Zoysia – Dethatching

Thatch is a layer of partially decomposed plant material that builds up on the soil’s surface. Usually, more than one half inch of thatch on general turf areas decreases the turf vigor by restricting movement of air, water and fertilizers into the soil. Excessive thatch also restricts root development and provides a suitable environment for insect and disease problems. Thatch can be removed by a vertical mower or by other means of dethatching such as a power rake.

Vertical mowing helps keep turf healthy by removing the vegetation from the thatch layer. This dead vegetation is lifted to the surface by the blades of the vertical mower. Vertical mowing can be done in the early spring on warm season grasses just before green up occurs, or when the grass is growing rapidly yet not so hot that the water requirements are high.

Dethatching equipment can be rented from local nurseries or rental companies. Remember to fertilize after dethatching because the turfgrass has been damaged and needs the stimulation to recover.

Dethatching is most beneficial on zoysiagrass and bermudagrass.

Q: My husband bought a dethatching blade for our lawnmower. How should it be used on a zoysiagrass lawn?

A: In the first place, do you have a thatch problem? Thatch is a layer of undecomposed stems and clippings on which a turfgrass is growing. Thatch is bad because the grass is more susceptible to drought and cold damage. zoysiagrass lawns don’t usually have a thatch problem unless they were over-fertilized and were mowed infrequently in the past. Before you dethatch, go out and examine your lawn grass. Is the grass growing on the soil or on a layer of thatch? If the thatch is less than ½ inch thick, you should not dethatch. A more practical reason is that dethatching is done only when the grass is vigorously growing in June. The best management of zoysiagrass right now would be to lower the regular mowing blade ½ inch and remove the brown winter stems.

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A local gardener sent me an e-mail saying that her home has ‘Emerald’ zoysia. She has been told it needs to be dethatched and aerated but she wanted my opinion about whether this was necessary.

In my opinion, dethatching is a service that is oversold to homeowners. True, some lawns develop thatch problems – - but it is usually a result of over-fertilizing and under-mowing the turf. Cathy is as qualified as anyone to determine the extent of her thatch problem. All she has to do is thrust a shovel or trowel into her lawn and lever up a patch of grass to examine. Thatch is a layer of dead stems and un-rotted grass clippings that lies on top of the soil. Grass may grow on top of it or be half-covered by it. A thick (one-half inch or more) layer of thatch encourages disease, insect and drought injury.

Once she inspects her grass, she can decide what to do. A thin layer of thatch is no problem; a thick layer might need attention. The nugget of information Cathy needs is the knowledge that aeration can accomplish more than allowing air to reach her grass roots. It helps manage thatch as well! A motorized core aerator pulls up thousands of soil plugs and deposits them on top of the grass. As the plugs dry and crumble, they innoculate the thatch underneath them with fungi and bacteria which consume the thatch. My advice to her was to check her thatch layer, have the zoysiagrass aerated in early summer and to make sure an average of twelve holes per square foot are left in her lawn when the aerator stops. I also suggested that if she found lots of thatch she should halve her fertilizer rate and double her mowing frequency this next season.

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