Alternatives to mulch and turf under trees
Story and Photos by Richelle Stafne
Trees are the backbone of the landscape and there are several reasons for landscaping under and around them with something other than lawn grass. Under shadier tree canopies, it can be difficult if not impossible to grow a lawn. A lack of rooted plant material results in exposed soil, which can eventually lead to erosion. Under a more open canopy or younger trees, using ornamental landscape plants rather than turf will reduce time spent mowing and will also eliminate mower and string trimmer damage to trees. Creating sweeping beds that connect multiple trees pulls a landscape together and can direct the eye to interesting views of the home and garden. These connected beds flowing through the landscape can also direct foot traffic. A landscape dominated by trees underplanted with ornamentals has greater value and curb appeal than those with turf or mulch alone.
One of the most important things to remember when planting ornamentals under and around trees is to avoid piling too much soil over the root area of the tree. Most tree roots are in the upper few inches of soil. Beds built on top of a tree’s root zone can, unfortunately, encourage tree roots to grow upward, into the bed. In addition, adding thick layers of soil and mulch can suffocate the tree’s root system, eventually leading to root death and tree failure. Try to add no more than 1-2 inches of soil, adding very little near the trunk and increasing the depth gradually as you move away from the trunk.
While adding a shallow layer of organic mulch (2-4 inches) will reduce mowing, erosion, and damage from string trimmers and mowers, it is as not as appealing as colorful ornamental plants. The ornamental plants chosen will depend on soil pH, soil type, tree species, density of the tree canopy, tree age, and the presence of allelopathic chemicals.
The amount of shade under a tree can be reduced somewhat by the pruning technique called thinning. In the thinning process, select branches are removed throughout the canopy to allow more sunlight to reach the ground below. A proper pruning job would likely be undetectable to the untrained eye.
Use caution when planting around crapemyrtles (Laegerstroemia spp.), as their roots tend to sucker endlessly once disturbed. Allow the old, barked, surface roots of large established trees to become part of the beauty of the bed. Do not force a new planting to grow where tree roots are thick. Instead, consider placing a garden bench, sculpture, or birdbath in those areas. Take photographs of the tree and potential bed from the road or sidewalk to help you visualize and accentuate the positive with your new under-canopy planting project.
Ornamental plants should complement their tree companions. Daffodils (Narcissus spp.), spring-blooming herbaceous plants such as Lenten rose (Helleborus spp.), creeping phlox (P. subulata), and annual plantings such as pansies (Viola x wittrockiana) and snapdragons (Antirrhinum majus) make a big impact under and around spring-flowering trees such as dogwood (Cornus spp.), redbud (Cercis canadensis), and saucer magnolia (Magnolia x soulangeana). Dwarf evergreen and broadleaf evergreens, especially variegated varieties, can provide interesting color, form, texture, and fruit and offer big winter impact when deciduous trees have lost their leaves. A single evergreen species planted en masse under a tree canopy can be dramatic, yet simple. Examples include cast-iron plant (Aspidistra elatior), variegated Liriope, and Asiatic jasmine (Trachelospermum asiaticum). Pachysandra terminalis and Ajuga reptans are also low-growing ground cover options.
Where garden beds incorporate very young trees, vegetable and herb gardens can be grown while there is limited tree root disturbance and maximum sunlight penetration. The possibilities are nearly endless and can add pizzazz and value to a landscape.
KNOW YOUR NEIGHBORS
Allelopathy is a common biological phenomenon by which one organism produces biochemicals that influence the growth, survival, development, and reproduction of other organisms. These biochemicals are known as allelochemicals and have beneficial or detrimental effects on target organisms.*
This may occur via gasses released from leaf openings, chemicals leached from fallen leaves, or chemicals exuded via roots. Examples of allelopathic trees that may inhibit the growth of nearby plants include hackberry (Celtis occidentalis and C. laevigata), Eucalyptus, black walnut (Juglans nigra), oak (Quercus spp.), and pecan (Carya illinoinensis).
*Cheng, F., & Cheng, Z. (2015). Research Progress on the use of Plant Allelopathy in Agriculture and the Physiological and Ecological Mechanisms of Allelopathy. Frontiers in Plant Science, 6, 1020. http://doi.org/10.3389/fpls.2015.01020.