For The Answer
Submitted by Calvin Finch, Ph.D., Manager, Conservation Division, Water Resources & Conservation Department, SAWS, and Horticulturist
One of the most frequent questions that Dr. Jerry Parsons and I receive on our gardening Q & A show is, Why isnt my St. Augustine grass growing well under our oak trees?
The usual answer is, There is too much shade. St. Augustine has good shade tolerance but does not prosper when the shade is too dense. Three or four hours of sun are required for St. Augustine to survive. With less sun than that, St. Augustine grass does not prosper. Expect the lawn to get sparse as time passes.
There are no lawn grass options if there is too much shade for St. Augustine grass. Zoysia has some shade tolerance but not as much as St. Augustine. Buffalo and Bermuda grass do not have any shade tolerance. Fescue and rye grass have considerable shade tolerance but they only last through the winter. Our climate is too hot for them to survive the summer. It is true that fescue survives in Arizona and California in the desert. The key to fescue survival is cool nights. It is hot in the Arizona desert during the day but it cools down at night.
There are a number of groundcovers that can tolerate more shade than St. Augustine grass. Consider monkey grass, Asiatic jasmine, vinca minor, or blue shade ruellia. It is true that the groundcovers do not make great surfaces for traffic or games but they produce a green attractive surface.
If you want a surface for activities in deep shade, consider hardscape. A flagstone patio set in sand surrounded by Asiatic jasmine is very attractive and perfect for a picnic table or for an area where the children can play with their toys. Bricks without mortar set in sand and held tight with steel edging on treated wood is very elegant and the surface is smooth enough for tricycles and roller skates.
I advocate hardscape without mortar because it is better for the oak tree roots. Treated wood, decomposed granite, flagstone, or brick without mortar is almost as good as mulch over oak roots.
Sometimes the problem with the St. Augustine grass in the shade is not too much shade. A virus disease called St. Augustine Decline (SAD) is common in old common St. Augustine grass lawns. The symptoms are often the same as too much shade. The grass is sparse and chlorotic and does not respond to watering or fertilization.
Recognize SAD by the mottled yellowing of the leaves. Regular chlorosis caused by lack of iron is characterized by lines of yellow running up and down the blade. SAD shows blotches.
SAD is relatively easy to cure. The named St. Augustine varieties are SAD resistant. Plug in squares of Raleigh in the worst patches and it will eventually take over the disease-weakened common St. Augustine grass. Better yet, use Floratam and you not only will have a SAD resistant sod, Floratam is the most drought and disease resistant St. Augustine grass.