Extension Educator, Horticulture
One word can bring a competitive grass grower 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 ½ inch, is good since it can insulate the soil and hold in moisture.
You can easily check the thatch depth in your lawn by going out with a pocket knife and digging out a plug in the lawn. Look at the cross section of the plug to determine the level of thatch.
Thatch more than one-half 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. Thick thatch can house pests and diseases and can lessen pesticide effectiveness.
The amount of thatch produced in a given lawn depends on the type of grass, how it is managed and environmental conditions. Grasses that spread by rhizomes such as Kentucky bluegrass and zoysia grass naturally produce thatch. Cultural practices such as heavy nitrogen fertilization and over-watering also increase thatch. If the grass is growing fast, thatch may not 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 produce clumps in the lawn and can smother the grass. Try to mow lawns on a regular basis and don't remove more than one-third of the leaf blade at one time.
Ok now we know thatch can be a problem and why, so what do we do now? If thatch is over one-half inch than it's 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 may also pull out a lot of good grass with it. The lawn may have to be reseeded after the destruction. This is best done in August when reseeding is optimal.
Another option, core aerification followed by topdressing, will generally correct the reasons thatch is accumulating. Core aerifying machines pull up small soil cores to the surface. It looks like a flock of geese has visited your yard. The cores of soil 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 or compost over the lawn. The soil or compost 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. Aerifying leads to better root systems and healthier lawns. Aerify in spring or fall when lawn is actively growing. The soil should be moist, but not wet. Water the lawn a few days before aerifying.
How often aerifying is done depends on the level of thatch, the amount of soil compaction and maintenance practices. For home lawns with no major problems aerifying may not be needed every year, but every other year. Holes should be about 2 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.