and Tree Selection
Sections Go directly to the Section by clicking on the title below
Note: Click on green text in each section for more information and photos.
Tree Diversification – Updated for 2019
Edited by Len Phillips and R W Gibney RLA - ISA
Nature has taught us that diversity is the rule and not the exception in the plant world. Even seemingly consistent natural landscapes such as grasslands, spruce forests and fields of lupine can appear to be mono-cultural. The diversity within these landscapes is wider than one might expect. When landscape architects and municipal arborists face the task of planting trees in urban settings, the importance of diversity cannot be overstated. This includes residential settings and any place where public trees are being planted. A broad diversity of trees is needed in our urban landscape to guard against the possibility of large-scale devastation by natural disasters as well as native and introduced insect pests and diseases.
While a street lined with a single type of tree has a certain classic elegance (fed to us over the centuries), the future does not bode well in that these trees can suffer the same consequences of aging, disease and pests simultaneously making a once canopied street void of trees.
History
The widespread planting of American elms (Ulmus americana) in the towns and cities of eastern United States by our forefathers was a nice idea. Our early horticulturists were taking advantage of the beauty and adaptability of a native tree that Thomas Jefferson called “nature’s noblest vegetable”. The accidental introduction of Dutch elm disease, and the consequential destruction of millions of elms, served not only to focus attention on the loss of urban forests but also to the qualities of the American elm. Norway Maple trees (Acer platanoides) were also looked at as the perfect street tree after WW2 and it was over-planted throughout the USA. As many of them reach the end of their life, we have discovered the detriments of this invasive species. It is now banned for sale in parts of the northeast.
Role of Diversity
We now realize that we need a diversity of trees in our urban forests, not only to guard against disasters like Dutch elm disease, but also to “put the right tree in the right place” as the evolution of our cities and suburbs creates challenges and new settings for tree installation.
Diversity plays an important role in the long-term stability of an ecosystem. When an area has a high diversity of tree species, it is less likely to suffer catastrophic loss from diseases or pests. For example, consider the impact the Emerald Ash Borer (EAB) (Agrilus planipennis) has had on ash trees. The EAB, an exotic wood-boring beetle discovered in southeast Michigan in the summer of 2002, has killed more than 10 million trees in southeast Michigan and is now spreading in all directions. Installing a diversity of hardy tree species throughout the urban forest can reduce the vulnerability of the trees to species specific pests.
Age Diversity
Consideration should be given to diversity by age within the urban forest. Planting the entire forest at one time will result in a future collapse of the entire forest at one time. A rule may also apply to age if the age of the existing forest and the number of existing trees are known. By using these two numbers one can prepare an age diversification factor for the tree replacement plan. For example: if the oldest tree in the urban forest died at 100 years of age and the tree inventory indicates a maximum capacity of 30,000 trees in the urban forest, then 300 trees should be planted every year.
Diversity by Shapes
If given enough space to grow, trees have characteristic shapes. Some shapes fit better in a particular space than others. For example, when vase-shaped American elms (Ulmus americana) are planted across the street from each other, their crowns form a cathedral-like arch over the street. A tree with a rounded crown like a maple (Acer sp.) will shade the yard. Pyramidal-shaped trees like linden (Tilia sp.) with dense persistent leaves provide less shade, but are better at breaking the wind speed nearer the ground. A pyramidal-shaped tree also takes up more space near the ground. Trees with a weeping stature provide focal points in a landscape as do columnar trees. Columnar trees are also suitable in locations where space is tight or a tall hedge is desired.
Diversity of Food Value
A diversity of trees with high food value is the single best way to increase wildlife populations. For example:
Keep in mind that every tree must be carefully selected for each location to be sure it is appropriate. For example, do not plant ash if the Emerald Ash Borer is on its way to your city, and sweetgums with their spiny seed balls should not be installed where pedestrians are walking.
A Diversity of Wildlife Habitats
Birds and small animals need concealed spaces for nesting and hiding to protect them from the eyes of predators. Planting conifers in groups, growing hedges with low branches, or using prickly or thorny plants in a few areas are all ways to provide wildlife cover.
Site Diversity Considerations
Not all sites are appropriate for tree plantings – even if another tree was removed from that site. Very narrow tree lawns between roads and sidewalks cannot support large trees without root conflicts. Soil volumes that support healthy root growth and tree potential are vital to success. Be aware of underground utilities, corner sight distances, driveway backing visibility and fire hydrant conflicts. Overhead wires call for careful selection of trees with limited height potential (“wire-friendly trees”). Future maintenance and removal costs can outweigh the advantages of planting trees at inappropriate locations. Again, the phrase “right tree in the right place” comes to mind.
The Ten-Percent Rule
The “10% rule” is a guide for diversification and a reaction to the likelihood that some major insect or disease pest could, at some point in time, wipe out most of the trees in a city. Landscape architects, urban foresters and municipal arborists should use the following guidelines for tree diversity within their areas of jurisdiction:
Individual trees as well as strips or blocks of uniformly planted species, cultivars, or clones with proven adaptability should be scattered throughout the city to achieve spatial as well as biological diversity.
Originally, the rule was considered a safeguard against a “new pest” that might be introduced from a foreign country. The American experience with Dutch elm disease and chestnut blight is sufficient to explain our concern about such epidemics. However, the spread of dogwood anthracnose disease on our native, Cornus florida, has caused great alarm because it did not come from an introduced pathogen. There are also many “native” insect problems that we are aware of and that must considered as potential threats to the urban forest. The quest for diversity does not rule out the planting of trees like the native Dogwood. While this fungus problem has dissipated over the years, Dogwoods planted in the last several years are thriving and new cultivars are being developed.
Enacting Species Diversity
While the “10% rule” may serve as a target or goal to soothe the consciences of city councils and municipal arborists, it will not solve all potential pest problems nor guarantee the long-term stability and aesthetics of the urban forest. We need to install more of the superior trees developed through genetic research. We need to utilize the practical experience from practitioners of urban forestry. We have to plan the installation of city trees, and understand the problems and potentials of our actions.
