Many landscapes have problem spots where nothing seems to grow properly or look attractive. The area below a large tree possessing a dense leaf canopy is a good example. Plants need light in order to thrive and, unfortunately, the area beneath a tree receives relatively little. Couple this with the competitive nature of the root system of many species of trees and one has a problem area to deal with. Fortunately, there are species of plants commonly referred to as ground covers that occupy areas such as the preceding quite well and serve as an attractive solution to the problem site.
By definition ground covers are (relatively) low, dense-growing plants requiring minimal maintenance that establish a monoculture in areas of the landscape. They can be classified into those that tolerate shade and those that require copious sunlight. The latter ground covers are often used for special purposes such as to prevent soil erosion or choke out weeds. For the sake of this article we will concentrate on those ground covers which can tolerate heavy shade such as in the situation described above.
Shade-tolerant ground covers fall into two distinct groups: evergreen and deciduous. Bugle weed (Ajuga reptans), creeping lily-turf (Liriope spicata), English ivy (Hedera helix), perwinkle (Vinca minor), Japanese spurge (Pachysandra terminalis), and wintercreeper euonymus (Euonymus coloratus) are among the best evergreen ground covers for shady areas. While most are planted for their foliage, several of the preceding (e.g. bugle weed and periwinkle) bear attractive flowers in the spring of the year as well.
Bishop's goutweed (Aegopodium podograria), lily-of-the-valley (Convallaria majalis), mock strawberry (Waldsteinia fragarioides), sweet violet (Viola odorata), sweet woodruff (Galium odoratum) and wild ginger (Asarum europaeum), and are deciduous ground covers that also thrive in shady areas. There are a number of other accent plants (e.g. hosta and fern) that can be used in heavily shaded areas, but they tend have lesser ground cover effect.
In addition to shade and roots, other factors may produce problems for plant growth and development beneath a tree. Trees with heavy canopies often shed water (especially during light rains) greatly reducing the availability of moisture to plants below. Hence, frequent watering of newly planted ground covers is needed for rapid establishment. Even after established, watering often becomes a necessity, especially in periods of dry weather. Adequate soil preparation through the incorporation of organic matter will help to encourage root growth and ease stress on newly planted ground covers. This, along with adequate water and fertility should cause them to establish themselves faster.
The accumulation of leaves is yet another problem for ground covers to contend with under trees, especially if the trees are large. Ground covers should never be allowed to be totally covered with leaves. This is especially critical for evergreen ground covers during the fall and winter. While a thin covering in the winter is not harmful, it should not be so think as to totally block light which evergreen plants need year around.
Leaves that are very soft (e.g. maple) and hold moisture should be totally removed from ground covers to prevent them from matting during wet weather those suffocating the plants beneath them. Leaves of species with stiffer leaves (e.g. oak) have a lesser tendency to suffocate plants below them when wet, if they are not too thick. Removing leaves from ground covers is more of a task than raking them from the lawn; therefore a leaf (lawn) vacuum can help reduce the work. A few leaves may be allowed to remain since they will not harm the plants below them and quickly break down during the ensuing growing season. This helps to add valuable organic matter to the soil occupied by the ground cover.
Growth rates vary by species, but most plants that tolerate shade do not grow quickly simply because there is little light for photosynthesis. Initial spacing of ground covers depends both on species planted and how rapid one wants a dense stand of that species. In most cases planting on six- to eight-inch centers is needed for quick cover. For those wishing to economize, wider spacing may be practiced with the realization it will require more time to establish a dense stand. If wider spacing is practiced, mulching the area between plants will help to conserve moisture, control weeds and make the area more attractive.
Under large trees with shallow, aggressive roots it might be difficult or impossible to improve the soil and plant the entire area. In such cases, it probably is more practical to establish pockets of improved soil where there are fewer roots and establish ground cover plants in those pockets. Once established, the ground cover's roots should compete fairly well with the tree's roots and establish an attractive, uniform stand. Remember, however, that the roots of trees like improved soil and will establish themselves in the pockets created. For this reason frequent watering and occasional fertilization especially is important for ground cover establishment and continued growth.
Finally, while ground covers tend to choke out competing weeds once established, shade-tolerant weeds can be problematic while the ground cover establishes itself. As mentioned above, mulching between plants can help prevent weeds. If weeds do appear they should be removed promptly to prevent them from slowing the development, speed of cover and general attractiveness of the desirable ground cover.
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REVISED: December 5, 2011