In my last article I talked about thatch, what it is, what problems it causes, what can cause its rapid buildup and what can be done to slow it down. Once thatch reaches a thickness of one inch or more it's time to remove it. The idea is to remove most, if not all, of the underlying brown stemmy growth and allow new growth to come back from the remaining stolons and rhizomes (above and below ground runners).

If necessary, scalping the grass down to a much lower level with a power mower or string trimmer may be used as a first step. This will leave some of the lower thatch intact. Thinning out the remaining thatch is done with the use of a heavy-duty vertical mower, often called a verticutter. This is a specialized piece of equipment that has evenly spaced, free swinging blades that rotate perpendicular to the soil surface and slice into the thatch. Verticutters can be rented at most rental shops.

Use the appropriate blade spacing and cutting depth when verticutting different types of turf grasses. For bermuda grass, zoysia and seashore paspalum use a blade spacing of 1 to 2 inches and a cutting depth set to just reach the soil surface. These grasses all have underground rhizomes and can survive if most of the above-ground stolons are removed. In this case all of the new growth will come from the rhizomes.

Centipede grass and St. Augustine have thicker stolons and do not form rhizomes, so blade spacing of 2 to 3 inches will work better for dethatching. The depth should be set so that the blades do not reach the soil surface, leaving a thin layer of stolons behind. If thatch accumulation exceeds 2 or 3 inches, which is common in St. Augustine grass, the lawn should be verticut at intervals of several weeks using gradually increasing depths of cut to prevent potential severe damage. Scalping with a rotary mower is not recommended for either of these two grasses.

After dethatching, rake up and remove all debris, consisting mainly of cut stolons. These stolons can be used as planting material for establishing another lawn if used within a day or two. Following cleanup, water the lawn well to prevent drying of exposed roots and rhizomes. Sometime in the next few days will be a good time to core aerate and top-dress with a thin layer of compost to fill the core holes. Core aerators are also available at rental shops. This is especially beneficial if the soil seems compacted and drainage is poor.

Apply one pound of nitrogen per 1,000 square feet (five pounds of ammonium sulfate will give one pound of nitrogen) to encourage new top growth. Continue to irrigate well until the new growth is well established, which should take only a few weeks if done at the right time of year. Start regular mowing as soon as the new growth begins to cover.

The best time to verticut is when the grass is healthy and actively growing, which in Hawai'i is April through September. Make sure your lawn is in good shape before starting. It is a good idea to give it a shot of fertilizer and irrigate regularly weeks beforehand.

Vertical mowing is an effective means of removing thatch, but if not done correctly the grass can be severely damaged. Unless you truly enjoy being your own yard guy, it might be advisable to have a reputable lawn maintenance company do it for you.