What To Know About Thatch In Lawns

It wasn’t too many years ago that if you had lawn problems in the late summer and you described the symptoms to an “expert” or brought a sample into your local garden center you were told that the problem was thatch. It was a catch-all term and a malady that nearly all our lawns seemed to have. The fact of the matter was that little was known about thatch just 20 years ago, and we’ve since found that the recommendations that were made often did more damage than good.

We hope that the vast majority of the “stuff” growing in our lawns are perennial grasses and that they are living in a state of continuous renewal. Old plants die and new ones are produced. Ideally, with the right care and oversight, the lawn achieves a balance between the rate at which organic matter is produced and decomposes.

This organic matter has value. It contains various plant nutrients which, after decomposition by microorganisms, may be returned to the soil for future use by the new grass plants. This is one of the reasons why I stress the importance of using a mulching mower instead of collecting the clippings. In essence, it’s perpetual composting. There are things that stop this process, such as excessive use of chemical fertilizers and pesticides.

At other times, problems develop when this organic material is produced faster than it can be decomposed. The turf develops an excessive amount of, you guessed it, thatch. Thatch primarily consists of partially decomposed stem and root tissues that develop in the organic layer between the base of the turfgrass plant and the soil. Stem, crown and root tissues are high in cell wall material called lignin that’s very slow to decompose.

There is a very common belief, almost an old wives’ tale, that thatch is caused by grass blades that are cut and don’t decompose or that the grass is being cut too tall and the clippings left behind don’t break down. This seems to be rare, but it does happen. The part of the grass plant that decomposes the fastest is the blade — the part that we cut. The grass clippings are largely water, and the balance of the organic material is broken down very quickly by bacteria and fungi.

A certain amount of thatch is actually desirable because it forms a cushion that increases wear tolerance in the lawn. It therefore follows that to some degree the more you expect to use your lawn (especially if you’re a party animal) the more thatch you want to cushion the traffic. Thatch also insulates the soil from high temperatures and reduces water evaporation (again, like compost or a mulch) from the soil surface. And on a golf course thatch adds resiliency to the course green and helps the ball “bite.”

It’s when thatch accumulations top a half-inch that problems begin to develop. A heavy layer reduces water movement to the soil and may actually divert water from the lawn. It also reduces soil aeration, which is necessary for good root growth, and one thing we’re learning about plants is how critically important the roots are.

Increases in disease and insect problems are also associated with a heavy thatch layer as more than a half-inch of thatch can create a barrier that prevents the movement of fertilizers and many pesticides. It hasn’t been that uncommon for homeowners to find grubs in their lawn and apply a grub pesticide only to get no results. The pesticide was so strongly bound in the thatch layer that none of it reached the grubs. It’s also possible for a thatch layer to be so deep that the roots of the grass plant can’t reach the soil, and on a hot dry day, the turf simply fries.

You may also notice that some lawn areas build up thatch faster than others as do some particular lawns. Turf heavy in bluegrasses — quite common out here — tend to build thatch faster than fescue and ryegrass lawns. Overwatering can also cause build-ups and the excessive use of pesticides (insecticides and herbicides) can cause thatch build-ups as the chemicals poison the soil microbes that would otherwise be digesting the thatch.

Most of the most recent research has shown that the major cause of thatch buildup is the overuse of soluble nitrogen fertilizers. Nitrogen is a plant nutrient that can stimulate high vegetative growth rates in lawns, and if the growth rate surges ahead of the lawn’s ability to decompose the thatch, you’ve got problems.

So how do you know what’s too much? Is your lawn spongy? When it’s mowed does it seem to get “scalped” a lot? To measure the thatch layer, cut a small square of turf with a sharp knife or spade and lift it out of the lawn. Look at the profile and you should see two obviously visible layers. The bottom, darker layer is the soil while the top, lighter layer is the thatch. If the thatch is thicker than a half-inch, you need dethatching.

For years, dethatching has been a mechanical process. For very small lawns you can buy a special dethatching rake that has sharp cutting tines that dig through the thatch layer and bring it up to the surface where it then has to be raked with a fan rake and collected. This is back-breaking work though. Having done it on my father’s lawn as a kid, I don’t recommend it.

The second method is to use a machine appropriately called a dethatcher or vertical rake. Tines spin on an axle whose height can be adjusted depending on the amount of thatch you want to remove and the depth of the thatch. The best time to do this work is late summer through early fall, when the turf is about to go through a vigorous growth cycle and overseeded grasses are germinating. Sometimes called a verti-cutter, these machines are great at bringing the thatch to the surface, but you still have to rake it up and cart it away. Again, this too is a back-breaking and tedious job that may best be left to your lawn care people, but the machines can be rented at most rental stores.

There is a downside to mechanical dethatching. There is some scientific evidence that using these machines can damage the grass plants more than they help and one suggestion is to make two shallow passes instead of one deep pass that may damage roots and crowns.

As an alternative, you can apply an organic product called Thatch-X or a similar product to your lawn next June. A combination of organic fertilizer, cold water sea plants, enzymes, microorganisms and biostimulants, these products can reduce thatch build-ups by as much as 50 percent when combined with aeration.

Then there is prevention. When seeding a new lawn or overseeding an older lawn, use lawn seed varieties that are not prone to thatch production. This means higher amounts of ryegrasses and fescues and reduced amounts of bluegrass. Also keep an eye on your soil’s pH and make sure it stays above 5.5. Also consider aerating every three to five years if you have compacted soil or a heavily used lawn. And as always, keep growing.