Several varieties of lawn grasses (such as zoysiagrass, bermudagrass, St. Augustinegrass and centipedegrass) are vigorous growers and produce large quantities of stems, rhizomes, stolons and roots that are resistant to decay. That layer of dead, non-decayed organic matter that develops between the green grass and the soil surface is called thatch.

A shallow layer (1/2” or less) of thatch on a warm-season lawn can help protect the turfgrass roots from fluctuations in temperature and soil moisture. More than 1” of thatch, however, can cause problems.

Thatch often develops over a period of years before you notice damage. But once it does, you may see more signs of disease, drought intolerance and an uneven height of growth that can lead to scalping. Thatchy lawns may look healthy in spring, only to lose large patches of turf in summer. That’s because as the thatch layer gets thicker, new grass plants grow in it (instead of in the soil) and then die when exposed to heat and drought. Also, thatch can harbor turfgrass insects and diseases.

Annual dethatching, followed by raking and removal of the thatch, will help improve the vigor of your lawn (and you can compost the material). Other cultural practices — such as fertilization, liming, topdressing or herbicide applications — should be done AFTER dethatching.

Preventing thatch

Good lawn-management practices may not prevent thatch development completely, but they can certain slow it down.

• Apply moderate (but appropriate) amounts fertilizer to maintain turf vigor without excessive growth.
• Mow as often as needed to avoid removing more than 1/3 of the length of the blades at one time. Allowing short clippings to remain on the lawn does not add to thatch.
• Do not overwater the lawn, which can cause excessive shoot growth and the development of disease.
• Core-aerify heavy, compacted soil to improve the movement of air, nutrients and water, which improves the natural, microbial decomposition of thatch.

Removing thatch

Thatch is best removed on dormant warm-season lawns before they green up well in spring. On cool-season lawns, dethatching should be delayed until late September or mid-October.

It’s best to remove thatch with a dethatching machine, vertical mower or power rake, which can often be rented from an equipment-rental store. Or, you may be able to purchase a dethatching blade for your mower.