Author: David R. Chalmers, Extension Agronomist
Posted April 1997
Thatch is more than just an accumulation of grass clippings at the soil surface. Thatch is an intermingled organic layer of dead and living shoots, stems, and roots that lies between the soil surface and the zone of green vegetation. Its development is related to a number of factors which affect the rate it develops or the rate it is decomposed by micro-organisms in the soil. A thatch layer up to 1/2 inch can be tolerated in lawn situations. In fact, a small thatch layer can help to increase turfgrass wear tolerance. However, if this layer is allowed to accumulate unchecked, lawn problems will develop. Excessive thatch layers result in increased insect and disease activity and mower scalping injury. Other problems related to excess thatch build-up include reduced tolerance to heat, drought stress, and reduced tolerance to low winter temperatures.
Whether thatch is a problem or a potential problem is determined by the positive and negative influences affecting thatch formation and decomposition. Foremost in this scheme is the inherent thatching tendencies of turfgrass species and even varieties of an individual species. Lawns containing bermudagrass, zoysiagrass, Kentucky bluegrass, or creeping red fescue have a strong potential for thatch development. Thatch is rarely a problem in tall fescue or perennial ryegrass lawns.
Thatch formation is further influenced through management practices. Increased stimulation of turf growth through excessive fertilizer use (especially nitrogen) or frequent irrigation will increase the rate of thatch development. Fertilizer and water use should therefore be programmed to produce a quality turf without over-stimulating its growth. Infrequent mowing or too high of a mowing height also favors thatch formation.
Soil micro-organisms, which act to decompose thatch, function best in warm, moist, well-aerated soils at a near neutral pH. Thatch accumulation increases in soils where micro-organism populations are inhibited by acidic soil conditions or poor soil aeration caused by poor drainage or soil compaction. Eliminating drainage problems, aerating to reduce soil compaction, and liming to adjust soil pH to the near neutral range (6.2 to 6.5) will favor the activity of thatch-decomposing micro-organisms.
Control of existing thatch layers may require further measures. As a thatch layer develops, it becomes further removed from soil micro-organisms and decomposition slows. Soil introduced into a thatch layer from a surface topdressing or from aerifying cores that are left to break down on the turf surface will serve to inoculate the thatch layer with micro-organisms and provide favorable conditions for decomposition. Physical removal of the thatch using a vertical mower (power rake) is always an option. However, without correcting the cause of the problem, thatch will continue to develop and detract from turf quality.
(Originally published as "Thatch - Is it a Problem on Lawns," by David R. Chalmers, Extension Agronomist, in The Virginia Gardener Newsletter, Volume 5, Number 8.)