Turf 101: Thatch

Last updated 5/14/03

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5/14/03

Turf 101: Thatch

There are many misconceptions about thatch. Thatch is a tightly intermingled organic layer of dead and living shoots, stems, and roots that accumulate just above the soil surface. It is not the leaf clippings left over after mowing or those that find their way to the soil surface. A small amount of thatch is desirable because it moderates soil temperature fluctuations and provides a cushion on the soil surface. Too much thatch interferes with water and air movement, reduces fertilizer and pesticide response, and increases disease and insect activity. The worse thing about thatch levels greater than ½ inch homelawns is roots begin growing in the thatch and since thatch doesn’t hold much water, the turf becomes susceptible to drought damage even under mild water stress.

Thatch builds up when growth or organic matter production is higher than the breakdown. Therefore, a neglected lawn will never accumulate a thatch layer whereas an intensely managed lawn will. Too much irrigation and too much nitrogen will increase organic matter production leading to thatch. Conversely, too much irrigation, compacted or heavy soils will also lead to thatch because it limits the microbial activity that breaks down thatch. Thatch management includes only watering as needed, moderate fertility, and aerifying at least once every year or two to decompact the soil. If a ½ inch of thatch has accumulated in the lawn, it’s time to take serious action and dethatch.

The best time to dethatch lawns is when they are actively growing in early May or late September, but when there is not much stress on the plants. Dethatching machines are power rakes with blades that cut through the thatch down to the soil surface. As the blades revolve, dead and live organic material is torn loose and brought to the surface. Dethatching machines that cut with knives or blades are preferred for their effectiveness. Avoid machines with flexible rake-type tines and dethatchers that attach to your rotary mower blade. Dethatching machines can be rented from rental companies, or dethatching can be done by a lawn care company. The organic material removed by the dethatcher must be raked and removed. The best place to put it is on the garden or in your compost pile.

If the thatch layer 1/2 inch or more thick, a number of passes with the dethatcher in different directions may be necessary. Several passes will produce large quantities of refuse, and the lawn will look very ragged for some time. Reseeding may be necessary after dethatching lawns with 1/2 inch or more of thatch. Thatch thicker than 1 inch is most easily removed with a sod cutter set at the soil surface which can remove sod in light and manageable strips. The lawn must then be reestablished and, although this is hard work, it will be worth it in the long run.

Zac Reicher, Associate Professor/Turfgrass Extension Specialist