What to plant under your landscape trees
Story and Photos by J.D. Willoughby
Mulch can be so plain, especially in a small garden that needs more color. One way that gardeners overcome this is by planting a variety of ground covers, perennials, and annuals that can thrive with competition and in the sometimes deep shade under trees.
Knowing what tree species you’ll be planting under and the level of soil compaction are the two main things to consider when planting under trees. Certain tree species, such as black walnut (Juglans nigra), are allelopathic, meaning they release chemicals that negatively impact the health of some plants. Tree-of-heaven (Ailanthus altissima) also releases chemicals detrimental to other plants.
Soil compaction is often an issue in suburban and urban landscapes. If your trees are well-established but have surface roots, consider hiring an arborist to remove the compacted soil from the critical root zone and replace it with high quality topsoil amended with compost. Surface roots are an indication that the soil is compacted and the roots need air. It also means that the soil has little room for water, which could stress the tree, particularly if more plants are added under the tree, competing for valuable resources. If the soil is compacted, plants will not thrive.
Piling soil or mulch over the root zone to create more planting space can cause the tree to decline. If the tree is new, till the soil and amend with compost to create a viable new bed that can support both the tree and plants. Large trees with extensive root systems, such as oaks (Quercus spp.), should not be disturbed unless they are deeply rooted and the surrounding soil can support additional plants. Smaller trees, such as crapemyrtle (Lagerstroemia spp.) are more likely to have space for planting.
Woody shrubs are not appropriate for planting under trees because of their own extensive root systems, however, some perennials and annuals will thrive. For planting under larger trees, consider a woodland garden with shade-loving perennials such as coral bells (Heuchera spp.), bleeding heart (Dicentra spectabilis), sweet woodruff (Galium odoratum), Hosta, common foxglove (Digitalis purpurea), oak sedge (Carex pensylvanica), and Lenten roses (Helleborus orientalis).
Sun-loving selections include black-eyed Susan (Rudbeckia hirta), purple coneflower (Echinacea purpurea), sweet alyssum (Lobularia maritima), creeping phlox (P. subulata), any Sedum, and ‘Walker’s Low’ catmint (Nepeta x faassennii ‘Walker’s Low’).
Trees that thrive in forest-like conditions are usually more accepting of understory plantings. Forests support spring ephemerals, perennials, annuals, shrubs, and trees of varying heights. Oaks in particular don’t have problems with understory plantings, provided their roots are not on the surface and there is adequate water and nutrients. If you’re unsure about which trees are better suited, it’s best to plant under and around a newly planted tree so the roots will grow together.
One of the best reasons for planting perennials and annuals under trees is because traditional turfgrass does not grow well in shady areas. There are several varieties of grasses and sedges that will grow in shade, including sea oats (Chasmanthium latifolium) and oak sedge. However, if the garden needs more color, perennials and annuals are the way to go.
If the bed will be surrounded by lawn, edge around the new garden with a flat shovel or other tool to sever the roots of any turfgrass. If there are no trees planted, a rototiller will make short work of the soil. Be sure to till in compost to provide extra nutrients for the plants and trees. If the tree has an extensive root system, determine the garden edge carefully. Avoid severing tree roots if possible. Probe the soil to find the tree roots. If they are not too extensive, plant new perennials and annuals in the pockets of soil that are available. Remember not to pile more soil over the tree roots to build it up to an appropriate depth. When all plants have been installed, spread 2-3 inches of mulch around the new garden. This will help the soil hold moisture, especially when the competition for resources becomes more substantial.
One of the most important things to monitor is water. New plants require a lot of water as they’re establishing a root system. Water deeply two or three times a week, depending on rainfall.
When planning a garden under trees, observe nearby woodland understories and try to mimic nature with your plantings. Notice the spacing and continuity of certain species to get an idea of what will work well in your garden and what will work well together. Do not discount spring ephemerals, such as Jack-in-the-pulpit (Arisaema triphyllum) or Virginia bluebells (Mertensia virginica), as they provide the first pops of color in an understory garden.
Cardinal flower (Lobelia cardinalis) blooms red in the shade and is a terrific source of summer color. Pink turtlehead (Chelone lyonii), also a shade-lover, blooms in late summer and early fall.
All of these woodland perennials would be excellent under established Japanese red maple (Acer palmatum), oaks, or dwarf red buckeye (Aesculus pavia).