Problems Associated with Shade
Tree leaves can substantially reduce the amount and quality of sunlight reaching turfgrass. Plants growing in very dense shade are low in food reserves, resulting in weak plants.
Shade varies with the season, the characteristics of the trees, and their location on the lawn. Maples, oaks, and beeches are examples of trees with dense canopies that intercept most of the light. Some evergreens such as firs and spruces have very dense canopies but affect small areas of turf because of their narrow canopy. Pines, poplars, ashes, and birches produce a more open shade than maples and oaks. Areas with an understory or with trees close together cast very dense shade. Leafless deciduous hardwood trees can block out nearly 50 percent of the sunlight in the winter, whereas the same trees in full leaf can block nearly 95 percent of summer sunlight.
Shrubs and shallow-rooted trees such as willows, maples, and beeches compete strongly with turfgrasses for nutrients and water. In clay soil, most feeder roots of shade trees grow in the upper 8 inches where turfgrass roots grow. Competition extends past the tree's drip zone, since roots can grow a considerable distance beyond this point. Reduced amounts of light, nutrients, and water produce succulent, weak turfgrass plants. They are slow to establish and are susceptible to insects, disease, and environmental stress as well as being less able to withstand traffic than plants grown in full sunlight.
Environmental conditions associated with shade favor some diseases. Poor wind movement and reduced sunlight moderate the temperature and increase the relative humidity in shady areas. As a result, foliage remains wet for extended periods. Although dew forms less frequently in shaded than sunny locations, it lasts longer because the trees hinder drying. Wet foliage encourages disease development, and thus it is important to select disease-tolerant turfgrasses.
Modifying the Environment
Turfgrasses will not grow in very heavy shade or under dense leaf cover. If an area gets less than 50 percent open sunlight or less than 4 hours of sunlight per day, it is much too shady for turfgrass to grow well. Consider removing selected trees, especially if existing trees are too close together and removing them will not detract from the landscape design. Use ground covers such as English ivy, ajuga, liriope, and pachysandra as well as pine bark and needles, crushed stone, and woodchips as a more attractive alternative to turf when shade is excessive.
A turf-free zone at least 2 to 4 feet in diameter around a tree can improve the growth rate of small plantings by minimizing competition between tree and turf roots for nutrients and water.
Removing tree limbs up to a height of 6 feet and cutting out unnecessary undergrowth will enhance wind movement and reduce the potential for disease. Selective pruning of the crown will open the tree canopy and allow more light to reach the turfgrass. Removing dead and diseased limbs can enhance the health and appearance of the tree if pruning is done selectively and with care. Avoid severe pruning.
Tree-root pruning also aids in lawn performance, but care must be taken not to injure desirable trees. Maples, beeches, oaks, and certain evergreens are very sensitive to extensive root pruning. Roots should be cut cleanly, and no more than 40 percent of the functioning roots should be removed at one time. Supplemental irrigation and fertilization help reduce the harmful effects of root pruning.
The depth of shade within the dripline of a tree can result in soil erosion, exposing surface roots. Willows, elms, and maples often have are surface roots which can be covered with 3 to 4 inches of mixed topsoil and organic matter. Shade-tolerant ground covers can be established in these areas to give a pleasing appearance and minimize mowing problems.
Often, lawns with trees that had enough sunlight to grow good quality turfgrasses a few years ago become too shady because the trees have enlarged.
Using shade-tolerant cultivars is important when growing turfgrass in partial shade. Mixtures of tall fescue in combination with shade-tolerant cultivars of Kentucky bluegrass (80 percent and 20 percent by weight, respectively) are the best choices in most locations where cool-season grasses can be grown. (See Table 1. Characteristics of Principal Lawn Grasses Grown in North Carolina). The addition of a fine fescue, specifically cultivars of hard fescue, is beneficial in areas that will receive little maintenance. A mixture of 80 percent tall fescue, 10 percent Kentucky bluegrass, and 10 percent hard fescue by weight seeded at 6 pounds per thousand square feet is recommended. Other fine fescues, such as certain cultivars of creeping red and chewings fescues that perform well under low light intensities in other states, are thinned by disease in North Carolina. Perennial ryegrass and Sabre rough bluegrass ( Poa trivialis ) have also performed poorly in shade trials in North Carolina.
