Made in the shade
While shade may be welcome relief on a hot summer day, landscape managers know it poses maintenance challenges for landscape plants. Shade limits plant growth because plants need light to photosynthesize and thrive. With less light, plants are less vigorous and can be prone to problems that would not be a factor in higher light levels. Various levels of shade combined with differing soil-moisture conditions complicate the picture, requiring landscape managers to carefully evaluate site conditions to create plantings that will succeed in a shady location.
Types of shade Dense, deep shade occurs under the canopies of trees with close-growing branches or in groves of closely spaced trees or shrubs. When such shade lasts all day, we call it full shade. Because so little light reaches the ground in full shade, it limits the growth of all but the most shade-tolerant plants. Partial shade refers to locations that receive some periods of sun during the day. Light or "dappled" shade may occur under more widely spaced plantings or under trees with airy branching structures, such as ginkgoes and honeylocust. These environments are still shady but brighter than deep shade, permitting more choices of plant material that will grow well there.
As with any other type of site, shaded locations can be either dry or wet. Sites may be persistently wet due to drainage or soil conditions, or dry due to heavy competition by roots of existing vegetation. Soil moisture adds another critical factor for your selections of plant material for shady locations.
Turf in the shade Some turf specialists estimate that up to 25 percent of the turf in the United States may be growing in the shade. Unfortunately, turf growing in the shade typically loses out. Pit tall fescue against a ponderosa pine, and the tree will win nearly every time. Of course, exceptions exist. A few species are adapted to moderate shade. Others, though they are not well adapted to shade, may at least persist there when adverse conditions in open, sunny areas are causing more exposed turf to perish.
Growing conditions are radically different in shade compared to sunny areas. Not only does the quantity of light decrease in shade, but light quality changes as well. After passing through canopy foliage, sunlight loses much of the near-red wavelengths predominant in sunlight and is proportionately richer in the less photosynthetically effective far-red wavelengths. Blue wavelengths are another component critical for growth that dense canopies largely filter out (particularly conifers). The result of these changes is a reduction in photosynthesis and its products, including carbohydrates and other constituents needed for growth. Leaves, leaf cuticles and stems are thinner in shade. Shoot density drops and rhizome and stolon numbers decrease. Shade-grown plant tissues are succulent and more susceptible to environmental stresses and disease. In addition, dew and transpired moisture take longer to dissipate in shade than in sunny locations. This additional moisture may contribute to increased disease incidence.
Tree roots compete with turf for water and nutrients. This competition can further weaken turf growing in shade. Allelopathic effects may also be a factor. Allelopathy is a negative influence of one living organism upon another by the production of suppressive or inhibitory substances. An example of an allelopathic effect is the inhibitory effect of silver maple on Kentucky bluegrass. Black walnuts are notoriously allelopathic to many turf and ornamental species.
Shade tolerance involves several factors, but prominent among them is the ability of some species (such as fine-leaf fescues) to maintain an adequate level of photosynthesis in shade that other species cannot attain. Also, resistance to powdery mildew and other diseases common in shade are important for shade survival. Selecting a cultivar with some tolerance of lower light levels, shade-prevalent diseases and other stressful conditions is key to successful management of turf in the shade. (See table, page XX, for a list of shade-tolerant species.)
Improving conditions for turf in a shady environment Pruning trees with dense canopies (for example, maples) allows additional light to pass through to the turf. Prune lower branches up to a height of 6 feet or more, but leave at least two-thirds of the tree's total height in branched canopy (that is, no more than one-third of the tree's total height should consist of trunk with no branches). Thinning out branches of trees and shrubs in the landscape also improves air circulation and may lower humidity.
In shade, mow turf at 3 inches to allow maximum interception of light by the thin turf. Avoid light, frequent irrigation. Rather, water infrequently, deeply and in the early morning to allow maximum time for drying. Irrigating in the evening may cause turf to remain wet and raise humidity through the night, increasing the chance for disease. Also avoid excessive nitrogen fertilization, which can promote shoot growth, lower carbohydrate levels and promotes soft, succulent tissue that is more susceptible to disease. If possible, rotate traffic patterns (for example, on shaded golf-course greens and tees) to minimize the contributing stresses of wear and compaction on the shaded turf.
When turf won't succeed... Despite the best turf-management practices, certain shady sites will not support acceptable-quality turf. Even the most highly adapted shade-tolerant turfgrass cultivars will not succeed in conditions darker than light to moderate shade. In moderate or dense shade, it may be impossible to grow high-quality turf. Under such conditions, you can either change the environment by removing or thinning some of the trees or use more shade-tolerant alternatives.
Using shade-adapted plants can cut site maintenance significantly. Because these plants are adapted to lower light levels, they will suffer fewer disease problems than sun-loving species growing in the same situation, reducing the need for disease spray programs. And because they thrive in the shade, they will not need replanting as light-hungry turfgrasses often do. In large masses, such as groundcover beds or shrub borders, shade-adapted plants reduce the amount of hand trimming and edging necessary around individual trees growing in turf. In addition, on steep, shaded slopes, groundcover beds can increase the safety factor by eliminating the risk of tipping mowers on the sharp incline.
Broadly speaking, you could call any plant material you use in mass to substitute for turf a "groundcover." Whether plants are shade-tolerant shrubs, vines or herbaceous species, they serve the purposes of filling in an area with attractive plant material, holding soil in place and adding textural and color contrasts in the landscape.
With a few notable exceptions, such as impatiens and wax begonia, most of the color in shade plantings will come from foliage rather than from blooms. Because of low light levels, flowering is usually less abundant in the shade than in sunnier areas. However, that doesn't mean shade plants have to be drab. Witness the striking pinks, reds, whites and greens in the foliage of shade-tolerant coleus and caladiums. Plants with variegated foliage stand out exceptionally well in the shade. The contrast between the white, cream or golden portions of the foliage and the deep-green or blue-green sections is most intense in the shade.
Dozens of shade-tolerant groundcovers are available. The table on page XX is a brief listing of some types to consider when shady conditions limit the possibilities for growing quality turf. Use a good horticultural reference or ask a Cooperative Extension agent for additional recommendations.
Whether you choose turf or some shade-tolerant alternative, it is possible to grow successful plantings in shaded areas. If you decide to use turf, careful cultivar selection and proper maintenance are key to ensuring the best possible sward in light-limited conditions. For areas where turf simply can't make it, a variety of shade-adapted plants are available for attractive, low-maintenance plantings.
Denny Schrock is an extension horticulturist and Dr. John Dunn is professor of turfgrass science, both at the University of Missouri (Columbia, Mo.).
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