Thatch in lawns is often misunderstood — both its cause and control. Some lawns have serious thatch problems while others do not. Excessive thatch (over 1/2” thick) creates an environment favorable for pests and disease and an unfavorable growing environment for grass roots, plus it can interfere with some lawn care practices.
Thatch is a layer of living and dead organic matter that occurs between the green matter and the soil surface. It accumulates as dead roots, lawn debris and dead turfgrass crowns build up faster than they break down. Thatch problems are due to a combination of biological, cultural, and environmental factors. Cultural practices can have a big impact on thatch. For example, heavy nitrogen fertilizer applications or over-watering frequently contribute to thatch, because they cause the lawn to grow excessively fast. Avoid over-fertilizing and over-watering. Despite popular belief, short clippings dropped on the lawn after mowing are not the cause of thatch buildup. Clippings are very high in water content and break down rapidly when returned to lawns after mowing — assuming lawns are mowed on a regular basis (not removing more than one-third of the leaf blade).
As thatch levels accumulate to greater than 1/2”, lawn problems begin, and the thatch needs to be controlled. Thatch may be torn out with a dethatcher or vertical mower, but will most likely return unless the cause is corrected. Mechanical dethatching is also very destructive to the lawn because roots are in thatch instead of soil, so plants tear out easily.
Overseeding is usually required afterwards. For this reason, it’s best to tear out thatch in early fall for optimum reseeding timing.
Core aeration and topdressing (applying additional soil) are two methods that will generally correct the reasons thatch is accumulating. Core aeration machines will pull up small soil cores to the surface that are left there to act like topdressing. The holes created help solve problems such as compaction or poor drainage. Topdressing is simply adding a thin layer (1/8” – 1/4”) of compatible soil over the thatch, which adds microorganisms to help in the breakdown process.
You can rent aeration equipment to do it yourself or you can get your local lawn care professional to provide this service.
If you think you want to do it yourself, consider that this piece of equipment weighs about 200 pounds, requires extensive manhandling to operate, usually has a minimum rental time of four hours, and typically costs more than having a professional do it for you.
Aeration is an excellent lawn practice with many benefits: It helps solve soil problems that in turn lead to better root systems and healthier lawns. Aerate in spring or fall (fall is generally the preferred time), making sure adequate moisture exists in the soil. Make two trips over the lawn, the second perpendicular to the first. An average of 15 to 20 aeration holes per square foot is suggested. Cores should remain on the surface and be allowed to air dry. These cores act as topdressing that helps degrade thatch. Additional topdressing material can be added after core aeration, if desired.
In his book Lawn Care for Dummies, garden expert/author Lance Walheim offers these dethatching tips:
The average lawn contains over 8,000,000 individual grass plants. These plants are always forming new parts and losing old ones. Thatch is the matted layer of dead and living organic matter that forms in most lawns above the soil. Thatch is a natural part of growing turf, and a small amount is actually healthy. It conserves moisture and provides a source of new humus as it decomposes.
When the soil in your lawn can’t break down thatch as fast as it builds up, a lot of problems can result. Thatch over 1/2” thick becomes a breeding place for both insects and diseases. Heavy thatch also encourages turf roots to stay in the thatch layer instead of pushing into the soil.
Another big problem with matted thatch is how it sheds water. The principle is the same as the thatched roofs used on old-style cottages. By being thick and heavily matted, the surface keeps water out. That’s fine on a house, but not on a lawn. For all these reasons, it’s important to manage thatch levels as part of a good turf maintenance program.
In severe cases (3/4”–1” or more of thatch), stripping or complete tilling and reseeding may be the only solution. Dethatching removes a lot of thatch but also disrupts the good turf. Slice seeding can also be considered.
Of all the choices for thatch control, aeration (or core cultivation) is the best way to reduce and control thatch. Aeration is simple, economical, and doesn’t tear up the entire lawn. Regular aeration benefits the entire lawn while solving moderate thatch problems. Aeration also keeps thatch from becoming a serious problem by speeding up the decomposition process and punching through the thatched roof over your soil.