Shade is a welcome feature of a landscape. Unfortunately, it can also create some unwelcome pest problems.
What shade does to a landscape
Trees and large shrubs that produce a shade canopy serve to moderate temperature fluctuations by lowering daytime temperature and keeping it warmer at night by preventing the release of heat. This moderating effect on temperature helps humidity remain high both day and night, and, as a result, the more consistent temperature along with the presence of high humidity and subdued light create conditions conducive to fungal activity. Shady landscape conditions also tend to cause a slowdown in the drying of soils and plant leaves due to decreased light penetration and reduced wind speed through the space.
Turfgrass plants growing in shade produce succulent, thin cell walls and elongated leaves that are heat, drought and disease susceptible. Moreover, the reduced sunlight leads to decreased carbohydrate and sugar production, resulting in plants that are less able to recuperate from foot traffic, mower damage or plant pests.
Pests favored by shade
Though not true insects, slugs and snails are synonymous with plants grown in shade, such as hostas and ferns. These pests are favored by the conditions outlined earlier: reduced sunlight and wind, cool to moderate consistent temperatures and moist soils. In fact, seasonal rainfall patterns are a good indicator of whether slugs and snails will be a pest in a particular year in that they seem to disappear in drought years, while they are plentiful during rainy growing seasons.
Unfortunately, there is no tried and true treatment for slugs and snails. The best approach to controlling them is to implement as many of the following practices as is practical, depending on the site and situation of the client and landscape space:
- Remove debris from flower beds to eliminate hiding places.
- Remove landscape fabric, bricks and garden art pieces.
- Reduce the layer of mulch surrounding ornamental plants. If necessary, remove it altogether until the slug and snail problem is solved.
- Thin-out ground cover plants or shrubs where slugs and snails are a problem to facilitate air movement and sunlight penetration to create drier conditions.
Chemical control may become necessary if the above practices fail to produce satisfactory results. The best approach is to thinly scatter bait products containing metaldehyde or iron phosphate around susceptible plants and moist, protected locations. As with all pest control products, read and follow all label directions thoroughly before applying.
In turf, scale insects sometimes become a problem, such as the Rhodes grass scale and the bermudagrass scale. The Rhodes grass scale is found in the Gulf States and other southern states toward California. It attacks turfgrass crowns, causing infested plants to wither and die. The bermudagrass scale infests bermudagrass, especially in shady areas.
Scale insects are mobile only in the young, immature stages. After that, they settle in a location on the leaf or stem, feed with needle-like mouthparts and usually cover themselves with an armored or shell-like protective covering. These tiny, globular or oval insects are easily overlooked.
When numerous, scale insects produce dead areas in turf, especially in the shade. Detection is difficult because of their small size and lack of activity on the crowns of turfgrass. Occasionally, scale populations can build up to the point where they cause enough damage for concern. Under these circumstances, modification of the turf canopy via close mowing and verticutting is usually sufficient for control.
In shady sections of the landscape, powdery mildew can become a problem on turf and ornamentals. In most cases, it’s a matter of a violation of “right plant, right place.” The disease is aptly named, in that a white powdery substance becomes coated on leaf surfaces as if a baker had dusted them with flour. Obviously, this becomes aesthetically unappealing, but also a concern in that the growth of the fungi blocks sunlight reception by the chloroplasts and photosynthesis is reduced. Powdery mildew can be treated with various fungicides, but a more effective course of control involves replacing infected turf or ornamental plants with ones that are more shade-tolerant.
Other diseases are also evident in shady landscapes. The hosta is a host for three pathogens in particular. Petiole rot, which is aptly named as the fungus invades plant tissues at the base of the leaf petioles, has symptoms that usually appear after rainy weather. When infected, the outer and older leaves turn yellow and wilt from the base, where tissues are soft and mushy. Phytophora blight is also a common pest of hosta. It produces leaf, root and crown rot symptoms. Early signs are slow, stunted growth and overall root systems that are smaller than normal. Close inspection of the roots indicates that root hairs are often absent and the roots themselves are discolored to a brown or black color. Third, Fusarium root and crown rot can become problematic under heavily shaded conditions. The most telling signs of infection are seen when the crown is split open to reveal softened brown or black inner tissues instead of firm white ones. Controlling these diseases involves good sanitation practices and treatment with fungicides for prevention. Fungicide applications made to wounded tissue surfaces, such as those created during division, can be helpful in preventing future infections.
More of a primitive plant, moss can be a pest in a landscape. In areas where turf is thin or poorly sited ornamentals are present, moss usually begins a slow and steady domination. This is most often the case in heavy shade, where no direct sunlight is present. When moss is established, consider changes in plant material. Moss killers, usually containing copper sulfate, can be applied to problem areas. However, in most situations they are only 50 to 60 percent effective. Moreover, because the existing plant material is not well-adapted to heavy shade, the moss regrows faster than the existing plant material to take its place.
Managing shady landscapes
As mentioned previously, endeavor to implement right plant, right place with clients. This means performing a site analysis on existing plants, and recommending plants well-adapted to the site in terms of sun/shade, soil preference, disease resistance, season of bloom, insect tolerance, eventual size and winter appeal, especially for ornamental plants.
For turf, a common response for a sun-loving species like Kentucky bluegrass is to become thin in shade. Because shade is generally a poor growing environment for most turf species, it’s important to think and act proactively. First, select shade-tolerant grasses such as chewings fescue, creeping red fescue and hard fescue. Turf-type tall fescue also performs well in light to medium shade. These are good choices for the North; in southern landscapes, centipedegrass and St. Augustinegrass offer the best chance for success.
Because turf typically grows slower in shade, the rate of photosynthetic activity is reduced as well. Avoid excessive nitrogen applications, as this produces a reduction in stored carbohydrate reserves. Thinner-than-normal cell walls also commonly develop in turf in the shade, which can lead to greater incidence of fungal disease.
For cool-season varieties, late fall fertilization has been shown to be beneficial, and it’s even more important for those in shade. At this time of year, trees offer less competition for the absorption of applied nutrients, and slow-release applications are well timed.
Advise your clients to reduce recreational activities, as thin-cell-walled grass plants with little food reserve can’t bear much traffic without sustaining damage. Any effort to minimize traffic in shaded areas is beneficial.
Remove leaves and debris promptly, all year long, as they shade the grass plant and reduce its food making potential.
Finally, if efforts to grow quality turf in the shade fail, consider using shade-tolerant ground covers. Again, right plant, right place is a good guiding principle for dealing with keeping shady landscapes healthy.
John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln.