Thatch

Thatch is a tightly intermingled layer of living and dead stems, leaves, and roots of  grasses that accumulates between the layer of actively-growing grass and the soil surface. Thatch is a normal component of an actively growing lawn. The overall effect of a thatch layer  is a lawn that does not respond well to management practices and is easily injured when conditions are optimum for growth. Leaving short clippings after mowing does not add to thatch development--soil microorganisms break the clippings down quickly and return a valuable source of balanced nutrients back to the turf. As long as the thatch is not too thick, it can increase the resilience of the turf to heavy traffic. Thatch develops more readily on highmaintenancelawns than on low-maintenance lawns.

To determine if your lawn has a thatch problem, cut out and examine a small, triangular- shaped plug of turf several inches deep. Note the spongy layer of material above the soil. If this layer is more than 3/4 to 1 inch thick when you compress it, you should  consider having your lawn dethatched or a management program that will encourage thatch decomposition. 

Factors Causing Thatch Development

Thatch accumulation tends to be governed by one or more of the following factors:

1. Grasses that produce a large amount of side shoots, such as Kentucky bluegrass (produces rhizomes) and creeping bentgrass (produces stolons), tend to producethatch readily.

2. The major causes of thick thatch accumulation are management practices that reduce the population of organisms (earthworms and microorganisms) which decompose thatch and/or that cause plant material to build up more quickly than the microorganisms that are present can break it down. Good aeration (avoid compaction), soil pH around 6.5 (avoid acidity), and adequate moisture favor the build-up and activity of beneficial microorganisms. Unbalanced fertilization and indiscriminate use of fungicides and insecticides are the most detrimental to beneficial organisms in turf.

3. Fertilizer deficiencies produce roots and stems that are more resistant to decay. Sufficient nitrogen is required for growth and to stimulate bacterial decomposition of thatch. The other essential plant nutrients must also be present in the proper balance. Too much nitrogen can promote disease development: high-nitrogen fertilizing speeds up lush, succulent growth that is more prone to insect pests and disease.

4. Disease-causing fungi can live on grass clippings as well as on living grass plants. These clippings are then added to the thatch layer that harbors disease organisms until the environmental conditions are right for disease development. Over-watering causes a reduction in soil oxygen, reducing microbial activity necessary for thatch decomposition. Extended periods of saturated soil will induce surface rooting thereby reducing or delaying the breakdown of the material responsible for the thatch buildup.

Thatch-Induced Problems

Excess thatch accumulation decreases the vigor of grasses by restricting the downward movement of air, water, plant nutrients and other materials applied to the soil. During wet periods, thatch may act as a sponge and hold excessive amounts of water, which can reduce the oxygen supply to the roots. During hot, dry weather, thatch may become dry and very resistant to wetting, even shedding water until it is re-moistened. When temperature and moisture conditions are right, a disease infestation can kill the already weakened turf.

Pesticide effectiveness is reduced since some of the materials can be absorbed to the organic matter and they will not reach and/or control the organism that it is intended to control.

Mowing height is also affected by thatch accumulations. As un-decomposed material builds up, the mower tends to ride on the thatch and does not cut at the desired height. If the cutting height is lowered in an attempt to overcome this problem, scalping and brown areas develop.

Thatch decomposition during periods of hot, moist weather can generate sufficient heat decomposition byproducts that injure or kill grasses.

Thatch Removal

Practice thatch removal on an annual basis. A good raking in the spring and/or fall will remove some of the  un-decomposed material. If excess thatch accumulates renovation may be the only answer. Normal thatch removal will not injure the lawn severely enough to necessitate reseeding. Spring is the best period for thatch removal, particularly if large amounts need to be removed. At this time, all dead stems and roots which accumulate throughout the fall and winter can be removed. Grasses can recover quickly at this time because the soil is warm and the rain is more likely to be regular. If properly de-thatched, grasses will recover quickly and resume normal appearance in a short period of time. If light thatch removal is all that is required, de-thatching can be done any time of the year.

Power rakes or other mechanically driven de-thatching machines are usually superior to hand rakes. Considerable force is necessary to slice or scratch into the grass mat and remove the dead material. Operate the de-thatching machines across the turf in a north-south direction and repeat in an east-west pattern if the thatch is very thick. Remove loosened material before changing directions. After thatch has been removed from the lawn, mow at the recommended mowing height. DO NOT dethatch when soil is wet.

Suggestions for Thatch Prevention

• Maintain soil moisture by watering thoroughly and infrequently, applying one inch of water per week in one or two applications. Never use light and frequent or daily irrigations.

• Aerate the soil with a core-type aerator if the soil is compacted or if water is not soaking into the ground.

• Maintain proper pH levels. Have the soil tested every three or four years. If the soil becomes too acid (pH of 5.8 or lower) apply lime to correct the condition. A slightly acid pH (pH of 6.5) provides the best environment for nutrient uptake by the grasses and for microbial activity.

• Maintain adequate nutrition for normal growth. Nitrogen is especially important to stimulate heavy populations of beneficial organisms.

• Mow frequently to maintain grass at a height of 2 ½ - 3 inches.

• Reduce nitrogen fertilization and amend the soil with phosphorus, potassium, and lime according to a soil test analysis.

Good turfgrass management programs will help prevent thatch accumulations and will aid in a healthy and good-looking lawn.

Adapted from the University of Massachusetts Extension, 1999; UConn College of Agriculture and Natural Resources, 2005