Thomas J. Koske | 4/19/2005 10:28:39 PM
Localized dry spot is one of the most serious summer problems during dry periods, according to LSU AgCenter horticulturist Dr. Tom Koske. Dry spot first appears as small patches of dead or dying turfgrass.
The spots will continue to enlarge in a circular pattern, as any number of diseases do. Soil beneath the spots will be powder dry to a depth of 1 to 2 inches. Koske says this is especially true of sandy or thatchy turf.
Dry spot most often occurs from one or more conditions. Fungi or humic acids can coat sand particles and make them repel water. Another possibility is a compacted 'plow pan' layer or buried debris that stops water percolation. Tree roots also will dry areas of soil as their feeder roots extend past the drip line of the tree. In addition, a type of non-toadstool fairy ring can leave arcs or rings of thin, dying grass because of its water-repellent fungal body.
For a simple test of dry spot, Koske says to pull a soil core or wedge and place a few drops of water on the first few inches beneath the turf. See if it beads up or penetrates. If it beads, you have a hydrophobic localized dry spot.
To correct dry spot, you must irrigate. Most sprinklers apply water very slowly, at rates of 1/4 to 1/2 inch of water per hour. If the water begins to run off before you have thoroughly wet the top 2 to 3 inches of soil, however, you should irrigate in cycles. Wait an hour or so before continuing to irrigate. To determine how deeply the soil has been wetted, probe down into the soil using a knife, screwdriver, soil probe or spade. Once the soil has been properly irrigated, do not irrigate until the surface begins to dry again.
If the weather is extremely hot and dry and the soil is sandy, you may need to irrigate again within four or five days. If, on the other hand, the weather is fairly cool at night, irrigation may not be necessary again for another one to two weeks.
After a good summer rain, the turf appearance will usually improve, but only for a few days. This improvement is short-lived, because little moisture actually penetrates the soil. If the soil can absorb moisture only at the rate of one-tenth inch per hour, rainfall at 1 inch per hour will be of little value since much of it will just run off. When a thatchy lawn becomes dry, the thatch becomes hydrophobic (water repellent). It could take several hours of light rainfall just to penetrate the thatch.
Forking or core aerifying those affected areas will greatly help to get water into the soil. If you have a fairy ring problem, Prostar or Heritage fungicide may be applied monthly to areas to remove the fungus problem.
For related topics, look for Gardening and Get It Growing links in the Feature section of the LSU AgCenter Web site: www.lsuagcenter.com. Additional yard and garden topics are available from an extension agent in your parish LSU AgCenter office.