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The cutting edge of mowing

It’s that time of year when many home lawns in our coastal area are looking pretty tough. Most have not been mowed for several months and our cool, wet, dark winter days have been ideal for moss invasion. WSU’s standard recommendation for moss control this time of year has been an application of Ferrous Ammonium Sulfate. The Iron in this mixture kills the existing moss, turning it black. The Ammonium Sulfate fertilizer stimulates grass growth by providing nitrogen. Good turfgrass health, however, depends on more than just moss killer and fertilizer. Most lawns can use a good thatching. The question often arises: Just how much thatch is too much?

Different species of grass accumulate thatch at different rates. Perennial ryegrass is a low thatch producer; bluegrass and fine fescues form a dense thatch that is difficult to remove. A layer of thatch ranging from a half to three-quarters of an inch is generally acceptable in most lawns, but anything greater can cause problems.

Once a thick thatch layer is formed it must be removed by vertical mowing. Use a machine that has rigid or flexible blades or tines (power rakes) that can cut into or pull turf out of the soil surface. You can rent a power rake, or you can hire a professional lawn service to do the job. Attachments to your lawn mower are not as effective as true vertical mowers. The best time to dethatch your lawn is now, which will allow the grass sufficient time to recover before the stress of summer heat. Thatching now will also remove the majority of moss that is present.

After de-thatching, always fertilize with a 3-1-2 ratio fertilizer. Be sure to water the fertilizer in thoroughly. De-thatching without followup fertilization results in increased weed encroachment and ugly turf. Once a thick layer has been removed, dethatching every year or every other year will ensure a healthy lawn that is easy to manage. Dethatching is less disruptive and a lot less work if it is done regularly. People who dethatch only after a severe thatch layer builds up usually destroy their lawns.

Question: Is there any way to prevent thatch from accumulating?

Answer: If your lawn is healthy, it naturally will produce thatch consistent with the type of grass you are growing. Thatch should be viewed as inevitable and desirable provided it isn’t too thick. Learn to think in terms of managing thatch rather than trying to get rid of it completely. Consider the following tips to help avoid encouraging too much thatch. Avoid over-fertilizing your lawn. The idea is to fertilize enough to provide a nice green lawn, but not so much that extra mowing is needed.

Mow frequently enough so that only one-third of the grass blade is removed at any one time. Normally, once a week is fine, but in spring when growth is vigorous, you may need to mow twice a week.

Removing clippings will have little effect on thatch development, but it will force you to fertilize the lawn more often. Leaving the clippings on the lawn will not contribute to thatch accumulation unless you also apply excess nitrogen fertilizer.

“Wonder amendments” such as bacterial, yeasts, enzymes and other “miracle cures” have no significant effect on thatch accumulation. Natural decomposition will occur from the organisms that are already in your soil when the lawn area is well drained and the soil pH is neutral (between 6 and 7). If your soil is acidic, periodically apply lime to help bring it up to neutral.

Donald D. Tapio is the Washington State University Extension regional specialist for Grays Harbor and Pacific counties. He can be reached at 482-2934 or