Enacting a species diversity plan implies three simple actions:
To achieve each of these elements, a plan containing eight tasks is necessary. The first two tasks focus on evaluating the current and potential tree resources. The remaining six tasks outline a systematic process for establishing thresholds for each species and circulating species on and off of the annual planting list.
Task 1 – Compile and assess data on the current tree population and program. An inventory is the preferred tool for collecting and summarizing this data. Necessary information includes:
Remember to advise your clients that an inventory must be a ‘living document’ that is updated annually and preferably after each tree related event including removals, new plantings and storm damage.
Task 2 – Develop a list of all possible species to potentially plant in the area. The list should include everything from small to large; and abundant to obscure. This list can be derived from a number of sources such as:
It is important to recognize availability of the trees you choose – preferably at regionally local nurseries when possible. By simply calling nurseries and asking about their inventories, availability can be established.
Task 3 – Establish a target for each species on your current list. To minimize an over emphasis of any one species on the list, a cap for each species should be established. This cap should be a percentage of the total possible number of trees that can be present in the community (existing number of trees plus the number of vacant planting sites).
Task 4 – Develop a short list of species to install each year. This list may have anywhere from five to twenty species. This is the list of species that are going to be emphasized in the current year's planting program. The list should incorporate a variety of species in various sizes. It should also recognize the soil volumes available at each planting site and the presence of overhead and underground utilities.
Task 5 – Create a threshold for each species on your annual planting list. To minimize an over emphasis of any one species on the annual list, a cap for each species should be established. As that threshold is reached, that particular species is taken off of the list for the remainder of the year. A careful balance of all of suggested species for the year will guarantee an even and equitable distribution of the trees.
Task 6 – Establish a planting cycle for each species. Each species stays on the annual list for a few years. The duration is a function of how much emphasis will be placed on that particular species (the current versus the target). For example, a city might have 16 Kentucky Coffeetrees currently and a target of 266, so 250 coffeetrees can be installed in the next few years, before this tree is removed from the tree list. Other cities have chosen to select the trees that the inventory indicates have the highest number, and place a ban on using these trees until the diversification formula has been met.
Task 7 – Rotate species on and off the annual list. Once a species has reached the end of its planting cycle, it is taken off the list for a few years. Other species are then placed on the annual list to encourage their emphasis. This down time is temporary and should last anywhere from three to five years for any one species.
Task 8 – Evaluate the program. No program is complete without the ability to gauge whether or not the program is successful or not. The best way to assess the success or failure of a species diversity program is to compare the change over time to the individual species count and compare that count against the targets you have established for each species. Continue to assess the suitability of species and incorporate changes due to insect/disease, cultural, or other realizations.
Benefits of Diversification
Sources
Tree Diversification – Updated for 2019
Edited by Len Phillips and R W Gibney RLA - ISA
Nature has taught us that diversity is the rule and not the exception in the plant world. Even seemingly consistent natural landscapes such as grasslands, spruce forests and fields of lupine can appear to be mono-cultural. The diversity within these landscapes is wider than one might expect. When landscape architects and municipal arborists face the task of planting trees in urban settings, the importance of diversity cannot be overstated. This includes residential settings and any place where public trees are being planted. A broad diversity of trees is needed in our urban landscape to guard against the possibility of large-scale devastation by natural disasters as well as native and introduced insect pests and diseases.
While a street lined with a single type of tree has a certain classic elegance (fed to us over the centuries), the future does not bode well in that these trees can suffer the same consequences of aging, disease and pests simultaneously making a once canopied street void of trees.
History
The widespread planting of American elms (Ulmus americana) in the towns and cities of eastern United States by our forefathers was a nice idea. Our early horticulturists were taking advantage of the beauty and adaptability of a native tree that Thomas Jefferson called “nature’s noblest vegetable”. The accidental introduction of Dutch elm disease, and the consequential destruction of millions of elms, served not only to focus attention on the loss of urban forests but also to the qualities of the American elm. Norway Maple trees (Acer platanoides) were also looked at as the perfect street tree after WW2 and it was over-planted throughout the USA. As many of them reach the end of their life, we have discovered the detriments of this invasive species. It is now banned for sale in parts of the northeast.
Role of Diversity
We now realize that we need a diversity of trees in our urban forests, not only to guard against disasters like Dutch elm disease, but also to “put the right tree in the right place” as the evolution of our cities and suburbs creates challenges and new settings for tree installation.
Diversity plays an important role in the long-term stability of an ecosystem. When an area has a high diversity of tree species, it is less likely to suffer catastrophic loss from diseases or pests. For example, consider the impact the Emerald Ash Borer (EAB) (Agrilus planipennis) has had on ash trees. The EAB, an exotic wood-boring beetle discovered in southeast Michigan in the summer of 2002, has killed more than 10 million trees in southeast Michigan and is now spreading in all directions. Installing a diversity of hardy tree species throughout the urban forest can reduce the vulnerability of the trees to species specific pests.
Age Diversity
Consideration should be given to diversity by age within the urban forest. Planting the entire forest at one time will result in a future collapse of the entire forest at one time. A rule may also apply to age if the age of the existing forest and the number of existing trees are known. By using these two numbers one can prepare an age diversification factor for the tree replacement plan. For example: if the oldest tree in the urban forest died at 100 years of age and the tree inventory indicates a maximum capacity of 30,000 trees in the urban forest, then 300 trees should be planted every year.
Diversity by Shapes
If given enough space to grow, trees have characteristic shapes. Some shapes fit better in a particular space than others. For example, when vase-shaped American elms (Ulmus americana) are planted across the street from each other, their crowns form a cathedral-like arch over the street. A tree with a rounded crown like a maple (Acer sp.) will shade the yard. Pyramidal-shaped trees like linden (Tilia sp.) with dense persistent leaves provide less shade, but are better at breaking the wind speed nearer the ground. A pyramidal-shaped tree also takes up more space near the ground. Trees with a weeping stature provide focal points in a landscape as do columnar trees. Columnar trees are also suitable in locations where space is tight or a tall hedge is desired.
Diversity of Food Value
A diversity of trees with high food value is the single best way to increase wildlife populations. For example:
- summer fruit – cherries, dogwoods, plums, apricots
- fall and winter fruit – apples, crabapples, dogwoods, hackberries, hawthorns, mountain ashes
- seeds – ashes, birches, firs, hemlocks, maples, spruces, sweetgums
- nuts and acorns – butternuts, walnuts, chestnuts, hazelnuts, hickories, oaks, pecans
Keep in mind that every tree must be carefully selected for each location to be sure it is appropriate. For example, do not plant ash if the Emerald Ash Borer is on its way to your city, and sweetgums with their spiny seed balls should not be installed where pedestrians are walking.
A Diversity of Wildlife Habitats
Birds and small animals need concealed spaces for nesting and hiding to protect them from the eyes of predators. Planting conifers in groups, growing hedges with low branches, or using prickly or thorny plants in a few areas are all ways to provide wildlife cover.
Site Diversity Considerations
Not all sites are appropriate for tree plantings – even if another tree was removed from that site. Very narrow tree lawns between roads and sidewalks cannot support large trees without root conflicts. Soil volumes that support healthy root growth and tree potential are vital to success. Be aware of underground utilities, corner sight distances, driveway backing visibility and fire hydrant conflicts. Overhead wires call for careful selection of trees with limited height potential (“wire-friendly trees”). Future maintenance and removal costs can outweigh the advantages of planting trees at inappropriate locations. Again, the phrase “right tree in the right place” comes to mind.
The Ten-Percent Rule
The “10% rule” is a guide for diversification and a reaction to the likelihood that some major insect or disease pest could, at some point in time, wipe out most of the trees in a city. Landscape architects, urban foresters and municipal arborists should use the following guidelines for tree diversity within their areas of jurisdiction:
- install no more than 10% of any species,
- no more than 20% of any genus,
- no more than 30% of any family.
Individual trees as well as strips or blocks of uniformly planted species, cultivars, or clones with proven adaptability should be scattered throughout the city to achieve spatial as well as biological diversity.
Originally, the rule was considered a safeguard against a “new pest” that might be introduced from a foreign country. The American experience with Dutch elm disease and chestnut blight is sufficient to explain our concern about such epidemics. However, the spread of dogwood anthracnose disease on our native, Cornus florida, has caused great alarm because it did not come from an introduced pathogen. There are also many “native” insect problems that we are aware of and that must considered as potential threats to the urban forest. The quest for diversity does not rule out the planting of trees like the native Dogwood. While this fungus problem has dissipated over the years, Dogwoods planted in the last several years are thriving and new cultivars are being developed.
Enacting Species Diversity
While the “10% rule” may serve as a target or goal to soothe the consciences of city councils and municipal arborists, it will not solve all potential pest problems nor guarantee the long-term stability and aesthetics of the urban forest. We need to install more of the superior trees developed through genetic research. We need to utilize the practical experience from practitioners of urban forestry. We have to plan the installation of city trees, and understand the problems and potentials of our actions.
Enacting a species diversity plan implies three simple actions:
- Minimize planting overused species.
- Increase the planting of underused species.
- Introduce new species into the landscape.
To achieve each of these elements, a plan containing eight tasks is necessary. The first two tasks focus on evaluating the current and potential tree resources. The remaining six tasks outline a systematic process for establishing thresholds for each species and circulating species on and off of the annual planting list.
Task 1 – Compile and assess data on the current tree population and program. An inventory is the preferred tool for collecting and summarizing this data. Necessary information includes:
- species count and percent representation in the population
- number of vacant planting sites in the community
- number of trees planted each year
- number of trees removed each year
- contents of the annual planting list
Remember to advise your clients that an inventory must be a ‘living document’ that is updated annually and preferably after each tree related event including removals, new plantings and storm damage.
Task 2 – Develop a list of all possible species to potentially plant in the area. The list should include everything from small to large; and abundant to obscure. This list can be derived from a number of sources such as:
- the current tree population highlighting all the species that currently appear to be doing well,
- planting lists from neighboring communities,
- local arboreta or botanical garden lists,
- tree, shrub, and landscape books,
- nursery catalogs that match zone restrictions for your area.
It is important to recognize availability of the trees you choose – preferably at regionally local nurseries when possible. By simply calling nurseries and asking about their inventories, availability can be established.
Task 3 – Establish a target for each species on your current list. To minimize an over emphasis of any one species on the list, a cap for each species should be established. This cap should be a percentage of the total possible number of trees that can be present in the community (existing number of trees plus the number of vacant planting sites).
Task 4 – Develop a short list of species to install each year. This list may have anywhere from five to twenty species. This is the list of species that are going to be emphasized in the current year's planting program. The list should incorporate a variety of species in various sizes. It should also recognize the soil volumes available at each planting site and the presence of overhead and underground utilities.
Task 5 – Create a threshold for each species on your annual planting list. To minimize an over emphasis of any one species on the annual list, a cap for each species should be established. As that threshold is reached, that particular species is taken off of the list for the remainder of the year. A careful balance of all of suggested species for the year will guarantee an even and equitable distribution of the trees.
Task 6 – Establish a planting cycle for each species. Each species stays on the annual list for a few years. The duration is a function of how much emphasis will be placed on that particular species (the current versus the target). For example, a city might have 16 Kentucky Coffeetrees currently and a target of 266, so 250 coffeetrees can be installed in the next few years, before this tree is removed from the tree list. Other cities have chosen to select the trees that the inventory indicates have the highest number, and place a ban on using these trees until the diversification formula has been met.
Task 7 – Rotate species on and off the annual list. Once a species has reached the end of its planting cycle, it is taken off the list for a few years. Other species are then placed on the annual list to encourage their emphasis. This down time is temporary and should last anywhere from three to five years for any one species.
Task 8 – Evaluate the program. No program is complete without the ability to gauge whether or not the program is successful or not. The best way to assess the success or failure of a species diversity program is to compare the change over time to the individual species count and compare that count against the targets you have established for each species. Continue to assess the suitability of species and incorporate changes due to insect/disease, cultural, or other realizations.
Benefits of Diversification
- Diversification will mean making better use of the tree species commonly planted in your city, while ensuring that they are well suited to the site in the context of predicted climate change.
- Take opportunities to use tree species not historically widespread in your city, but which will become more suitable under current climate change predictions.
- Make sure that any group mixtures are of compatible species, and they will meet the long-term management objectives.
- Use existing or expected natural regeneration to diversify stands where the species you expect will be able to meet the set objectives and will remain well suited to the site, even where you are not practicing diversification.
- When thinning or re-spacing, take the opportunity to retain minor species where they will contribute to the long-term objectives of the urban forest.
Sources
- ”A guide for increasing tree species diversity in Wales” Forestry Commission on Wales, November 15, 2010
- Duntemann, Mark, “Urban Forestry Management Series: Implementing a Species Diversity Program”, Natural Path Urban Forestry Consultants, July, 2004.
- Santamour, Frank S., Jr., “Uniformity and Common Sense”, METRIA 7 Proceedings, 1990.
- National Arbor Day Foundation, “Tree Diversity Activity”, 2009.
Invasive Plants
Edited by Len Phillips
Arborists and landscape architects should be aware of the invasive plant problem and know what to do when an invasive is found. Invasive plants affect everyone. When settlers in America began gardening, they had a constant struggle to keep the natural ecosystem from invading and overwhelming their small cultivated patches. Today, Americans have so altered the landscape that the reverse is true. Many of the plants we have cultivated have escaped and are now threatening to invade more favorable native plants in conservation and wild areas. Additionally, the U.S. Geological Survey reports that some $20 billion in environmental damage can be linked to invasive trees, plants, and wildlife. This affects not only those making a living from the land, but people everywhere.
Researchers believe there are several factors that contribute to a plant becoming invasive. These include:
Invasive plants are opportunistic plants that take advantage of areas with disturbed soil. They spread rapidly and crowd out desirable native species. They also compete with native species for water, nutrients, sunlight, and space. As a result, invasive species can:
Categories of Invasive Plants
Invasive Prevention
The best defense against invasive plant takeovers is constant surveillance of landscaped and natural areas and continued annual follow-up treatments of all infested sites until the problem is controlled. Early detection and treatments will minimize the costs and efforts that must be dedicated to eliminate invasive plant infestations. The longer the plants are left to establish the more rapidly they spread; the more difficult they are to control; and the more time that is needed for follow-up treatments.
Invasive Plant Control Plan
Early invasive detection and treatment along with an effective control program are the main components of a complete prevention plan. There are a few effective biological agents for invasive plant control, but they are only useful where containment is the objective. For control of heavy plant infestation, herbicides are most often used in combination with other types of control such as mechanical and cultural methods, hand pulling, mowing or cutting, and burning. Mechanical methods may disturb a site and encourage the invasive plants instead of eliminating them. Some southern cities are using goats to control Kudzu and many other communities are using goats to control poison ivy. They have found that not only do the goats eat the plants but their cloven hooves sever the plants root system. Plus the goats are not harmed from eating the poison ivy.
Therefore, herbicides are preferred because they are the least disruptive and the most effective and economical means for invasive plant control. They are often applied as a direct control application method for single and small infestations. Without the use of herbicides, native plants would be the losers and invasive plants would take over.
An effective control plan must begin with training for plant identification; education on the harm of invasive plants to the environment; herbicides and their uses; application techniques; and application timing. All landscape area managers must be able to identify invasive plants so they can be spotted for early control. Adjacent landowners should also be able to identify the plants and be encouraged to report them. Extra employee training cannot be overemphasized whether vegetation management specialists, university experts, county agents, or consultants provide it.
List of Invasive Trees, Shrubs, and Vines
The following list was prepared for Central Illinois and may need to be adapted to your area:
Amur Maple Acer ginnala
Boxelder Acer negundo
Norway Maple Acer platanoides
Porcelain Vine Ampelopsis brevipedunculata
Japanese Barberry Berberis thunbergii
Oriental Bittersweet Celastrus orbiculatus
Autumn-olive Elaeagnus umbellata
Burning Bush Euonymus alatus
European Privet Ligustrum vulgare
Japanese Honeysuckle Lonicera japonica
Amur Honeysuckle Lonicera maackii
White Mulberry Morus alba
Amur Corktree Phellodendron amurense
Common Buckthorn Rhamnus cathartica
Glossy Buckthorn Rhamnus frangula
Black Locust Robinia pseudoacacia
Multiflora Rose Rosa multiflora
Poison Ivy Toxicodendron radicans
Siberian Elm Ulmus pumila
European Cranberry-bush Viburnum opulus
Conclusion
The majority of exotic plants is not problematic and poses no threat to our natural areas. Exotic plants play an important role in modern day landscapes offering tough, resilient, and aesthetically pleasing additions to yards, streets, and urban landscapes. Prevention is obviously a moot point in many areas. While herbicides are one of the most effective and economical ways to control invasives and reclaim areas, think what the outcome would be if herbicides were not used at all.
Sources
Edited by Len Phillips
Arborists and landscape architects should be aware of the invasive plant problem and know what to do when an invasive is found. Invasive plants affect everyone. When settlers in America began gardening, they had a constant struggle to keep the natural ecosystem from invading and overwhelming their small cultivated patches. Today, Americans have so altered the landscape that the reverse is true. Many of the plants we have cultivated have escaped and are now threatening to invade more favorable native plants in conservation and wild areas. Additionally, the U.S. Geological Survey reports that some $20 billion in environmental damage can be linked to invasive trees, plants, and wildlife. This affects not only those making a living from the land, but people everywhere.
