At the height of summer, sitting in the cool shade of a mature tree may sound wonderful, but not when the ground beneath it is an unappealing patch of bare dirt. Even if your plans don't include summer lounging, staring at a barren landscape is probably not high on your garden activities list. Don't despair. Regardless of your tree species or where you live, many ground covers are available to solve this potential problem.
Sharing Garden Space with Trees
Those spreading branches so wonderful for lounging under also cast dense shade in which most plants won't thrive. So when shopping for ground covers, No. 1 on the considerations list is finding shade-tolerant plants.
Because trees also compete with other plants for water and nutrients, add drought tolerance and easy growth habits to that list of potential ground cover candidates.
In addition, certain trees are alleopathic, which means they produce toxins that inhibit the growth of plants beneath them. Alleopathic trees include walnuts (Juglans spp.), such the black walnut (Juglans nigra), which is hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9a, and eastern red cedar (Juniperus virginiana, USDA zones 2 through 9).
Consideration of your tree species' susceptibility to root damage will influence your ground cover choice, too, because some trees -- such as maples (Acer spp., USDA zones 3 through 9) and elms (Ulmus spp., USDA zones 3 through 9) -- tolerate less soil disturbance at their roots than other trees.
To avoid disturbing roots when establishing a ground cover beneath a tree, sow seeds or plant very small seedlings, and dig only as deeply as absolutely necessary.
Ground Covers for Shade
All shade is not created equal; some ground covers make the transition from sun to shade or tolerate partial or dappled shade while others require deep shade. Additionally, some trees have more open growth habits than others, allowing less shade-tolerant ground covers to flourish beneath them. Keep the degree of shade under your trees in mind when choosing ground covers.
Common periwinkle (Vinca minor, USDA zones 4 through 9) is a particularly versatile ground cover, able to spread its glossy, green leaves and blue, starlike flowers from sun to deep shade with equal facility. In fact, it is such a persistent grower -- even invasive in some areas of the United States -- that it often oversteps its bounds and grows where not wanted. Place a physical barrier, which extends below its root zone as well as several inches above soil level, around common periwinkle's entire perimeter.
For heavy shade, choose wild ginger (Asarum canadense, USDA zones 4 through 8); lily of the valley (Convallaria majalis, USDA zones 4 through 9) with its delicate, bell-like, white flowers; the prolific evergreen standby English ivy (Hedera helix, USDA zones 5 through 11); or one of the most fragrant of flowering plants, sweet woodruff (Galium odoratum, USDA zones 4 through 7). English ivy is also listed as an invasive species in many states.
For light shade, consider mondo grass (Ophiopogon japonicus, USDA zones 6 through 11) or the grasslike creeping liriope (Liriope spicata, USDA zones 6 through 11), which also is invasive in some U.S. locations. For a vibrant two-season display in light shade, pair the heuchera cultivar 'Peppermint Spice' (Heuchera sanguinea, 'Peppermint Spice,' USDA zones 4 through 9), which has silvery-green and red leaves and a springtime splash of coral-pink flowers, with the autumn-flowering heuchera 'Lime Ruffles' (Heuchera sanguinea 'Lime Ruffles,' USDA zones 4 through 9), which has white blooms and brilliant-lime leaves. Both are mounding heuchera varieties.
For moderate shade, creeping wintergreen (Gaultheria procumbens, USDA zones 3 through 5) is an evergreen option with deep-red berries in winter. Another evergreen option is the Japanese pachysandra cultivar ‘Green Carpet' (Pachysandra terminalis 'Green Carpet,' USDA zones 5 through 9), which has glossy, deep-green leaves and is quite versatile, tolerating light shade as well as deep shade.
Ground Covers Compatible with Walnut Trees
In addition to requiring a high degree of shade tolerance, plants grown under walnut trees must not be sensitive to juglone, a toxin produced by walnut roots and leaves. Juglone inhibits the growth of other plants, reducing vegetative competition for walnuts. Many plants, however, exhibit juglone resistance or tolerance as well as shade tolerance.
Most ferns thrive in shade, and many will grow under walnuts. They include Christmas fern (Polystichum acrostichoides, USDA zones 3 through 9) and cinnamon fern (Osmunda cinnamomea, USDA zones 3 through 9).
Small and large hosta varieties (Hosta spp.) grow in walnut-tree shade and offer showy, spikelike flowers, usually in summer or early autumn. For deep shade, choose one of the small, blue-leafed hostas, such as ‘Baby Bunting’ (Hosta 'Baby Bunting,' USDA zones 3 through 8) or 'Popo' (Hosta 'Popo', USDA zones 3 through 8). Gold or lighter-leaved varieties such as 'Little Aurora' (Hosta 'Little Aurora,' USDA zones 3 through 8) tolerate a bit more sun. Most hostas thrive in USDA zones 3 through 8, but research your desired variety for its compatibility with your area because some varieties are tropical.
V**iolets (Viola spp.) of one kind or another grow in every U.S. state. The variety commonly called Heartsease (Viola tricolor Heartsease, USDA zones 3 through 9), with its three-colored blooms and heart-shaped leaves, offers a long flowering season and readily self-seeds, Astilbes, such as the snow-white flowering Japanese astilbe cultivar 'Deutschland' (Astilbe japonica 'Deutschland,' USDA zones 4 through 9), though not a conventional ground cover at 24 to 30 inches tall, really lights up shady areas beneath trees. The Siberian iris** (Iris sibirica, USDA zones 3 through 9) is another option for under walnut trees. Heartsease, 'Deutschland' and Siberian iris thrive in shade and tolerate walnut trees.
Ground Covers Compatible with Eastern Red Cedar
Due to the exceedingly low branches and tear-drop shape of eastern red cedar, the question of which ground cover to plant beneath it seldom arises. If, however, you have a large cedar pruned to hold its branches higher than normal, then certain native grasses may grow well under the tree. Grasses that evolved where eastern red cedar is abundant and, therefore, are able to handle its plant-inhibiting toxin, thujone, are your best bets. They include little bluestem (Schizachyrium scoparium, USDA zones 3 through 9). Also try the switchgrass cultivars 'Summer' (Panicum virgatum 'Summer,' USDA zones 3b through 6a) and 'Carthage' (Panicum virgatum 'Carthage', USDA zones 6 through 8).