HAWAI'I'S GARDENS
Shade-friendly grasses key to covered areas
By Jay Deputy
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During the hot, muggy days of late summer, you've got it made — if you've got shade. Good landscape design should incorporate — whenever possible — a cool, shady area into the living or recreational space of a home. Shade may be provided by the roof of an enclosed patio, or by trees and hedges surrounding an open-air patio.
While we love shade, however, lawns don't. Lawn grasses need sunlight to photosynthesize carbohydrates that fuel growth. As shade increases, turf growth slows down, resulting in weaker leaves and shallower roots. The turf begins to thin out and shade-loving weeds gradually take over.
In constant-shade areas — such as, say, under large banyan or monkeypod trees — a lawn will often be completely replaced by weedy grasses. It often becomes pointless to try to fight the weeds because grass will not grow anyway.
You now have three choices: mow the weeds (at least they are green), mulch the area under the tree (arborists favor this option) or put in a shade-tolerant ground cover. Several commonly used choices include mondo grass or one of the vine-type ground covers such as pothos or philodenron. Wedelia doesn't do as well under heavy shade.
If your shady areas are not that heavy, and you want to keep a grass lawn, then turf species selection becomes very important. Remember, there are only five species of turf, called warm-season turf, which will survive in Hawai'i. Do not use the grass seed sold for "shady grass lawns." This is a mixture of cold-season grasses (Kentucky bluegrass, fescue, rye, etc.) that will die out in Hawai'i during the hot summer months.
The grasses to use for very sunny areas are:
It is interesting to note the effects of shade on the growth, appearance and performance of all of these turf grasses. As light penetrates the leaf canopy of trees and shrubs, it is selectively absorbed and filtered. By the time it reaches the grass surface, the light wavelengths have been altered, affecting the growth of the grass leaves. The result is longer and thinner leaves with a thinner waxy cuticle and a more upright growth. This gives the grass a softer feel and results in succulent growth leading to drastically reduced tolerance to foot traffic, shallower rooting and reduced drought and disease resistance. All of these symptoms increase as the shade becomes heavier or more constant and is more severe in the species with lower shade tolerance.
What can you do to improve grass performance under shady conditions? Most important: choose a grass species that will tolerate your level of shade. This in itself may be enough. If the grass continues to struggle in heavier shade, selective pruning of the trees and shrubs to thin out the canopy and allow more light penetration is effective, but may need to be repeated annually. A certified arborist should be consulted.
Increasing the mowing height in the shady area will provide more leaf area and greater carbohydrate reserves. Decrease nitrogen fertilization; this will increase carbohydrate reserves in the roots. Reduce wear damage in the area by keeping foot traffic to a minimum. Irrigate less often, but water more deeply with each watering, and avoid watering in the evening. This will help prevent fungal or bacterial diseases. Watch for the appearance of diseases and treat with fungicides early on.
Jay Deputy is an education specialist in landscape horticulture and turf at the University of Hawai'i-Manoa, Department of Tropical Plant and Soil Sciences, and Hawai'i administrator for the Certified Landscape Technician Program. Send lawn or turf questions to deputy@hawaii.edu.
