Classics
The following article has been selected because it is deemed very important to the arboricultural profession
and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Recommending Street Trees
Edited by Len Phillips
What is an ideal street tree?
It should tolerate the local urban environment and pollution problems.
It should live at for least 50 and preferably 100 years.
It should be beautiful all year round.
It should have few pest problems.
It should be disease resistant.
It should tolerate local soils.
It should be hardy.
It should tolerate road salt.
It should have seasonal color.
It should not be dropping fruit.
It should need little maintenance.
It should not be susceptible to die back.
It should not have brittle limbs or anchorage problems.
It should not have surface roots, especially for growing near sidewalks.
However, since one perfect street tree does not exist, tree selections should be made by using the best trees that we do have. The first step consists of creating a list of trees that are currently doing well in your city. The list can be composted of the tried and true from previous planting experience and the results of the best trees found in the street tree inventory. This list is modified by removing those materials that experience has indicated do poorly in local roadside or urban conditions. Also remove from this list, trees with high maintenance costs, surface roots, intolerance to storms, and undesirable fruiting habits.
Then add natives, cultivars of natives, and hardy trees that are likely to tolerate the local conditions. Add trees that are tough in terms of pest resistance and those that should do well in the city. It is advisable to compare the selections with what other authorities and neighboring cities have recommended. Also review books, local universities, and extension services. Local nurseries can be used in order to determine what species are available as well as those that the nursery has recommended. The internet might be helpful for providing information about specific trees provided the source is fairly local such as this University of Florida website.
Once a list of trees is compiled, it should remain open so new trees can be added and failures can be deleted as the situation warrants. The list of trees to be used should cover half the trees being ordered for the next planting season. The other half of the order should include 2 – 10 trees each of species or cultivars of trees that might be suitable but are untried in your community. These trees should be considered as a trial to see how well they tolerate your city. Those that look great can be added to the recommended list of trees and the trees that do poorly can be added to a list of trees that should not be used again. The reason for selected 2 – 10 trees is to reduce the expense if the trees are a failure and have to be replaced. Two trees would be suitable on a limited budget and you are planting 300 trees or less. Ten trees would be suitable for the largest cities where the trees could be planted in different locations all around the city to see if one spot might be better than others.
Once the tree planting list is established it can be used to fit the trees to the planting sites in accordance with the municipal regulations, utilities, structures, climate, arboricultural standards, and other urban and environmental factors. The size of the tree at maturity must also be considered when locating a tree at its permanent site. The planting list should be made a part of the Street Tree Master Plan.
Unacceptable Trees
There may be species that are known to be unacceptable in your region or have failed in your city. These should be posted on a list of unacceptable trees and made a part of your Street Tree Master Plan so future removal expense is postponed. Any failures from your trials should also be added to this list. By keeping track of the failures, new cultivars might come along that will tolerate the reason for failure. Since you know what the problem is, you should try the new tree to see if it is successful in avoiding the problem.
Diversification Formula
The Diversification Formula has been developed to prevent an over dependence on a single genus such as ash or American elm. The Diversification Formula was established by the International Society of
Arboriculture (ISA) and is now an accepted standard throughout the United States and other countries around the globe. The Formula is defined as a planting plan containing:
Enacting a species diversity plan implies three simple actions:
The Diversification Formula should be carried one step further. It should be applied to the existing street tree inventory so the dependence on a single species from previous or natural plantings will not be carried forward into future planting. The Diversification Formula was set up so if, for example 3% of a city’s tree population contained red oak and a disease or insect killed them all, then 97% of the trees would still remain. The American elm was as much as 50% of the urban tree population before Dutch elm disease devastated landscapes across North America. The Emerald Ash borer is repeating the process and going for the ash trees that many communities planted to replace the American elms. The Diversification Formula may be difficult to use in places where the number of trees is severely restricted due to soil or climatic conditions, but that does not mean that diversification can be ignored.
The City of Lansing, Michigan has a diversification policy which states: “No Tree will be planted next to a tree of the same species and at least four genus will be planted on every street block.” This adaptation of a diversification policy works very well. The result is that Lansing still has tree-lined streets despite the fact that the Emerald Ash borer has been in the area since 2002 and most cities in the region have lost almost all of their trees.
Aesthetic Value
Aesthetics and function should work together to determine the selection of an urban tree species. What is the purpose of the tree? Is it to screen views, provide shade, enhance the aesthetics, enclose a space or improve the site and environment? Definite shaped trees and ornamental flowering or fruiting trees should be carefully selected and located to avoid distractions to a driver while at the same time used to improve aesthetics. The tree itself can become the focal point through unique color, spectacular flowers, large fruit, or fall foliage. Trees should be used to assist in defining the edge of the roadway and enhancing spatial qualities. Spacing between trees should vary from 30 to 150 feet apart to create a naturalized appearance, and if one tree dies or is removed it does not spoil the continuity of the planting.
Trees need sufficient unpaved areas and good soil for maximum health and vigor. They should be allowed to develop into perfect specimens and true representatives of their species. They should not be dangerously close to traffic in the street and could, if local ordinances allow, be planted behind the sidewalk to take advantage of good soil, open space, and a better growing environment.
Blocks versus Mixed Design
Street tree plantings can be designed in one of two layouts depending upon the city, past practice, and location in the US. Block planting means that an entire block of a city street will have only one species of tree planted on that block. Ideally the trees are all planted at the same time so they look the same throughout their lifetime. Aesthetically, all trees look identical on the block, which unifies a neighborhood with a common species. The space can be equal and in the mid-western and western states, this planting concept is quite compatible with the grid of the street patterns. This design provides a feeling of unity, equality, and aesthetic compatibility. This makes the maintenance requirements considerably less because all maintenance would be approximately the same on each tree. Theoretically, the Diversification Formula is not violated since the next block would contain a totally different species. However, this planting style can mean planting trees in inappropriate locations and a disease or pest will wipe out the whole block at once.
Mixed plantings mean diversity because each tree on a city block is different than the tree beside it. The trees can be planted with different spacing, sizes, and shapes. Many cities prefer this design approach since each house is different; the tree can also be different. This design is customarily found where the streets flow in random or curvilinear patterns. This planting concept also tends to keep the trees healthier because diseases will not progress from one tree to the next and trees grow naturally in random sizes, spacing, and species. However, maintenance costs will be higher and there may not be the aesthetic harmony that would be apparent with block planting. Many people prefer this concept because it seems more natural and sustainable.
The following article has been selected because it is deemed very important to the arboricultural profession
and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Recommending Street Trees
Edited by Len Phillips
What is an ideal street tree?
It should tolerate the local urban environment and pollution problems.
It should live at for least 50 and preferably 100 years.
It should be beautiful all year round.
It should have few pest problems.
It should be disease resistant.
It should tolerate local soils.
It should be hardy.
It should tolerate road salt.
It should have seasonal color.
It should not be dropping fruit.
It should need little maintenance.
It should not be susceptible to die back.
It should not have brittle limbs or anchorage problems.
It should not have surface roots, especially for growing near sidewalks.
However, since one perfect street tree does not exist, tree selections should be made by using the best trees that we do have. The first step consists of creating a list of trees that are currently doing well in your city. The list can be composted of the tried and true from previous planting experience and the results of the best trees found in the street tree inventory. This list is modified by removing those materials that experience has indicated do poorly in local roadside or urban conditions. Also remove from this list, trees with high maintenance costs, surface roots, intolerance to storms, and undesirable fruiting habits.
Then add natives, cultivars of natives, and hardy trees that are likely to tolerate the local conditions. Add trees that are tough in terms of pest resistance and those that should do well in the city. It is advisable to compare the selections with what other authorities and neighboring cities have recommended. Also review books, local universities, and extension services. Local nurseries can be used in order to determine what species are available as well as those that the nursery has recommended. The internet might be helpful for providing information about specific trees provided the source is fairly local such as this University of Florida website.
Once a list of trees is compiled, it should remain open so new trees can be added and failures can be deleted as the situation warrants. The list of trees to be used should cover half the trees being ordered for the next planting season. The other half of the order should include 2 – 10 trees each of species or cultivars of trees that might be suitable but are untried in your community. These trees should be considered as a trial to see how well they tolerate your city. Those that look great can be added to the recommended list of trees and the trees that do poorly can be added to a list of trees that should not be used again. The reason for selected 2 – 10 trees is to reduce the expense if the trees are a failure and have to be replaced. Two trees would be suitable on a limited budget and you are planting 300 trees or less. Ten trees would be suitable for the largest cities where the trees could be planted in different locations all around the city to see if one spot might be better than others.
