Why, When, and How to Dethatch a Lawn
Restore your grass so it is lush and green for the long-term by learning why, when, and how to dethatch a lawn. These lawn dethatching tips will help you become an expert in no time.
Thatch is a matted layer of grass stems, roots, stolons, rhizomes, and other organic debris that builds up faster than it can decompose. Thatch accumulates immediately below the green surface of the lawn—on top of the soil at the base of blades. A layer of thatch that’s less than ½ inch thick is normal and does not present cause for action. In fact, a thin layer of thatch is a good thing: It reduces soil compaction and increases tolerance for cold and heat by protecting the crown (the point on the plant where grass growth originates). Thatch can also reduce water loss through evaporation.
If the thatch is more than ½ inch thick though, it may cause trouble. Think of it as a layer of plastic wrap on top of the soil. Your grass will not have access to air, water, and nutrients because thatch blocks them from reaching the roots. A thick layer of thatch can harbor disease-causing insects and fungi, prolong high humidity that promotes disease, and bind or tie up pesticides. And as thatch builds, grass roots knit into the thatch layer instead of the soil, where they easily succumb to environmental stresses such as prolonged periods of hot, dry weather.
What causes thatch?
There are numerous causes for excessively thick thatch. The most common ones are poorly aerated soil, excess nitrogen (which leads to excessive growth), and too much water (which reduces the amount of oxygen in the soil). Thatch may also result from activities like the overzealous use of pesticides that kill the earthworms and microorganisms that break down organic matter.
Some varieties of lawn grass are simply more prone to developing thatch than others. Among the cool-season grass species, Kentucky bluegrass is a prime offender for spreading aggressively and accumulating thatch. (Tall fescue may be a better choice for avoiding thatch.) Among the warm-season grass species, Bermuda is more likely to accumulate thatch than zoysia. (Check with your local cooperative extension service to better understand the grass species that work in your area.)
There are multiple ways to figure out if your lawn has a thatch problem. The easiest is to simply check to see if your lawn is green on top but brown below. The best time to check is after you mow when you have just cut off the top green growth so the lawn will look brown and/or dead. Either of those conditions indicates thatch. Another test is to walk across the lawn: If it feels spongy, it may be due to thatch.
If you are still in doubt, use a spade or knife to cut several small plugs from the lawn. Measure the thatch, which will appear as a layer of spongy brown material between the grass and the soil. If it is more than ½ inch deep, your lawn needs to be dethatched.
When is the best time to dethatch your lawn?
Lawns should be dethatched only when conditions are best to promote rapid recovery of your grass type. The best time to dethatch cool-season lawns is late August to early October, depending on your location, when the grass is growing vigorously and few weed seeds are likely to germinate. A light application of fertilizer (½ to ¾ pound actual nitrogen per 1,000 square feet) and regular, deep watering will speed the lawn’s recovery.
Dethatch warm-season lawns in late spring or early summer after they’ve completely greened and are growing rapidly.
How do I dethatch my lawn?
There are several ways. For lawns with moderate levels of thatch, aerating (see below) may do the trick. Or use a cavex rake (aka thatching or lawn dethatcher rake)—which has unusual semicircular tines. Those knifelike blades cut through the sod and pull out thatch. For large lawns with serious thatch problems, the most effective solution is a vertical mower (aka power rake). Resembling a heavy-duty power mower but with a series of spinning vertical knives, it cuts through thatch.
Dethatching often creates a large volume of debris that must be removed. If the debris is weed-free and you have not used herbicides or pesticides on your lawn, compost it. Or check to see if your city has a composting program for yard waste.
How do I reduce future thatch accumulation?
Thatch is inevitable, but appropriate lawn care practices will help reduce the severity in the future. Use the right type and amount of fertilizer for normal growth. Water deeply, thoroughly, but infrequently. Maintain proper pH levels; adjust them if needed. And aerate your lawn on a regular schedule to make room for new growth.
How do I aerate my lawn?
The roots of all plants need air as much as water, which is why aerating is so important. It’s a simple process of perforating the soil (and any thatch) while removing plugs of lawn that leave behind small holes that allow water, air, and fertilizer to get to roots. Aerating enables the roots to grow deeply, producing a more vigorous lawn.
Multiple tools are available for aerating lawns. If your lawn is small, a foot-press aerator that you push into the soil like a spade may suffice. For large lawns (more than a few hundred square feet), you may want to buy or rent an engine-powered core aerator that resembles a lawn mower. You steer it across the lawn—several times in different directions—to break up compacted soil as much as possible. (Aim for 20 to 40 holes per square foot.) Either method requires moist soil, so aerate a day or two after a soaking rain or water the lawn deeply prior to aerating. Soil that is too wet or too dry will prevent the tool from working properly.
Aerating tools remove thin, cigar-shaped plugs of soil and deposit them on the lawn’s surface. (Avoid tools that are designed to punch holes without removing cores.) Let the plugs dry for a few days, then break them up with energetic raking to create a thin, beneficial topdressing.
The best time to aerate cool-season lawns is between late August and early October. The second best time is in the spring. (In spring, wait until you’ve mowed the lawn twice before aerating.) Follow with regular, frequent watering. The best time to aerate warm-season laws is in late spring or early summer—or anytime followed by four weeks of good growing weather. Lawns with severe thatch problems may need twice-yearly aeration.