KENNEWICK -- They stand out like sore thumbs: dry spots in the lawn that seem to appear right after hot, dry weather begins in our area.
Dry spots can have several causes.
The most likely cause of these dry spots either is a poorly designed or a poorly functioning irrigation system. There are several things to do to check if the irrigation system is at fault.
1. Run the system and check to see if the sprinkler heads are functioning properly. Are the nozzles adjusted correctly to cover the prescribed area? Are any nozzles plugged?
2. Has a plant grown and blocked a sprinkler's output.
3. Is the water pressure adequate for each sprinkler head to cover the prescribed areas?
4. Are the right sprinkler heads being used for the design and is there adequate overlap?
You can check the system's coverage by placing empty straight-sided cans in the area of the dry spots as well as in the areas that aren't dry. Run the system for a set amount of time and then check the depth of water in the cans. Is the amount of water in the dry spots' cans the same as the amount in the green areas? If the amount varies greatly, a tune-up of your irrigation system is due.
If the amounts are the same, you will need to investigate other possible causes for the dry spots.
Another reason may be excessively thick thatch that has dried out and is now resisting wetting. Dry thatch will repel water applied to the area. A lawn wetting agent (surface active agent) can be purchased at garden stores. These agents cut surface tension and improve the movement of water through the thatch. (Only use nonionic organic wetting agents, not soaps or detergents.) If your thatch is thicker than a half-inch, you should dethatch or power-rake your lawn next spring.
If the dry area is on a slope, the sprinklers may be applying water too quickly for the water to penetrate the soil and it runs off instead. To avoid this situation, apply the needed water in several short sets, allowing time between each set to allow the water to soak in instead of running off.
Sometimes dry spots are the result of soil compaction. Compaction happens when soil particles get pushed together from foot traffic, play and mowing, or it may have been compacted before the lawn was established. The spaces between compacted soil particles are limited, making it difficult for water to infiltrate the soil. Lawn aeration removes cores of soil and helps to partially relieve soil compaction and can help with water penetration. Aeration also can be a temporary solution when thick thatch becomes a problem in the middle of summer.
Occasionally, a dry spot is the result of something buried in the soil. Sometimes during home construction materials get buried creating a shallow area of soil. Dig down in the soil beneath a dry spot to probe what might be underneath. One local resident investigated and found that extra concrete had been left behind in a big lump under the lawn; another found a 2-by-4!
Applying extra water to a dry spot is a bandaid approach that doesn't fix the real problem and wastes water. If dry spots are showing up in your lawn, investigate the cause so you can apply the right cure.
* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension.