Underplanting Trees - Respect the Roots

Roots are vital for growth and survival; they anchor a tree, absorb water and nutrients, and exchange carbon dioxide and oxygen from pore spaces in the soil. Some trees send their roots deeply; others are shallow-rooted, with roots atop the soil surface. Tree species exhibit a range of reactions to soil disturbance; some are sensitive, while others may tolerate such disturbance. A caution regarding planting under black walnut (Juglans nigra), its roots are allelopathic, preventing the growth of many plants within their root zone. To establish plantings under black walnut, research lists of plants adapted to growing alongside its roots.

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 herbicide for plant removal. Do not cut exposed roots with an ax, which could make a tree unstable.

Preserve the root flare on your tree. Adding a large volume of soil or compost atop the roots compacts the soil and prevents the essential gas exchange by root hairs. This leads to the tree's death, 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 two inches of good soil or compost to the planting area. Do not try to cover exposed roots or add soil within one foot of the trunk. Moisture retained by the new soil could cause tree bark to rot.

Plants grown under trees must tolerate shade, dry soil, and varying degrees of moisture and sunlight during the seasons. Tree roots absorb the available moisture and, when competing with other plants, always win. Dry shade perennials, once well 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 adds structure to the planting.

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), add early floral interest and bloom before the tree canopy leaves out. Ephemerals are just that; they die back in summer to return the following spring. In autumn, tuck a few snowdrops (Galanthus spp.) into the space, adding early-season flowers.

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 evergreen barrenworts (Epimedium spp.) or the gold of a Japanese forest grass cultivar (Hakonechloa macra) to brighten the planting.

Begin planting about one 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 are easier to work with and tuck into place. Fine-haired tree roots will regenerate if disturbed, but if you uncover a root two inches or more in diameter, 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 for the first year or even two years to ensure establishment. Avoid the occasional sprinkle with the garden hose; instead, water deeply once a week to encourage deep rooting.

Underplanting areas collect fallen leaves each autumn. Species native to woodland areas do well with a light, insulating blanket for evergreens and young plants. Leave the leaves as soil organisms will slowly break them down to compost. If the leaves accumulate deeply, gently rake some to avoid smothering tender plants. Use the relocated leaves as mulch in other planting beds or store them in their own area. Don't shred them! Keep the leaf litter intact to protect overwintering wildlife and pollinators.

Each spring, fertilize with a slow-release, organic fertilizer that plants and tree roots can use, reapply a thin layer of mulch or compost, and water well. Dry shade plants should be drought-tolerant after establishment. Still, monitor the garden for soil moisture and water during periods of drought stress. Then, step back and enjoy your garden for years to come.

Trees Tolerant of Some Root Disturbance

• red maple (Acer rubrum)
• river birch (Betula nigra)
• hornbeam (Carpinus spp.)
• hickory (Carya spp.)
• eastern redbud (Cercis canadensis)
• Washington hawthorn (Crataegus phaenopyrum)
• honeylocust (Gleditsia triacanthos var. inermis)
• Kentucky coffeetree (Gymnocladus dioicus)
• American holly (Ilex opaca)
• crabapple (Malus spp.)
• American hop-hornbeam, ironwood (Ostrya virginiana)
• Norway spruce (Picea abies)
• sycamore (Platanus occidentalis)

Trees Sensitive to Root Disturbance

• sugar maple (Acer saccharum)
• buckeye and horse-chestnut (Aesculus spp.)
• dogwood (Benthamidia spp., formerly Cornus spp.)
• beech (Fagus spp.)
• larch (Larix spp.)
• magnolia (Magnolia spp.)
• pine (Pinus spp.)
• cherry and plum (Prunus spp.)
• oak (Quercus spp.)
• linden (Tilia spp.)
• hemlock (Tsuga spp.)

Dry Shade Perennials

• black cohosh (Actaea racemosa)
• columbine (Aquilegia spp.)
• Lady's mantle (Alchemilla mollis)
• Canada anemone (Anemone canadensis)
• wood anemone (Anemone nemorosa)
• pussytoes (Antennaria spp.)
• bearberry (Arctostaphylos uva-ursi)
• wild ginger (Asarum canadense)
• Lady fern (Athyrium filix-femina)
• Siberian bugloss (Brunnera macrophylla)
• Pennsylvania sedge (Carex pensylvanica)
• Appalachian sedge (Carex appalachica)
• green and gold (Chrysogonum virginianum)
• Dutchman's breeches (Dicentra cucullaria)
• fringed bleeding heart (Dicentra eximia)
• Asian bleeding heart (Lamprocapnos spectabilis formerly Dicentra spectabilis)
• marginal wood fern (Dryopteris marginalis)
• barrenwort (Epimedium spp.)
• white wood aster (Eurybia divaricata, formerly Aster divaricatus)
• snowdrops (Galanthus)
• sweet woodruff (Galium odoratum)
• bigroot geranium (Geranium macrorrhizum)
• wild spotted geranium (Geranium maculatum)
• Lenten rose (Helleborus orientalis)
• coral bells (Heuchera spp.)
• Japanese forest grass (Hakonechloa macra)
• plantain lily (Hosta spp.)
• dwarf crested iris (Iris cristata)
• twinflower (Linnaea borealis)
• Virginia bluebells (Mertensia virginica)
• partridgeberry (Mitchella repens)
• Allegheny pachysandra (Pachysandra procumbens)
• golden groundsel (Packera aurea)
• woodland phlox (Phlox divaricata)
• Christmas fern (Polystichum acrostichoides)
• lungwort (Pulmonaria spp.)
• bloodroot (Sanguinaria canadensis)
• woodland stonecrop (Sedum ternatum)
• foamflower (Tiarella spp.)
• large-flowered trillium (Trillium grandiflorum)