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The Honolulu Advertiser
Posted on: Friday, July 11, 2008

This is the time to improve your lawn

By Jay Deputy

As we get into the hot summer months, the lawn is once again growing at full speed, and mowing, irrigation, fertilization and other chores can become a weekend routine. Now is a good time to take a close look at the condition of your lawn before we get any further into the summer.

One potential problem can come from excessive thatch buildup. Thatch is the brown stemmy layer that builds up between the soil surface and the green leafy part of the grass. When thatch is more than an inch thick, it begins to make the lawn feel soft and spongy, which often results in damage due to mowing, called scalping. Heavy thatch also interferes with movement of water and fertilizer into the soil and provides a good hiding place for harmful lawn insects.

Excess thatch can be removed in several ways. 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.

One option is to mow the grass down to a much lower height. You can use a rotary mower or good gas-powered string trimmer. If a rotary mower is used, the grass can only be cut down to one inch because that's as low as that type of mower will cut. This will leave much of the thatch intact. Cutting lower with a string trimmer will give a very uneven cut, resulting in a clumpy regrowth, and is also very time-consuming. However, scalping the lawn down with a rotary mower is a good first step to prepare for a more effective method called verti-cutting.

This preferred method of thatch removal is done with the use of a heavy-duty mower, often called a verticutter. This specialized piece of equipment has evenly spaced, knifelike blades that swing perpendicular to the soil surface. The blades slice into the thatch and remove many pieces of runners growing near the soil line. Since this process is usually needed no more than once a year, it is much more economical to rent a verticutter rather than to buy one.

It is very important to use the correct blade spacing and cutting depth when verticutting different turfgrasses. Use a blade spacing of one to two 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 because most 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 two to three inches will work better for dethatching. Cutting 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 two to three inches, which is common in St. Augustine grass, the lawn should be verticut at intervals of several weeks using gradually increasing depths. This will prevent potential severe damage by removing too much at one time. Scalping with a rotary mower is not recommended for St. Augustine or centipede grass.

After dethatching, all of the loose debris, consisting 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.

Following cleanup, the lawn can be closely mowed to remove any remaining debris, then thoroughly watered to prevent drying of exposed roots and rhizomes. Sometime during the next few days will be a good time to core aerate, then spread a thin layer of compost to fill the core holes. This is especially beneficial if the soil seems compacted and drainage is poor. Apply three or four pounds of a good turf fertilizer for every 1,000 square feet of lawn to encourage new top growth. Continue to irrigate well until the new growth is well 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 that your lawn is not too stressed. It would be a good idea to fertilize and irrigate regularly for several weeks before.

Vertical mowing is an effective means of removing thatch, but will result in at least a temporary disruption. If not done correctly, the grass can be so severely damaged that it might not survive. Experience with the method and equipment, knowledge of the type of grass being renovated and doing it in the right season all are essential.

It is also a physically demanding job. Unless you truly enjoy being your own yard guy, 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 do it and if the follow-up 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 & Soil Sciences.