Do not permit leaves to accumulate on the new lawn. As leaves fall, they become layered and create a barrier that blocks light, air, and water movement. Remove leaves frequently until the grass is established. Some seedlings may be torn out by the rake; however, more seedlings will be lost if the leaves remain.
In general, warm-season grasses often suffer more winter injury in shaded areas than in open, sunny locations. St. Augustinegrass is the most shade-tolerant of the warm-season grasses, followed closely by zoysiagrass. Both Emerald and Meyer varieties of zoysiagrass are more widely used in North Carolina because they tolerate cold better than St. Augustinegrass. Centipedegrass and bahiagrass perform well under light pine-tree shade but are not as shade tolerant as St. Augustinegrass and zoysiagrass. Bermudagrass is the least shade tolerant of the turfgrasses and should not be considered for use in shady areas. Table 6 lists shade tolerant grasses and grass mixtures.
Even shade-tolerant grasses prefer sunny locations. Shaded lawns must be managed more carefully because they are often weaker than turf grown in full sun. Cultural practices must be altered to help ensure survival and enhance performance. Mow grasses at the top of the recommended mowing height range to promote deep rooting and to leave as much foliage as possible to manufacture food for the plant.
Lawn grasses grown in the shade should generally be fertilized at the same time as turf grown in the sun, but at a lighter rate (see Table 7). Lawn fertilization is not harmful to trees and shrubs and may actually be beneficial. Fertilizers associated with turf, such as 12-4-8 and 16-4-8, can help to meet the requirements of trees and shrubs, thus preventing a nutrient deficiency.
If trees require more nutrients than can be supplied by the turfgrass fertilizer, apply additional fertilizer by soil injection or drill coring to reduce the amount of area affected and minimize the potential for turf injury or loss. Keep track of the total amount of fertilizer applied to a given area so the total recommended amount for any plant is not exceeded. Over-fertilization may occur if different people are responsible for the trees, shrubs, and lawn.
Irrigate the lawn deeply and infrequently to encourage deep rooting of trees and lawn grasses, reduce soil compaction, and minimize the time that the foliage is wet.
Powdery mildew, brown patch, leafspot, and melting out are the major turfgrass diseases associated with shade. Powdery mildew is particularly severe on Kentucky bluegrass; however, most shade-adapted cultivars exhibit good tolerance to this disease. Brown patch is often associated with tall fescue, whereas leaf spot and melting out are associated with fine fescue, St. Augustinegrass, and Kentucky bluegrass. Planting improved, shade-adaptable lawn grasses and using good cultural practices can help reduce damage from diseases. A blend or mixture of improved, adapted cool season grasses, rather than a single cultivar, can help reduce the potential of turf loss from disease.
Table 6. Shade-Tolerant Cultivars
|Kentucky Bluegrass||Tall Fescue||Fine Fescue||St. Augustine- grass||Zoysiagrass||Centipedegrass|
Pennlawn creeping red
Table 7. Suggested fertilization schedule for shaded lawns (lb N/1000 sq. ft.)(1)
|Kentucky bluegrass + fine fescue||
|Kentucky bluegrass + tall fescue||
|Tall fescue||½||1||1||2 ½|
Dates suggested for central Piedmont. For the west, dates may be one to two weeks later in spring and earlier in fall. For the east, they may be one to two weeks earlier in spring and later in fall.
(1) Multiply by 43.5 to convert to a per acre basis. Follow soil test results if available.
(2) Centipedegrass should be fertilized very lightly after establishment. An additional fertilization in August may enhance performance in coastal locations. Do not apply any phosphorus unless suggested by soil test results.
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