I’m often asked about thatching versus aerating lawns. Thatch is the spongy layer of compacted roots and stems that builds up between the green vegetation of a lawn and the soil surface. It can be a problem. Thatch heats up easily and can dry out quickly, stressing grass whose roots struggle to penetrate the thatch layer. Yet, in wet weather, or where there’s poor drainage, thatch can stay soggy like a sponge and rot roots. Diseases and pests often thrive in thatch.
Thatch develops for a number of reasons. Certain grass types, such as Kentucky bluegrass or creeping red fescue, are more likely to develop thatch than bunch-type grasses, such as perennial rye or tall fescue. Compacted soils and soils low in organic matter and soil microbial activity are prone to thatch. Shallow watering and excessive nitrogen fertilizer also encourage thatch build up.
Managing thatch starts with selecting the appropriate species to plant when starting a lawn; preparing soil thoroughly before planting to allow good air and water penetration; using fertilizers and pesticides correctly and at the right time of year; and irrigating properly. Yearly core aeration reduces thatch development in an established lawn. Pulling plugs out of the soil reduces compaction and improves air and water movement in a soil. Spiking machines that don’t pull out a plug actually increase compaction and decrease turf health. Plug removal, followed by a top-dressing of organic matter and a deep irrigation, increases microbial action. Microbes decompose thatch. Spring is an excellent time to aerate in Northern Nevada.
Leaving short clippings on the lawn after mowing doesn’t contribute to thatch, unless there’s a dense existing thatch layer. If thatch reaches an inch or more, preventative measures alone won’t eliminate it. Then, a vertical mower with blades perpendicular to the soil or a power rake will be needed to slice down and through the thatch to bring it up to the surface where you can rake it up and carry it away. Power-rake only a small amount of thatch in your first pass in order to see how much thatch remains and whether a second pass is needed. Too much thatch removal can damage a lawn and allow weeds to grow.
Don’t remove thatch from a droughty, heat-stressed or weak lawn. Unhealthy lawns may not recover from the thatching process. The best time for thatching is early fall when the weather is cool so the lawn can recover quickly without being stressed.
For information, see “Managing Thatch in Lawns” from Penn State Plant Science Department:
JoAnne Skelly is the Associate Professor & Extension Educator Emerita University of Nevada Cooperative Extension.