Researchers believe there are several factors that contribute to a plant becoming invasive. These include:
- a fast growth rate
- seeds that germinate quickly in high percentages
- prolific seed production, which begins within the first few years of the invasive plant’s life
- easy seed dispersal by animals, water, and wind
- the ability to reproduce by seed as well as vegetatively, through suckering
- longer flowering and fruiting periods
- an adaptability to a wide range of soil and growing conditions
Invasive plants are opportunistic plants that take advantage of areas with disturbed soil. They spread rapidly and crowd out desirable native species. They also compete with native species for water, nutrients, sunlight, and space. As a result, invasive species can:
- displace native species
- reduce plant diversity and create monocultures
- alter ecosystem processes
- hybridize with native plants, changing their genetic makeup
- destroy and degrade the wildlife habitats that support native animals, insects, and micro-organisms
- create ecosystems that support aggressive, non-native plants, animals, and pathogens
Categories of Invasive Plants
- Exotics — These are primarily European or Asian species that have been accidentally or intentionally introduced to the US for their ornamental value or their ability to provide shade, windbreaks, and erosion control
- Translocators — These are native species that have moved out of their original range into an area that favors their over-production
- Opportunists — These are native colonizing species that previously may have been held in check by natural processes
Invasive Prevention
The best defense against invasive plant takeovers is constant surveillance of landscaped and natural areas and continued annual follow-up treatments of all infested sites until the problem is controlled. Early detection and treatments will minimize the costs and efforts that must be dedicated to eliminate invasive plant infestations. The longer the plants are left to establish the more rapidly they spread; the more difficult they are to control; and the more time that is needed for follow-up treatments.
Invasive Plant Control Plan
Early invasive detection and treatment along with an effective control program are the main components of a complete prevention plan. There are a few effective biological agents for invasive plant control, but they are only useful where containment is the objective. For control of heavy plant infestation, herbicides are most often used in combination with other types of control such as mechanical and cultural methods, hand pulling, mowing or cutting, and burning. Mechanical methods may disturb a site and encourage the invasive plants instead of eliminating them. Some southern cities are using goats to control Kudzu and many other communities are using goats to control poison ivy. They have found that not only do the goats eat the plants but their cloven hooves sever the plants root system. Plus the goats are not harmed from eating the poison ivy.
Therefore, herbicides are preferred because they are the least disruptive and the most effective and economical means for invasive plant control. They are often applied as a direct control application method for single and small infestations. Without the use of herbicides, native plants would be the losers and invasive plants would take over.
An effective control plan must begin with training for plant identification; education on the harm of invasive plants to the environment; herbicides and their uses; application techniques; and application timing. All landscape area managers must be able to identify invasive plants so they can be spotted for early control. Adjacent landowners should also be able to identify the plants and be encouraged to report them. Extra employee training cannot be overemphasized whether vegetation management specialists, university experts, county agents, or consultants provide it.
List of Invasive Trees, Shrubs, and Vines
The following list was prepared for Central Illinois and may need to be adapted to your area:
Amur Maple Acer ginnala
Boxelder Acer negundo
Norway Maple Acer platanoides
Porcelain Vine Ampelopsis brevipedunculata
Japanese Barberry Berberis thunbergii
Oriental Bittersweet Celastrus orbiculatus
Autumn-olive Elaeagnus umbellata
Burning Bush Euonymus alatus
European Privet Ligustrum vulgare
Japanese Honeysuckle Lonicera japonica
Amur Honeysuckle Lonicera maackii
White Mulberry Morus alba
Amur Corktree Phellodendron amurense
Common Buckthorn Rhamnus cathartica
Glossy Buckthorn Rhamnus frangula
Black Locust Robinia pseudoacacia
Multiflora Rose Rosa multiflora
Poison Ivy Toxicodendron radicans
Siberian Elm Ulmus pumila
European Cranberry-bush Viburnum opulus
Conclusion
The majority of exotic plants is not problematic and poses no threat to our natural areas. Exotic plants play an important role in modern day landscapes offering tough, resilient, and aesthetically pleasing additions to yards, streets, and urban landscapes. Prevention is obviously a moot point in many areas. While herbicides are one of the most effective and economical ways to control invasives and reclaim areas, think what the outcome would be if herbicides were not used at all.
Sources
- “Invasive Plants: Weeds of the Global Garden”, Brooklyn Botanic Garden 2009.
- “Invasive Trees, Shrubs, and Vines”, Morton Arboretum, 2012.
- Williamson, Max, "Invasive Q&A", Right of Way Vistas, Dow AgroSciences, Vol. 18. 1, 2005.
- “Wisconsin Manual of Control Recommendations for Ecologically Invasive Plants”, Bureau of Endangered Resources, Wisconsin Department of Natural Resources, 2012.
Tree Selection Using Cultivars, Natives and Clones – Updated for 2019
Edited by Len Phillips and R W Gibney RLA - ISA
This article was presented in an earlier course and the information is very relevant to tree selections for the sake of diversification and dealing with the consequences of climate change. Knowing and understanding the differences in how trees are developed and grown can give the landscape architect the insight and knowledge to select trees that will suit the needs of the site while having characteristics that enhance the environment while resisting pests and disease. Nurseries are striving to produce trees that are healthier, live longer and have resistance to pest problems. Trees are also being developed to meet growth needs whether it is for maintenance under utility wires, restricted root space, heat tolerance, columnar form, crown density. Some trees have been developed to minimize fruiting (Liquidambar), enhance flowering (many Crabapples), and increase variety (Rutgers Dogwoods).
While there is a need for the preservation and continued planting of native species, these trees often don’t fill the needs of the built, non-native environment of our urban and suburban spaces. Also, when native and non-native pests are apt to damage and kill native selections, the alternatives provide the resistance to these problems making more sustainable landscapes without the threat of being invasive or a danger to the healthy native plants in the area.
Often we will use the terms “cultivars” and “clones” interchangeably. As you will see there is a difference between these and other closely related terms that professional arborists should be aware of. Clones may not be cultivars, and cultivars may not be clones, but nativars can be cultivars. Few, if any, of our currently popular trees are clones, but with increased success in biotechnology (tissue culture), some will certainly be cloned in the future. All it takes to make a cultivar a clone is naming it a clone and growing it on its own roots.
Definitions
Clones
A clone consists of a group of plants of absolute genetic uniformity and is the ultimate in monoculture. Commercial farmers extensively utilize clones and plant large monocultures with minimal genetic diversity. One of the principal reasons commercial farmers utilize genetic uniformity is that most commercial crops are the products of generations of genetic research, bred and selected to be resistant to major pests and specific herbicides, are adapted to specific localities, and/or are drought and heat tolerant. Commercial farmers plant clonal monocultures to capture the advertised superiority of these plants and uniformity in sowing, culture, and harvesting. Also, as most agricultural plants are annuals, when pest problems do arise, an army of scientists are ready to battle the pest with new genetic combinations, chemicals, or bio-control agents.
The landscape architects and city foresters are not as fortunate as the commercial farmer. Few trees currently grown and sold as clones in the nursery trade have been purposely developed and thoroughly tested for pest resistance. The trees must endure for decades, often in difficult situations where environmental and biotic stresses are continually changing. The development of a new “replacement” for a clone, with similar characteristics of growth and pest resistance may require decades of research. Some of the clones now available for city planting were originally selected for certain aesthetics, propagated by budding and grafting, and marketed as named cultivars.
Cultivars
The use of seedling understock, whether of the same or a related species, introduces an element of diversity that might affect tree performance. Tree monocultures may only pose major problems when the numbers of trees are large and the area occupied by the trees is restricted. In other words, twenty to fifty trees of a single species, or even a single clone, planted along a few blocks of city streets do not constitute a “dangerous” monoculture. If 75% of the trees in the city were the same cultivar, that might become a problem.
Advantages of Cultivars
The most obvious advantage of cultivars is their reliability, especially those cultivars that have been in the nursery trade for 20 years or more. They can be counted on to develop the form, color, and growth rate for which they were selected. Their longevity in the trade and their widespread planting has provided the testing necessary to determine both their good and bad characteristics. The urban tree planter knows what to expect of such trees.
One other characteristic of most cultivars, especially those that had been traditionally propagated by budding and grafting, is their genetic capacity for strong wound compartmentalization. Studies have shown that every cultivar tested, in a wide range of genera and species, were strong compartmentalizers. The conclusion was made because the grafting and budding process constituted an inadvertent “screening” and only strong compartmentalizing trees would be amenable to long-term commercial propagation by these techniques. Some of the cultivars formerly propagated by budding and grafting are now propagated on their own roots and have retained this important trait.
Uncertainties of Cultivars
The major uncertainties of cultivars relate to the possibility of long-term graft incompatibility. Recent research has determined that graft compatibility is dependent on the similarity of stock and scion in cambial peroxidase enzymes, which mediate the production of lignin. In Castanea mollissima, Quercus rubra, and Acer rubrum graft incompatibility is a major problem because there is considerable tree-to-tree variability in enzyme patterns. On the other hand, Acer saccharum, Acer platanoides, and Gleditsia triacanthos appeared to be quite uniform in enzyme pattern and no graft incompatibility has been reported.
Another disadvantage of cultivars that are grafted to root stocks of a stronger species is that growth from below the graft can develop and change the intended characteristics of the tree. A good example of this would be the Higan Weeping Cherry (Prunus x subhirtella). These are often grafted high to create a taller single stem weeping specimen. Branches forming at or below the graft grow tall and straight with more vigor than the top thereby destroying the intended form and causing an unsightly tree. With proper maintenance and pruning, this is easily controlled but it does challenge the concept of sustainability.
Limited work on Cornus, Fagus, Fraxinus, and Koelreuteria indicated that graft incompatibility could cause problems in these genera. For those genera and species that are difficult to propagate from cultivars or by micro-propagation, the careful matching of enzyme patterns of stock and scion will produce graft-compatible combinations.
However, there are still many species that have not been studied and the landscape architect/city forester should be cautious in their use of new grafted cultivars of Celtis, Maclura, and Tilia.
Conclusion
It is recommended that you plant clones and cultivars that have proven their reliability. Use, somewhat cautiously and on a trial basis, some of the untested new cultivars of “unfamiliar” species or genera. These newer introductions have been developed through scientific research and have been selected for survival traits such as pest resistance or salt tolerance, but should be tested in your region for trial.
For uniformity, use seedlings of known geographic origin (or, in the case of exotics like Tilia cordata, from proven seed sources) so the plants will be able to tolerate the general climatic conditions in your area. For diversity, use the best clones, cultivars, and seedlings of many species and genera distributed throughout the city or as mixes of individual trees.
And, by all means, don’t stop planting natives when the site and climate will support them. Native habitats are moving northward due to climate change causing some plants to succumb to heat and pest stresses that they did not face in the past. Landscape architects, nurseries and horticulturists can adapt to this phenomenon by adjusting their climate zone maps accordingly and recognizing native habitats surrounding their project sites to determine appropriateness of selections for sustainability.
For the education of the next generation, plant a catalpa, a hickory, a horse-chestnut, a sassafras, and even a thorny honeylocust in park areas that can and should be used to stimulate an interest in the diversity of nature.
Sources
The test that follows contains 30 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
LA CES will award 1.0 PDH (HSW) credit for a passing grade. North Carolina Board of LA and New Jersey Board of Architects will award 1.0 credits for a passing grade. Approval is pending by .
The cost for taking this test is $20 per credit. If you purchase an annual subscription for 12 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report your passing test score to LA CES. If you are also ISA* certified we will report your passing score to ISA for no additional cost. Please be sure to add your ISA Certification number when you sign in. Tests with passing scores may be submitted only once to each organization.
*ISA has approved this course for .5 CEUs per section for a total of 1.5 CEUs. The Tree Diversification section is applied toward Certified Arborist, Municipal Specialist, Utility Specialist, or BCMA practice credits. The Invasive Plants section is applied toward Certified Arborist, Municipal Specialist, Utility Specialist, or BCMA science credits. The Tree Selection section is applied toward Certified Arborist, or BCMA science credits.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to Merchant' / gibneyCE.com. That will take you to the test sign in page followed by the test. If you are an ISA and/or CLARB member, please be sure to include your certification/member number(s) along with your LA license and ASLA numbers.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see our Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks after that.
LA CES maintains a record of earned PDH credits on their website http://laces.asla.org/
ISA maintains a record of earned CEU credits on their website http://www.isa-arbor.com/
Edited by Len Phillips and R W Gibney RLA - ISA
This article was presented in an earlier course and the information is very relevant to tree selections for the sake of diversification and dealing with the consequences of climate change. Knowing and understanding the differences in how trees are developed and grown can give the landscape architect the insight and knowledge to select trees that will suit the needs of the site while having characteristics that enhance the environment while resisting pests and disease. Nurseries are striving to produce trees that are healthier, live longer and have resistance to pest problems. Trees are also being developed to meet growth needs whether it is for maintenance under utility wires, restricted root space, heat tolerance, columnar form, crown density. Some trees have been developed to minimize fruiting (Liquidambar), enhance flowering (many Crabapples), and increase variety (Rutgers Dogwoods).
While there is a need for the preservation and continued planting of native species, these trees often don’t fill the needs of the built, non-native environment of our urban and suburban spaces. Also, when native and non-native pests are apt to damage and kill native selections, the alternatives provide the resistance to these problems making more sustainable landscapes without the threat of being invasive or a danger to the healthy native plants in the area.
Often we will use the terms “cultivars” and “clones” interchangeably. As you will see there is a difference between these and other closely related terms that professional arborists should be aware of. Clones may not be cultivars, and cultivars may not be clones, but nativars can be cultivars. Few, if any, of our currently popular trees are clones, but with increased success in biotechnology (tissue culture), some will certainly be cloned in the future. All it takes to make a cultivar a clone is naming it a clone and growing it on its own roots.
Definitions
- Cultivar or Grafted Cultivar: Cultivars are named selections. In landscape trees, cultivars are generally propagated by some asexual means, be it cuttings, budding, grafts, divisions, or tissue culture from the preferred tree onto seedling rootstock of the same species. The above ground portion of all trees will be genetically identical, but there will be genetic variability among rootstocks. What you lose with these genetically identical plants is diversity. Over a series of generations, if the plants can not reproduce, you decrease the gene pool and the ability of the population to adapt to change.
- Nativars: Nativars are cultivated varieties of native trees. They are usually a selection from a native tree that has a unique form or a unique color in autumn, or resistance to pests, diseases, and pollutants to the native tree. Nativars are propagated as cuttings or some other asexual means.
- Natives: Native trees are those growing in the wild and in proximity to the nursery growing native trees. Nurseries collect native tree seedlings from a local forest and are to be sold to local customers only. Natives are propagated by seed.
- Clone: A clone is propagated by asexual propagation (rooted cuttings or micro propagation) from a single tree and grown on its own roots. All members of the clone will be genetically identical from root tip to apical meristem. More often than not, a cultivar name will have been given to the clone.
- Seedlings: These plants are derived from seed collected from a single tree. Granted that most landscape trees are naturally cross-pollinated and are highly heterozygous (dissimilar pairs of genes for any hereditary characteristic), the progeny from a single parent tree will be more genetically uniform than seedlings derived from several parent trees.
- Seed-Orchard Seedlings: Seedlings derived from pollination among trees selected for certain desirable attributes (such as pest resistance) and propagated and planted in a “seed orchard”, specifically for seed production. There will be a high degree of genetic uniformity with regard to the traits that constituted the original selection criteria but, still, a reasonable amount of genetic diversity.
- Provenance Seedlings: Seedlings coming from native trees in a particular geographic area, assuring that the seedlings will be adaptable to the general climatic conditions pertaining to that area.
- Mixed Seedlings: Seedlings of unknown geographic origin with unknown adaptability or seedlings of different known provenance. This potentially high level of diversity may occur among the products of a single nursery and will nearly always occur when seedlings are obtained from several different nurseries.
Clones
A clone consists of a group of plants of absolute genetic uniformity and is the ultimate in monoculture. Commercial farmers extensively utilize clones and plant large monocultures with minimal genetic diversity. One of the principal reasons commercial farmers utilize genetic uniformity is that most commercial crops are the products of generations of genetic research, bred and selected to be resistant to major pests and specific herbicides, are adapted to specific localities, and/or are drought and heat tolerant. Commercial farmers plant clonal monocultures to capture the advertised superiority of these plants and uniformity in sowing, culture, and harvesting. Also, as most agricultural plants are annuals, when pest problems do arise, an army of scientists are ready to battle the pest with new genetic combinations, chemicals, or bio-control agents.
The landscape architects and city foresters are not as fortunate as the commercial farmer. Few trees currently grown and sold as clones in the nursery trade have been purposely developed and thoroughly tested for pest resistance. The trees must endure for decades, often in difficult situations where environmental and biotic stresses are continually changing. The development of a new “replacement” for a clone, with similar characteristics of growth and pest resistance may require decades of research. Some of the clones now available for city planting were originally selected for certain aesthetics, propagated by budding and grafting, and marketed as named cultivars.
Cultivars
The use of seedling understock, whether of the same or a related species, introduces an element of diversity that might affect tree performance. Tree monocultures may only pose major problems when the numbers of trees are large and the area occupied by the trees is restricted. In other words, twenty to fifty trees of a single species, or even a single clone, planted along a few blocks of city streets do not constitute a “dangerous” monoculture. If 75% of the trees in the city were the same cultivar, that might become a problem.
