Thatch is a layer of living and dead roots, crowns, and
lower shoots that forms between the soil surface and
green vegetation. Moderate thatch formation is a normal
development in lawns; however, problems can occur
when the thatch layer is excessive. A balance must
be maintained with grass clippings and thatch. Seldom is thatch directly
responsible for turf death; however, it usually leads to
other disease, water, and rooting problems that eventually kill grass.
Too much thatch can cause turf problems,
but a moderate amount of thatch is beneficial.
Beneficial aspects of moderate thatch:
- Supplies food source and habitat for beneficial microbes.
- Supplies organic matter that earthworms incorporate into the soil.
- Acts as a living filter to reduce groundwater contamination.
- Provides a protective layer that reduces turf injury from traffic.
What causes thatch?
Like many systems in nature, a balance must be maintained. Lawns with less than a 1⁄2 inch of thatch seldom have thatch-related problems. When the rate of shoot production is equal to the rate of microbial decomposition, thatch is properly balanced and an excessive build up is avoided. There are two ways your lawn’s thatch can be out of balance. First, plant material is produced too fast for a normal population of microbes to break it down. Second, the grass may be growing at a normal rate, but the microbes are not functioning properly. Biomass production can be influenced by
controlling fertilizer and irrigation. Microbial populations are more responsive to uncontrollable environmental factors such as temperature and soil type.
Grass clippings are not a primary cause of thatch…David D. Minner, associate professor, Department of Horticulture, Iowa State University and Eldon Everhart, Iowa State University Extension Horticulture field specialist.
Infrequent mowing of tall grass will contribute to thatch accumulation. The rule for mowing is to mow at a height and frequency so that no more than one third of the grass height is removed at any single mowing. For example a lawn that grows to a 3-inch height can be cut back to a 2-inch height, leaving the one-inch clippings to easily filter into the grass canopy. Grass clippings are not a primary cause of thatch, and they should not be collected in an attempt to prevent thatch buildup. Leaf tissue readily breaks down in a lawn compared to the more stable crown and shoot issue. Mulching-type mowers chop clippings into smaller pieces that easily filter into the grass canopy and decompose faster.
Do grass clippings contribute to excessive thatch?
Clippings will only contribute to thatch accumulation if there is already a preexisting thatch problem. Once thatch builds to beyond one-half inch there is minimal contact between soil microbes and the top of the thatch surface. Consequently, clippings that fall on top of thatch are slow to break down and may contribute to further thatch development. Where thatch is known to be a problem, clippings can be removed along with other thatch-reducing measures. Once thatch has been sufficiently reduced, clippings should no longer be collected.