The area beneath a mature tree presents one of the most complex environments for planting in a landscape. This space is defined by deep shade, dry soil resulting from the rain shadow effect, and intense root competition for limited water and nutrients. Older trees have expansive, shallow root systems that aggressively absorb available moisture and minerals, creating a challenging habitat known as dry shade. Successfully introducing new plants requires selecting species that tolerate these conditions while protecting the established tree’s long-term health.
Preparing the Planting Area and Protecting Tree Roots
The primary rule when planting under a tree is to avoid disturbing the tree’s root system. Roots are largely concentrated in the top 12 to 18 inches of soil and extend well beyond the drip line. Deep digging with spades or shovels can sever these essential roots, causing significant stress to the tree. Use a hand trowel for small, precise excavations to place new plants in pockets of soil between existing roots. Never cut any root larger than the diameter of your thumb, as this can severely compromise the tree’s stability and health.
Adding too much new soil over the root zone is a major hazard, as it can suffocate the tree by blocking the exchange of oxygen and carbon dioxide. The absolute maximum amount of soil to add is four inches. A thin layer of one to two inches of compost or topsoil is much safer. This light layer can be gently worked into the existing surface soil to provide an enriched planting medium without inhibiting gas exchange. Avoid creating a raised bed or berm, which traps moisture against the trunk and can lead to bark rot and root death.
After planting, apply a two-to-three-inch layer of organic mulch to retain moisture and regulate soil temperature. This mulch layer also reduces weed competition. Keep the mulch several inches away from the tree trunk to prevent moisture buildup that invites pests and disease. Consistent, deep watering is necessary for the first year or two to ensure the underplantings establish successfully against the mature tree’s competition.
Low-Growing Groundcovers for Deep Shade
Functional groundcovers are the ideal choice for creating a cohesive carpet of foliage under trees. They often possess shallow root systems that can coexist with the tree’s roots. These spreading plants cover large areas and suppress weeds in challenging dry shade conditions. Vinca minor, or periwinkle, is a reliable, evergreen choice that forms a dense mat and tolerates dry soil once established. Check local guidelines for invasiveness before planting, as its vigorous, vining nature can become overly aggressive.
Pachysandra terminalis, or Japanese spurge, is another popular, mat-forming option that thrives in deep shade. It features upright stems with whorls of glossy green leaves. Its dense habit provides year-round coverage, making it an excellent alternative to turfgrass that struggles under a canopy. For a more delicate look, Epimedium, or barrenwort, is recommended for dry shade. Its wiry rhizomes spread slowly without competing aggressively with the tree. Barrenwort is prized for its drought tolerance and attractive, heart-shaped foliage that often changes color in the fall.
Galium odoratum, or sweet woodruff, forms a low, fragrant mat and produces tiny white flowers in spring. This plant excels in the acidic conditions often found under pine and fir trees, though it may become invasive if the soil is consistently moist. When selecting any spreading plant, choose varieties with a proven tolerance for the specific conditions of dry, deep shade.
Perennials and Bulbs for Seasonal Color
While groundcovers provide the foundational green layer, accent perennials and bulbs introduce pockets of texture and seasonal color. Hosta species are a staple for this environment, offering a variety of leaf sizes, colors, and textures, from large blue-green foliage to small, variegated forms. Their broad leaves contrast beautifully with the finer textures of ferns. Hostas are typically planted in small groupings rather than as a uniform groundcover.
Ferns are essential for adding a woodland aesthetic. Species like the Japanese Painted Fern (Athyrium niponicum) provide a delicate, silvery texture and color without aggressive root competition. While many ferns prefer consistent moisture, the Christmas Fern (Polystichum acrostichoides) is drought-tolerant once established, offering year-round evergreen interest. For early spring blooms, shade-tolerant bulbs like Snowdrops (Galanthus) or miniature Daffodils are effective. They complete their flowering and energy storage cycle before the tree canopy fully develops and casts its deepest shade.
The Lenten Rose (Helleborus orientalis) is valuable for its evergreen foliage and blooms that emerge in late winter or early spring, providing color when little else is active. Brunnera macrophylla, especially silver-leafed cultivars like ‘Jack Frost,’ offers vibrant, heart-shaped foliage that brightens dark corners and produces sprays of tiny blue flowers in spring. These accent plants are best placed in the limited spaces between established groundcovers, allowing their unique characteristics to stand out against the functional green backdrop.