Advantages of Cultivars
The most obvious advantage of cultivars is their reliability, especially those cultivars that have been in the nursery trade for 20 years or more. They can be counted on to develop the form, color, and growth rate for which they were selected. Their longevity in the trade and their widespread planting has provided the testing necessary to determine both their good and bad characteristics. The urban tree planter knows what to expect of such trees.
One other characteristic of most cultivars, especially those that had been traditionally propagated by budding and grafting, is their genetic capacity for strong wound compartmentalization. Studies have shown that every cultivar tested, in a wide range of genera and species, were strong compartmentalizers. The conclusion was made because the grafting and budding process constituted an inadvertent “screening” and only strong compartmentalizing trees would be amenable to long-term commercial propagation by these techniques. Some of the cultivars formerly propagated by budding and grafting are now propagated on their own roots and have retained this important trait.
Uncertainties of Cultivars
The major uncertainties of cultivars relate to the possibility of long-term graft incompatibility. Recent research has determined that graft compatibility is dependent on the similarity of stock and scion in cambial peroxidase enzymes, which mediate the production of lignin. In Castanea mollissima, Quercus rubra, and Acer rubrum graft incompatibility is a major problem because there is considerable tree-to-tree variability in enzyme patterns. On the other hand, Acer saccharum, Acer platanoides, and Gleditsia triacanthos appeared to be quite uniform in enzyme pattern and no graft incompatibility has been reported.
Another disadvantage of cultivars that are grafted to root stocks of a stronger species is that growth from below the graft can develop and change the intended characteristics of the tree. A good example of this would be the Higan Weeping Cherry (Prunus x subhirtella). These are often grafted high to create a taller single stem weeping specimen. Branches forming at or below the graft grow tall and straight with more vigor than the top thereby destroying the intended form and causing an unsightly tree. With proper maintenance and pruning, this is easily controlled but it does challenge the concept of sustainability.
Limited work on Cornus, Fagus, Fraxinus, and Koelreuteria indicated that graft incompatibility could cause problems in these genera. For those genera and species that are difficult to propagate from cultivars or by micro-propagation, the careful matching of enzyme patterns of stock and scion will produce graft-compatible combinations.
However, there are still many species that have not been studied and the landscape architect/city forester should be cautious in their use of new grafted cultivars of Celtis, Maclura, and Tilia.
Conclusion
It is recommended that you plant clones and cultivars that have proven their reliability. Use, somewhat cautiously and on a trial basis, some of the untested new cultivars of “unfamiliar” species or genera. These newer introductions have been developed through scientific research and have been selected for survival traits such as pest resistance or salt tolerance, but should be tested in your region for trial.
For uniformity, use seedlings of known geographic origin (or, in the case of exotics like Tilia cordata, from proven seed sources) so the plants will be able to tolerate the general climatic conditions in your area. For diversity, use the best clones, cultivars, and seedlings of many species and genera distributed throughout the city or as mixes of individual trees.
And, by all means, don’t stop planting natives when the site and climate will support them. Native habitats are moving northward due to climate change causing some plants to succumb to heat and pest stresses that they did not face in the past. Landscape architects, nurseries and horticulturists can adapt to this phenomenon by adjusting their climate zone maps accordingly and recognizing native habitats surrounding their project sites to determine appropriateness of selections for sustainability.
For the education of the next generation, plant a catalpa, a hickory, a horse-chestnut, a sassafras, and even a thorny honeylocust in park areas that can and should be used to stimulate an interest in the diversity of nature.
Sources
- Adams Garden, “Dealing with Cultivars and Clones”, Native Plants with Adams Garden, 2012.
- Hansen, Jolene, “Natives Vs Nativars”, Nursery Management, March 2018.
- Santamour, Frank S., Jr., “Uniformity and Common Sense”, METRIA 7 Proceedings, 1990.
The test that follows contains 30 questions. Before taking the test be sure you have read the article carefully. The passing grade is 80% on the entire test.
LA CES will award 1.0 PDH (HSW) credit for a passing grade. North Carolina Board of LA and New Jersey Board of Architects will award 1.0 credits for a passing grade. Approval is pending by .
The cost for taking this test is $20 per credit. If you purchase an annual subscription for 12 credits, the cost per credit is reduced by 50% (see Annual Subscription link below). We will report your passing test score to LA CES. If you are also ISA* certified we will report your passing score to ISA for no additional cost. Please be sure to add your ISA Certification number when you sign in. Tests with passing scores may be submitted only once to each organization.
*ISA has approved this course for .5 CEUs per section for a total of 1.5 CEUs. The Tree Diversification section is applied toward Certified Arborist, Municipal Specialist, Utility Specialist, or BCMA practice credits. The Invasive Plants section is applied toward Certified Arborist, Municipal Specialist, Utility Specialist, or BCMA science credits. The Tree Selection section is applied toward Certified Arborist, or BCMA science credits.
To take the test by the pay per test option, click on the 'Pay Now' button below where you can send payment securely with your credit card or Pay Pal account. After your payment is submitted, click on ‘Return to Merchant' / gibneyCE.com. That will take you to the test sign in page followed by the test. If you are an ISA and/or CLARB member, please be sure to include your certification/member number(s) along with your LA license and ASLA numbers.
To take the test as an annual subscriber with reduced rates, click on Password and enter your test password which will take you to the test sign in page. If you would like to become a subscriber see our Annual Subscription page for details.
When you have finished answering all questions you will be prompted to click ‘next’ to send your answers to gibneyCE.com. You can then click ‘next’ to view your test summary. A test review of your answers is available upon request. You can spend as much time as you would like to take the test but it is important not to leave the test site until you have answered all the questions and see the 'sending your answers' response.
Test re-takes are allowed, however you will have to pay for the retake if you are using the pay per test option.
All passing test scores are sent from gibneyCE.com to your organization(s) at the end of every month and they will appear on your certification record 4 to 6 weeks after that.
LA CES maintains a record of earned PDH credits on their website http://laces.asla.org/
ISA maintains a record of earned CEU credits on their website http://www.isa-arbor.com/