Mature trees bring ornament and structure to a residential landscape. But the ground beneath their dense canopies can be dry and barren. Establishing a planting under trees is not easy but can be achieved — with care.

Roots are vital for growth and survival; they anchor a tree, absorb water and nutrients from the soil and take up oxygen from the atmosphere. Some trees send their roots deep; others have theirs near the soil surface. Some trees are sensitive to soil disturbance while others tolerate plants at their feet. And a few, including black walnut (Juglans nigra) release chemical compounds that prevent the growth of many plants within their root zone. If you want to establish plantings under black walnut, look for plants adapted to grow there. (See below for lists of trees that will and won’t tolerate root disturbance.)

To prepare the planting area under a tree, remove any grass or weeds using a hand trowel. Shovels or spades can nick roots or trunk bark, exposing the tree to disease and insect infestation. Never use a chemical herbicide for plant removal. Do not cut exposed roots with an ax, as this could make a tree unstable.

Preserve the root flare on your tree. Do not try to cover exposed roots and do not add soil within 1 foot of the trunk. Moisture from the new soil could cause tree bark to rot, and soil next to the trunk will encourage insect nesting.

Adding a large volume of soil or compost atop the roots will prevent the uptake of oxygen and lead to the death of the tree, so do not be tempted to create a raised bed around a tree. It is best to work with existing site conditions. Add no more than 2 inches of good soil or compost to the planting area. 

Plants grown under trees must tolerate shade, dry soil and varying degrees of moisture and sunlight during the seasons. Tree roots will take most available moisture and, when competing with other plants, trees always win. Dry shade perennials, once established, are the best plants for this harsh environment.

Limit your plant choices to a few and plant them in swaths for a more naturalistic look. To unify the landscape, repeat plants that are already growing in shady parts of your garden. Include evergreen perennials, such as Lenten roses (Helleborus orientalis) and ferns that tolerate dry conditions such as Christmas fern (Polystichum acrostichoides) or marginal wood ferns (Dryopteris marginalis). Their evergreen presence will add structure to the planting. Barrenwort (Epimedium  and crested iris (Iris cristata) also do well.

For early floral interest and blooms before trees leaf out, try spring ephemerals such as bloodroot (Sanguinaria canadensis), Virginia bluebells (Mertensia virginica), Dutchman’s breeches (Dicentra cucullaria), bleeding heart (Dicentra spectabilis), wood anemone (Anemone nemorosa) and trillium (Trillium grandiflorum). Ephemerals are just that; they die back in summer only to return the following spring. In fall, tuck a few small spring-blooming bulbs into the space, adding a bit more early-season color.

Follow ephemerals and bulbs with plants whose foliage fills in and looks good all summer. Texture and foliage color create interest in the shade when flowers are sparse. Try the silver and red of the Japanese painted fern (Athyrium niponicum var. pictum) or the gold of a Japanese forest grass cultivar (Hakonechloa) to brighten shady areas. (See below for more plant suggestions.)

Begin planting 1 foot from the trunk and do not nick or split tree roots. Dig planting holes as small as possible. Landscape plugs, spring seedlings or small divisions of perennials have smaller root mass and will be easier to tuck into place. Fine-haired tree roots will regenerate if disturbed, but if you hit a root of 2 inches or more, refill the hole and move to a new location.

After planting, water and spread an inch or two of finely shredded bark mulch or compost over the area to keep moisture in the soil and deter weed growth, then water again. Water consistently the first year and water deeply once a week to encourage deep rooting. Each spring, fertilize with a slow-release, organic fertilizer that can be used by both plants and tree roots, reapply mulch/compost and water in well. Dry shade plants should be drought-tolerant but monitor the garden for soil moisture and water during periods of drought stress. Then, step back and enjoy your garden for years to come.

Elise Ford is a Penn State Master Gardener. This volunteer program supports the outreach mission of Penn State Extension and provides research-based information on best practices in sustainable horticulture and environmental stewardship. Information: alleghenymg@psu.edu or 412-482-3476.

Dry shade perennials

barrenwort (Epimedium)

black cohosh (Actaea racemosa)

bigroot geranium (Geranium macrorrhizum)

Christmas fern (Polystichum acrostichoides)

columbine (Aquilegia)

crested iris (Iris cristata)

foamflower (Tiarella)

green & gold (Chrysogonum virginianum)

Japanese forest grass (Hakonechloa macra)

Japanese painted fern (Athyrium niponicum)

lady’s mantle (Alchemilla mollis)

Lenten rose (Helleborus orientalis)

lilyturf (Liriope)

lungwort (Pulmonaria)

marginal wood fern (Dryopteris marginalia)

Pennsylvania sedge (Carex pensylvanica)

plantain lily (Hosta)

Siberian bugloss (Brunnera macrophylla)

sweet woodruff (Galium oderatum)

wild ginger (Asarum canadense)

white wood aster (Aster nemorosa)

Small spring bloomers

crocus 

English bluebell (Hyacinthoides non-scripta)

glory of the snow (Chionodoxa)

grape hyacinth (Muscari)

Siberian squill (Scilla siberica)

snowdrops (Galanthus)

Spanish bluebells (Hyancinthoides hispanica)

spring beauty (Claytonia virginica)

winter aconite (Eranthis hyemalis)

Trees that tolerate underplanting

American holly (Ilex opaca)

crabapple (Malus)

Eastern redbud (Cercis canadensis)

hickory (Carya)

honeylocust (Gleditsia triacanthos var. inermis)

hornbeam (Carpinus)

ironwood (Ostrya virginiana)

Kentucky coffeetree (Gymnocladus dioicus)

Norway spruce (Picea abies)

red maple (Acer rubrum)

river birch (Betula nigra)

sycamore (Platanus occidentalis)

Washington hawthorn (Crataegus phaenopyrum)

Trees with sensitive roots

beech (Fagus)

buckeye (Aesculus)

cherry/plum (Prunus)

dogwood (Cornus)

hemlock (Tsuga)

larch (Larix)

linden (Tilia)

magnolia (Magnolia)

oak (Quercus)

pine (Pinus)

sugar maple (Acer saccharum)

First Published: October 15, 2021, 10:00 a.m.