Common ailments and solutions for sun-deprived areas

An example of dappled shade.
In many landscape designs, shady areas are added for a place for clients to relax.

All landscape designers and managers want to create a space for their clients that is functional and relaxing. In most of these scenarios, some form of sun protection, at least for part of the day, is necessary. Creation of such a space involves the inclusion of a shade tree, arbor, pergola or other element. However, too much of a good thing is possible, and it’s important to strive for balance between shade and sun to create a healthy landscape.

Summer is the season of the year when the balance between shade and sun is most important. In early summer, the maximum amount of shade is produced from trees as late-leafing trees, such as oaks, are now fully emerged, and disease-susceptible trees, such as sycamore, haven’t starting defoliating from foliar diseases yet.

Common shade problems

The most common problem encountered in a shady landscape is thin turf. Most turf species require at least four hours of direct sun to grow well. In many landscapes, the areas where turf is currently planted used to be in full sun, but as the tree canopies expanded and small buildings were installed, the amount of sun that is received has diminished.

When sunlight is limited, moss can be an unwanted invader. The north side of buildings and houses, shady flowerbeds and pathways/patios are common locations for moss. Moss is not easy to get rid of; the herbicides labeled for its control don’t work at a rate that most green industry professionals have come to expect. Physical removal through scraping and soil replacement is almost always necessary.

Wherever you find moss, slugs and snails are usually close behind. Dim light, low air movement and moist soils favor their development. Baits containing metaldehyde and carbaryl have a high degree of efficacy, however, slugs and snails will continue to be a problem unless a less suitable microenvironment is created. Increasing air flow, reducing or eliminating mulch and tree/shrub removal are options.

Because of the reduced level of sunlight and wind movement in shady environments, it’s more common to encounter foliar diseases on annuals, perennials, groundcovers and turf. Root rot is more likely as well, particularly if irrigation is provided at a rate suitable for sunny areas.

Another common problem in shade is poor growth rate. Plants that are chosen without regard to the amount of sun struggle in shade and result in slow growth. In certain scenarios, where sunlight is only moderately limited, shrubs and perennials that should bloom fail to do so. Instead, they only produce leaves, resulting in an understated appearance.

Problems from shade

As mentioned above, reduced air flow is a major factor in the development of foliar diseases. For example, the growth of the fungi that cause black spot on roses and apple scab on crab­apples are enhanced by cool, stagnant and moist conditions.

Monitor the number of hours of sun each site gets.
Sharp contrasts between sun and shade can cause problems in a landscape.

The season of the year is also influential. In spring, the sun is lower on the horizon than in midsummer, creating less intense rays. In late summer, sunlight might actually increase due to the defoliation of trees infected with leaf spot diseases.

The transition from sun to shade in a landscape space may be gradual or sudden. Either can cause problems for the landscape manager. As well, when each of these factors—season of the year, air flow, plant choice and transition from sun to shade—are combined, the effects are more dramatic and problematic.

Degrees of shade

Varying degrees of shade exist in the landscape depending on the sun angle, the number of buildings and other objects that cast a shadow and the density of the tree canopy. The green industry has been focused on three levels: full sun, partial sun/shade and shade. While this is true for the most part, shade can be further subdivided into three levels: filtered, dappled and heavy.

Filtered shade (sometimes referred to as light shade) is best characterized by a light blockage of the sun. In plant terms, the shade produced by a honeylocust tree is filtered shade. Dappled shade (sometimes referred to as medium shade) is greater than filtered shade. Tree species such as Kentucky coffee tree and river birch usually produce dappled shade. Heavy shade is just as it sounds, thick, dense and maximum in terms of light blockage. Oaks and maples tend to produce heavy shade, as does a house or garage.

Partial sun/shade is more meaningful in terms of the number of hours each day that the area receives full sun, such as if a landscape bed receives morning sun and afternoon shade. Determining the type and amount of shade is critical to the plant selection process, as well as the success of the overall landscape.

Dealing with shade

Success with dealing with problems in shade involves several steps. First, conduct a site analysis to determine the level of shade in each part of the landscape at each time of the year. This is best done at several parts of the day to monitor changes in light intensity from dawn to dusk. Note factors such as drainage, canopy differences, the health of the existing plant material and the condition of different cultivars of the same plant species. Above all, acknowledge that each site, each customer, each property is different and should be cared for individually.

After the site has been inventoried and assessed, redesign the landscape to match the site conditions with the adaptability and preference of the plant materials. Use the phrase “right plant, right place,” as a guiding theme. Start by replacing poorly sited plants with those from the Can’t Miss List (see sidebar). Use them as a foundation for the renovation. After they have been installed, experiment with the latest new releases to increase the chance for success.

Choosing the best adapted turfgrass for shady sites is equally as important. In wet shady areas, consider rough bluegrass. However, if turfgrass fails, the best step is to forget about turf altogether and incorporate large groundcover areas divided by pathways to accommodate foot traffic.

Hostas are great shade plants for many regions.

Can’t Miss List

Ornamental Plants for Shade

  • Trees: redbud, serviceberry, hornbeam, witchhazel
  • Shrubs: coralberry, snowberry, blue holly, boxwood, bottlebrush buckeye, alpine currant
  • Perennials: hostas, bleeding heart, Japanese anemone, ligularia, bugloss
  • Groundcovers: Japanese spurge, Bishops goutweed, spotted deadnettle (lamium), lily of the valley, lady’s mantle, periwinkle.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb.