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.
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.
Note: Click on green text in each section for more information and photos.
Living Mulch
By Len Phillips
In the practice of general plant care, a living mulch is a ground cover plant growing at the base of a tree. Living mulches are sometimes used at locations where the tree is growing in a sidewalk tree cut-out and the living mulch (plants) are growing around the tree. The living mulch is intended to beautify and replace the need for a tree grate or conventional mulch by using plants to provide weed suppression, control of the soil temperature, and a visual means to discourage people from walking close to the tree and causing soil compaction. Living mulches can reduce water runoff and erosion, and filter surface water runoff for use by the tree. Living mulches have also been shown to increase the population of organisms which enhance the quality of the soil. Living mulches are intended to grow for many years in harmony with the tree. Opponents of this concept believe the living mulch robs the tree of soil moisture, nutrients, and microorganisms. But the fact is that these plants actually help the tree by preventing evaporation of soil moisture and converting nutrients into forms that are useful for the tree.
Plant selection for creating a living mulch is not an easy task. While just about any plant might grow in harmony with a tree in a conventional landscape, very few will thrive living in the sidewalk cutout surrounded by concrete. The planting of the ground covers may occur anytime including right after the tree has been planted. The plants should be planted on a grade of no more than 1 inch (2.5 cm) below the sidewalk elevation to hold runoff water for the plants and tree. The living mulch plants should be carefully placed around the outside edge of the sidewalk cutout to avoid any conflict with the newly developing tree roots and the underground root stabilizing system.
Never plant anything within 12" (30 cm) of the flare. No part of the tree's trunk and bark should ever be covered with anything that retains moisture. Doing so encourages rot. In addition, any planting of small ground covers needs to be done carefully so as not to damage the existing tree's roots. Open areas should still be mulched with composted wood chips so the soil is never left bare.
Most Desirable Living Mulches
The following, in this author's opinion, are the best living mulches because they are vigorous ground covers and will tolerate pedestrian traffic, winter snow plow damage, and erratic weather conditions:
White clover Trifolium repens - Clovers with their deep roots, provide resistance to soil compaction. Legume living mulches also have some important positive effects on the nutrient cycling that benefit the tree. For example, clover fixes the atmospheric nitrogen that is important for tree growth and it is effective for enhancing and recycling the soil's nutrients. Clover has the ability to grow in sun or shade, maintain a low profile, thrive if repeatedly mowed and it will withstand foot traffic. It is considered by many as the best choice for a living mulch. In established stands, it also may provide some nitrogen suitable for use by the tree.
Vinca Vinca minor - Vinca does well in all light conditions and, once established, is fairly tolerant of a less than desirable site and soil conditions. Vinca is a shallow rooted creeper that tolerates soil compaction and pedestrian foot traffic, once it is established. It should be planted with quart size plants that have large root systems to reduce the time for the plant to become established. These larger plants are quite developed and fewer plants are needed so there's a lot less disturbance in the sidewalk cutout area. Impact on the tree is minimal.
Creepers
Creepers are plants that can grow to cover the soil from the outside edge of the sidewalk cutout where there is more light, and they may grow toward the trunk of the tree with time. These are all shallow-rooted perennials with roots in the top 6 inches (15 cm) of soil, and therefore, above the tree's main roots. They are all tolerant of varying sun exposures. Creepers do require weeding and extra water for the first few years but once established, the plants look outstanding all year. All the plants cool the roots and discourage any compacting foot traffic. Reports indicate that there has to be some assumed nutrient competition between tree and the ground cover but these reports also indicate that both the tree and the creepers appear to be living in harmony. Some of the best creepers for this function include:
Ajuga Ajuga reptans
Peanut Arachis repens
Epimedium Epimedium spp.
Wild Woodland Strawberry Fragaria vesca
Coral Bells Heuchera spp.
Mini and dwarf Hosta vareities Hosta spp.
Yellowroot (Golden Seal) Hydrastis Canadensis
Liriope Liriope muscari
Cupflower Mazus repens
Pachysandra Pachysandra
Ginseng Panax quinquefolius
Woodland Phlox Phlox divaricate
Stonecrop Sedum spp.
Comfrey Symphytum officinale
Creeping Thyme Thymus serpyllum
Turkish Speedwell Veronica liwanensis
Barren Strawberry Waldsteinia ternate
Hardy Ferns Several species are suitable for living mulch, especially in somewhat shady areas.
Undesirable Living Mulches
Not only are the plants listed below considered invasive in many locations, they can also cause moisture retention, root rot, and other problems for the tree. Many of these plants can also climb over the entire tree. NOT recommended are:
Porcelain Berry Ampelopsis brevipedunculata
Climbing Euonymus Euonymus fortunei
English Ivy Hedera helix
Kudzu Pueraria lobata
Bittersweet Solanum dulcamara
Poison Ivy Toxicodendron radicans
Grape Vitis spp.
Wisteria Wisteria spp.
Annuals – Growing annuals under young trees is the exception to being good for trees. Annuals are aggressive growers and they can cause growth reduction to the tree because of the competition for soil nutrients. Annuals also need lots of water and weekly maintenance.
Shrubs – Often young trees have been planted and combined with nice flowering shrubs. Over time the trees will win the competition and the shrubs look terrible and need to be removed, so they are not a good choice in the sidewalk cut out areas.
Creeping Juniper Juniperus horizontalis is a very popular group of junipers that require a lot of maintenance to keep them from growing over the flare and sidewalk, while also
encouraging pests and disease problems causing harm to the tree. These problems are in addition to the problems mentioned above with annuals and shrubs.
Caution
Not all trees are created equal. Each requires specific light, soil, and moisture conditions to survive and remain healthy. Some tree species are extremely sensitive to major soil disturbance. Planting beneath trees that are sensitive to having their roots disturbed may need to have the proposed plantings scaled back or carried out over a few years.
Source
Special thanks to members of LinkedIn's Urban Forestry discussion group for sharing their favorite living mulches.