Extension Educator, Horticulture
One word can bring a competitive grass grower literally to their knees – thatch.
A thick thatch is good for roofs and aging men's heads, but not so good for lawns. Thatch is a layer of living and dead organic matter that occurs between the grass leaf blades and the soil. It's spongy and looks a lot like peat moss. It's mostly old grass stems and roots. A little thatch up to 1/2 inch is good. It can help insulate the soil and hold in some moisture. You can easily check the thatch depth in your lawn by going out with a pocket knife (don't use the good kitchen knife) and digging out a plug in the turf. Look at the cross section of the plug to determine the level of thatch.
Thatch more than 1/2 inch thick can cause severe problems. The grass tends to root in the thatch rather than the soil. This makes the grass more susceptible to drought and winter injury. Thatch can also make a nice place for pests and diseases and can lessen the effectiveness of some pesticides.
The amount of thatch produced in a given lawn depends on the type of grass, how it is managed and environmental conditions. Grasses such as Kentucky bluegrass and zoysia grass naturally produce thatch. Cultural practices such as heavy nitrogen fertilization and over watering also increase thatch. The grass is growing fast and the thatch doesn't have time to break down adequately. Environmental conditions such as wet clay soils, high pH soils and soil compaction can also contribute to thatch buildup.
Despite popular belief, short clippings left on the lawn after mowing do not cause thatch. Clippings are very high in water and breakdown rapidly. Long clippings that are left as clumps in the lawn can be a problem due to smothering the grass. Lawns should be mowed on a regular basis and should not have more than one-third of the leaf blade removed at one time.
Ok now we know thatch can be a problem and why, so what do we do now? If thatch is over 1/2 inch than its time to consider some management options. First avoid over fertilizing and over watering. Equipment can be used to remove thatch. It can be torn out with a dethatcher or vertical mower. Unfortunately this method also pulls out a lot of good grass with it. The lawn would have to be reseeded after the destruction. This is best done in August when reseeding is optimal. Also dethatching really doesn't correct the underlying problem.
Core aerification, followed by topdressing are two methods that will generally correct the reasons thatch is accumulating. Core aerifying machines pull up small soil cores to the surface. It kind of looks like a flock of geese have visited your yard. The cores are left on the surface to serve as topdressing. The holes created help solve problems such as compaction or poor drainage. Topdressing is simply adding a thin layer (1/8 to 1/4 inch) of soil over the lawn. The soil contains microorganisms which help to breakdown thatch.
Aerifying equipment can be rented or services are available from lawn care companies. Aerifing has many benefits. It helps solve soil problems that in turn leads to better root systems and healthier lawns. Aerify in spring or fall. The soil should be moist but not wet so watering the lawn a few days before may be needed. How often aerifying is done depends on the level of thatch, the amount of soil compaction and maintenance practices. Holes should be about two inches apart. Make two trips over the lawn, the second perpendicular to the first. Cores should remain on the surface to dry to act as topdressing. Additional topdressing material could be added after core aerifying.