Every year at this time, lawn enthusiasts (and lawn worriers too!) turn their thoughts to mowing, fertilizing and controlling weeds. Often, thatch comes up in conversations between neighbors as a point of concern as well as what is it and do we want it or not?
In short, thatch is the brown shredded wheat like material that collects in a lawn over time just under the grass crowns, and just above the soil line. It accumulates when sheaths that cover rhizomes and crowns die and don’t break down. Old dead crowns contribute to thatch as well. It’s more common in Kentucky bluegrass and zoysiagrass lawns than with tall fescue or perennial ryegrass.
Is it good or bad? Do we want it or not? Even though many lawn owners have been taught to be scared of thatch, a little bit – about 1/4th inch – is good for the lawn. It’s a good buffer against the extremes of heat and cold and can capture stormwater and fertilizer runoff.
Thatch becomes a problem when it builds up to excessive levels; two thirds of an inch or more is too much. Too much thatch increases disease susceptibility, reduces drought and high temperature tolerance, restricts rooting, impairs water, fertilizer and pesticide infiltration and in general, reduces overall vigorous lawn growth.
To check the thatch level, cut a vertical wedge out of your lawn and look at it from the sideways profile. You’ll see grass blades, grass crowns, thatch and soil, in that order. You can use a pocket knife or a square nosed shovel to extract a sample.
Now, to the main question…should you de-thatch? Because dethatching is a fairly destructive process, try to slow down thatch development first with a regular aerating program. This procedure will remove soil cores and mix soil with the existing thatch, allowing the naturally occurring microorganisms to break it down, much like what occurs in a compost pile.
In many cases, aeration is not enough and it must be ripped out with a power rake. These machines use vertical knives that rotate, prying loose and pulling upward the thatch. In the process, some healthy turf shoots, crowns and roots are also pulled loose; if power raking is done, we strongly recommend overseeding afterwards to thicken the stand and allow the lawn to recover. After raking off the pulled up debris, apply 6-7 pounds of turf type tall fescue or 2 pounds of Kentucky bluegrass per 1,000 sq. ft. Keep the new seed moist, not soggy or dry. An application of mesotrione and starter fertilizer is also recommended to suppress annual grassy weeds and get the new seedlings off to a good start.
As mentioned, power raking is harmful to the existing lawn, but if the thatch is excessive, removing it and overseeding will be beneficial in the long run. For cool season lawns, it should be done in April or September, when the weather is conducive for healthy growth and the lawn has time to recover.