Barbara J. Bromley, Mercer County Horticulturist 2004
Ideal growing conditions for lawn grasses include fertile, well-drained soil, sun, and enough rain for the lawn to grow. Occasionally it is desired or necessary to grow grass in less-than ideal locations such as a shaded area under trees or at the base of a north-facing wall. A once-lush lawn may decline as the trees growing over it or shrubs abutting it increase in both size and the amount of shade cast.
- The grass may not get enough sun to manufacture food efficiently through photosynthesis to support the growing grass plant.
- If a tree is casting shade, it may also be competing with the grasses for available water and nutrients, and may block water from getting to the lawn during light rains.
- Lawns are often mowed lower than 2 1/2 inches; too short for shady areas.
- Lawn care professionals and homeowners tend to treat grass growing in the sun and grass in shaded areas with the same maintenance program.
- Many grass species grow poorly in shade, especially zoysia, perennial rye, and most Kentucky bluegrasses.
- There are different “grades” of shade: light or intermittent, open, medium, and dense. The denser the shade cast, the more difficult (or impossible) to grow grass.
- Lawns are usually considered areas for play, recreation, and pedestrian activity. Shady areas are less able to tolerate rough or constant activity, because the grasses are already growing under stressful conditions.
- Moss, which prefers shade, compacted soil, poor air circulation, poor drainage, acid soil, and/or low soil fertility may out-compete grass. (Moss can be an attractive ground cover in a shaded area and might be a useful alternative to grass.)
- Soil under old shade trees may be severely compacted making the development of grass roots difficult.
To overcome these problems, we have to think differently about the maintenance of grass grown in the shade and about the best grasses to grow.
Watering: Because of competition with tree roots and deflection of water off the tree canopy or in times of drought stress, additional water may have to be applied under the tree. This water should be applied early in the morning so the grass blades have a chance to dry before evening to reduce the incidence of disease problems.
Mowing: Mowing is often not required as often as on grasses growing in full sun, because grass under stress grows more slowly. Mower height should be set at 3 inches or higher, so there will be more leaf blade remaining for photosynthesis.
Fertilizing and Liming: Lime and fertilizer programs must be adapted to the soil type and the grass varieties grown. Fine fescues are much less tolerant of heavy applications of high nitrogen fertilizers than perennial ryegrass and bluegrass. A spring and a fall application of lawn fertilizer at recommended rates is usually sufficient. Allow clippings to remain on the lawn after mowing to help return some nutrients and organic matter to the soil.
Both soil pH and nutrient levels should be determined by soil test. Apply agricultural limestone, according to test results, to bring the soil pH to between 5.8 and 6.5. (The lower pH is appropriate for fine fescues growing under trees, such as oaks, that prefer a more acid soil.)
SEEDING SHADE TOLERANT GRASSES
The ideal time to seed or reseed grasses in shady areas is late August or early September, about the same as for seeding sunny spots. This timing minimizes heat and drought stress, competition from weeds, and competition from falling leaves, which smother young grass. Seeding may also be done in February or March (“honeycomb” or “frost” seeding) before trees leaf out, allowing some direct sunlight to help the young grasses. Seeding at other times of the year is generally less successful.
Grass species and varieties that are tolerant of shade must be used. Usually the seed mix will contain at least 60% fine fescues (hard, Chewings, and creeping red fescue varieties) that are shade and drought tolerant. The turf-type tall fescues also have some shade tolerance. A few varieties of Kentucky bluegrass (Glade, Eclipse, Ram II, Warrens A-34) are tolerant of light or intermittent shade. Zoysia, a warm-season grass, is not tolerant of shady conditions in New Jersey. Poa trivialis or roughstalk bluegrass is tolerant of shady and very moist conditions and is the best choice where both conditions exist.
RENOVATION OF A SHADED LAWN
- Have a soil test run to determine whether lime is needed and the most appropriate fertilizer based on nutrient need.
- Remove as many weeds as possible, either by hand or with an appropriate weed control chemical. READ THE LABEL! Some broadleaf herbicides that are safe for open lawn use can cause damage to plants other than “weeds” and may be absorbed by tree roots. Glyphosate (Roundup, Kleenup, and others), a foliar applied herbicide, is not soil active or root-absorbed. Wait the time recommended on the label before seeding.
- Spread a 1-3 inch layer of organic matter (compost, leaf mold, humus) and the recommended rates of lime and fertilizer over the area to be seeded and till or spade it into the soil. If tree roots make spading impossible, spread the organic matter, lime, and fertilizer over the area and rake to mix. Do not spread more than ½” of topsoil over the area containing tree roots and, even then, only if necessary. A change of grade can kill some trees. If buttressed or protruding tree roots are in the way, do not plant grass there. Consider an alternative to grass. Subsequent mowing will probably damage the roots
- Spread the grass seed selected for tolerance of shaded conditions and rake lightly to make sure seed has soil contact. Water lightly, but thoroughly. Keep soil evenly moist until the seed has germinated. With seed mixes this will occur over a period of weeks. (Perennial rye germinates in about 5-7 days, fescues in about l4 days and bluegrass in about 21 days.)
- When the grass is 4-5 inches tall, it can be mowed. Do not mow shorter than 3 inches. Both young and established shade grasses that are cut too short may weaken or die.
- In autumn gently rake or use a leaf blower to remove fallen leaves so they do not smother developing grasses. Use only a leaf blower on lawns seeded in late summer or early fall. Raking will rip out young grass plants.
ALTERNATIVES TO TURFGRASS
If grass does not grow well after one or more attempts, or if newly seeded grasses grow in spring but die after trees leaf out, consider an alternative ground cover. In densely shaded areas use an organic mulch such as shredded hardwood, pine needles, or bark chips. Where there is foot traffic install paths, use mulches, or allow moss to grow.
Ground cover plants that are suitable for shady areas that receive no foot traffic include: variegated goutweed (Aegopodium podagraria “Variegatum’– may be invasive), bugleweed (Ajuga reptans), lady’s mantle (Alchemilla mollis), leatherleaf bergenia (Bergenia spp.), leadwort (Ceratostigma plumbaginoides), green and gold (Chrysogonum virginianum), lily-of-the-valley (Convallaria majalis), Bishop’s hat (Epimedium spp.), sweet woodruff (Galium odoratum), English ivy (Hedera helix), yellow archangel (Lamiastrum galeobdolon), spotted dead nettle (Lamium maculatum), lily turf (Liriope spp.), mondo grass (Ophiopogon japonicus), Japanese spurge (Pachysandra terminalis), creeping phlox (Phlox stolonifera), myrtle or periwinkle (Vinca spp.), hosta and ferns.
|For more information, see the Rutgers Cooperative Extension Fact Sheets:
FS684 Turfgrass Seed Selection for Home Lawns
FS584 Seeding Your Lawn
FS738 New Jersey Seed Standards for Sod Certification.