Dealing With Thatch Buildup in Lawns

Thatch is one of those turfgrass issues where a little is good for the lawn, but too much causes problems.

A thin layer of thatch actually helps reduce wear and tear and can increase the life of your lawn. But when thatch is allowed to build up to more than ½-inch thick, it provides a safe haven for lawn diseases and insect pests, plus it’s more difficult to keep watered. The result is a stressed lawn that’s prone to fungal problems, such as pythium and dollar spot, that are made worse by the hot, humid weather we often get in summer.

What is Thatch?

If you look at a cross-section of turf, thatch is the layer of compacted living and dead organic matter between the green grass blades on top and the soil on the bottom. This thatch layer is made up of the fibrous parts of grass plants, such as stems and roots, that build up faster than they can decay.

In a healthy lawn, the thickness of the thatch layer generally stays fairly constant; as new organic matter builds up, older material decomposes. But when things are out of balance, thatch builds up too quickly (or doesn’t decompose), leading to lawn problems.

Top Causes of Thatch

The three contributors to thatch buildup include biological, cultural, and environmental causes.

Turfgrass Variety

Biological factors have to do with the turfgrass variety. Some varieties of grass are more susceptible to thatch because they grow faster and/or produce more tough fibrous stems and roots. Cool-season grasses that are more likely to form thatch include Kentucky bluegrass, creeping red fescue, and creeping bentgrass. Others, such as perennial ryegrass and tall fescue, do not produce significant thatch build-up. That’s one of the reasons why tall fescue is our top recommendation for lawns in central NJ and eastern PA.

Excessive Use of Fertilizers & Overwatering

Aside from the type of grass planted, cultural practices are a prime cause of thatch buildup. Any lawn care that encourages grass to grow faster contributes to thatch, such as over-fertilizing with high-nitrogen fertilizer and overwatering. Infrequent mowing also adds to thatch buildup because it allows more plant stems to form and then cuts them off.

Synthetic Pesticides & Herbicides

Another lawn care practice that promotes thatch is applying excessive amounts of synthetic pesticides and herbicides. These chemicals limit the microorganism activity in the soil that helps break down thatch, and have also been shown to reduce earthworm populations.

Poor Soil Conditions

The environmental causes of thatch include soil that’s too acidic, as it cannot sustain the microorganisms needed to break down thatch. Compacted soil and soil with poor drainage are other factors in thatch buildup.

Grass Clippings Do Not Cause Thatch

There’s one lawn care practice that does not cause thatch: using a mulching mower and leaving grass clippings on the lawn. Grass leaves contain very little lignin, the tough fiber that prevents stems and roots from decomposing. As a result, grass blades break down very quickly. Recycling grass clippings to the lawn when mowing returns valuable nutrients to the soil that help reduce the need for fertilizing.

How to Remove Thatch

Before removing thatch from your lawn, take a wedge-shaped grass sample 2 inches deep. Measure the thatch layer; if it’s more than 1 inch thick then it’s time to remove it.

The best way to dethatch the lawn is a one-two punch of core aerating followed by power raking, also called vertical mowing.

Core aerating pulls plugs of soil from the lawn to reduce compaction and improve drainage. Power raking uses metal tines to dig in, break up the thatch and bring it to the surface. Be sure to remove all the thatch debris after raking.

When to Remove Thatch

Dethatching can be tough on a lawn so only do it when the turf is healthy and weather conditions won’t unduly stress it. In our area, the best time to remove thatch from a cool-season turfgrass lawn is late summer or early fall.

This is also a good time for lawn fertilization and overseeding, so take the opportunity to give your lawn a “makeover” that will give it the best chance of being lush and green next spring.

How to Prevent Thatch Buildup

To keep your lawn thatch-free, top dress with a thin layer of healthy topsoil or compost after aerating and dethatching to boost the number of beneficial soil microorganisms and to encourage earthworm activity.

Because acidic soil can lead to thatch buildup, it’s a good idea to periodically take a soil test to check nutrient and pH levels. This will help you keep soil pH at a level that supports the microorganisms needed to decompose thatch, and will let you know whether or not your lawn needs to be fertilized.

Then be sure to keep up with good lawn care practices to prevent thatch from returning. Follow a schedule for fertilizing in the recommended amounts based on the soil test (this is where organic fertilization can make a big difference), avoid overwatering, be selective in using pesticides (there are organic options for many lawn problems) and mow the lawn to keep grass blades 3-4 inches long.