Once the tree planting list is established it can be used to fit the trees to the planting sites in accordance with the municipal regulations, utilities, structures, climate, arboricultural standards, and other urban and environmental factors. The size of the tree at maturity must also be considered when locating a tree at its permanent site. The planting list should be made a part of the Street Tree Master Plan.
Unacceptable Trees
There may be species that are known to be unacceptable in your region or have failed in your city. These should be posted on a list of unacceptable trees and made a part of your Street Tree Master Plan so future removal expense is postponed. Any failures from your trials should also be added to this list. By keeping track of the failures, new cultivars might come along that will tolerate the reason for failure. Since you know what the problem is, you should try the new tree to see if it is successful in avoiding the problem.
Diversification Formula
The Diversification Formula has been developed to prevent an over dependence on a single genus such as ash or American elm. The Diversification Formula was established by the International Society of
Arboriculture (ISA) and is now an accepted standard throughout the United States and other countries around the globe. The Formula is defined as a planting plan containing:
- no more than 10% of any species,
- no more than 20% of any genus,
- no more than 30% of any family.
Enacting a species diversity plan implies three simple actions:
- minimize planting of overused species,
- increase the planting of underused species,
- introduce new species and cultivars into the landscape.
The Diversification Formula should be carried one step further. It should be applied to the existing street tree inventory so the dependence on a single species from previous or natural plantings will not be carried forward into future planting. The Diversification Formula was set up so if, for example 3% of a city’s tree population contained red oak and a disease or insect killed them all, then 97% of the trees would still remain. The American elm was as much as 50% of the urban tree population before Dutch elm disease devastated landscapes across North America. The Emerald Ash borer is repeating the process and going for the ash trees that many communities planted to replace the American elms. The Diversification Formula may be difficult to use in places where the number of trees is severely restricted due to soil or climatic conditions, but that does not mean that diversification can be ignored.
The City of Lansing, Michigan has a diversification policy which states: “No Tree will be planted next to a tree of the same species and at least four genus will be planted on every street block.” This adaptation of a diversification policy works very well. The result is that Lansing still has tree-lined streets despite the fact that the Emerald Ash borer has been in the area since 2002 and most cities in the region have lost almost all of their trees.
Aesthetic Value
Aesthetics and function should work together to determine the selection of an urban tree species. What is the purpose of the tree? Is it to screen views, provide shade, enhance the aesthetics, enclose a space or improve the site and environment? Definite shaped trees and ornamental flowering or fruiting trees should be carefully selected and located to avoid distractions to a driver while at the same time used to improve aesthetics. The tree itself can become the focal point through unique color, spectacular flowers, large fruit, or fall foliage. Trees should be used to assist in defining the edge of the roadway and enhancing spatial qualities. Spacing between trees should vary from 30 to 150 feet apart to create a naturalized appearance, and if one tree dies or is removed it does not spoil the continuity of the planting.
Trees need sufficient unpaved areas and good soil for maximum health and vigor. They should be allowed to develop into perfect specimens and true representatives of their species. They should not be dangerously close to traffic in the street and could, if local ordinances allow, be planted behind the sidewalk to take advantage of good soil, open space, and a better growing environment.
Blocks versus Mixed Design
Street tree plantings can be designed in one of two layouts depending upon the city, past practice, and location in the US. Block planting means that an entire block of a city street will have only one species of tree planted on that block. Ideally the trees are all planted at the same time so they look the same throughout their lifetime. Aesthetically, all trees look identical on the block, which unifies a neighborhood with a common species. The space can be equal and in the mid-western and western states, this planting concept is quite compatible with the grid of the street patterns. This design provides a feeling of unity, equality, and aesthetic compatibility. This makes the maintenance requirements considerably less because all maintenance would be approximately the same on each tree. Theoretically, the Diversification Formula is not violated since the next block would contain a totally different species. However, this planting style can mean planting trees in inappropriate locations and a disease or pest will wipe out the whole block at once.
Mixed plantings mean diversity because each tree on a city block is different than the tree beside it. The trees can be planted with different spacing, sizes, and shapes. Many cities prefer this design approach since each house is different; the tree can also be different. This design is customarily found where the streets flow in random or curvilinear patterns. This planting concept also tends to keep the trees healthier because diseases will not progress from one tree to the next and trees grow naturally in random sizes, spacing, and species. However, maintenance costs will be higher and there may not be the aesthetic harmony that would be apparent with block planting. Many people prefer this concept because it seems more natural and sustainable.