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The Honolulu Advertiser
Posted on: Friday, August 5, 2005 Posted on: Friday, August 5, 2005

When lawn thatch gets too thick, it has to go

By Jay Deputy

Operating a vertical lawn mower is physically demanding and requires experience. This is a job you might want to hire someone for.

In last week's column, 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 an inch or more, it's time to remove it. There are several methods that can be used. 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.

Close mowing or scalping, in which the grass is mowed down to a much lower height than normal, is one option. You can use a rotary mower or good gas-powered string trimmer. However this is not the recommended way to dethatch. With a rotary mower, grass can only be cut to a height of an inch. This leaves much of the thatch intact. Cutting lower with a string trimmer will give a very uneven cut, resulting in a clumpy regrowth. It's also very time-consuming.

If necessary, scalping with a rotary mower may be used as a first step to prepare for a more preferred method, which is using a heavy duty vertical mower, often called a verticutter.

This is a specialized piece of equipment; it has evenly spaced, knife-like blades that revolve perpendicular to the soil surface. The blades slice into the thatch and remove it. Since this process is usually no more than an annual event, it is much more economical to rent than to buy a verticutter.

It is very important to use proper blade spacing and cutting depth when verticutting different turf grasses. Use a blade spacing of 1 to 2 inches and a cutting depth set to just reach the soil surface for Bermuda grass, zoysia and seashore paspalum. These grasses all have underground runners called rhizomes and can survive if most of the above ground stolons are removed.

In this case, all new growth will come from the rhizomes.

If the thatch of these grasses is several inches thick, it will be easier to use the verticutter if some of the excess growth is first removed by scalping down to one inch with a rotary mower.

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. This will prevent potential severe damage by removing too much at one time.

Scalping with a rotary mower is not recommended for either of these two grasses.

After dethatching, all loose debris — mainly of pieces of cut stolons — should be raked up and removed from the lawn. An average-size lawn may fill several pickup trucks.

These stolons can be used as planting material for establishing another lawn, if used within a day or two. After cleanup, the lawn can be closely mowed to remove any remaining debris, then thoroughly watered to prevent drying of exposed roots and rhizomes.

Sometime within the next few days to a week will be a good time to core aerate and top-dress with a thin layer of compost to fill the core holes. 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 established. This should take only a few weeks if done at the right time of year.

The best time to verticut is when the grass is healthy and actively growing. In Hawai'i, the best time is from April through September.

Make sure your lawn is not too stressed. It's a good idea to fertilize and irrigate regularly for several weeks before.

Vertical mowing is an effective means of removing thatch, but if not done properly, the grass can be so severely damaged that it might not survive. Experience is essential.

Vertical mowing also is physically demanding. It might be advisable in this case to have a reputable lawn maintenance company do it for you. But ask them ahead of time how they plan to remove it, and whether the followup care is included.

Jay Deputy is an education specialist in landscape horticulture and turf at the University of Hawai'i-Manoa, Department of Tropical Plant and Soil Sciences, and state administrator for the Certified Landscape Technician Program sponsored by the Landscape Industry Council of Ha-waii. Reach him at deputy@hawaii.edu.