When moss or algae invade a lawn, they weaken the grass plants by forming barriers to air and water movement into the soil. This often becomes a problem during wet winters, or when lawns growing under shaded conditions are heavily watered.

Mosses are primitive, nonflowering, rootless green plants with simple stems and leaves. Under favorable conditions they grow like a fine-textured mat on the soil, preventing water and air from reaching grass roots. Algae are a group of primitive, single-celled threadlike plants which are rootless, leafless, stemless and nonflowering. In wet lawns they form a thick, dense green-brown scum which also obstructs air and water movement into the soil. Mosses and algae are not plant parasites, and do not attack and harm grass directly.

Moss and algae require specific soil and climatic conditions to develop; they are favored by soil compaction and poor drainage, too much water, and a thick thatch layer. Also, moss is favored by low soil fertility, acidic soil conditions and heavy shade. Unlike moss, algae is favored by high soil fertility.

The best way to control moss and algae in your lawn is to eliminate the causes of their growth while keeping your grass growing vigorously. Some of the things you can do include the following:

• Fertilize the lawn, if moss is the problem, or withhold fertilizer, if algae is the problem.

• Aerate or core the lawn to reduce compaction and improve water penetration.

• Improve soil drainage by installing drain tiles or dry wells.

• Reduce the amount of water you apply.

• Remove excess thatch.

• If moss is present, reduce shade by selective pruning of trees and shrubs.

Where moss and algae are thick, you may want to apply certain chemicals for complete control. Spray applications of copper sulfate, ferrous sulfate or ferrous ammonium sulfate at the rate of 2 or 3 ounces dissolved in 4 gallons of water per 1,000 square feet works well. You may need to repeat the application if the infestation is severe. Copper sulfate may temporarily damage the grass, so use it only on very severe moss or algae infestations. Keep in mind that chemical treatments provide only temporary control, and that long term control depends upon eliminating the cause of the problem.