Gardening: Control your thatch before it controls your lawn

EILEEN WARD

Thatch is that brown layer of living and dead organic material that develops between the green vegetation and the soil surface. It imports a sponginess to the turf. Thatch consists of dead and dying leaves, stems, stolons, rhizomes and roots. Its purpose is to insulate the grass plant against sudden temperature changes, to cushion against wear, to reduce excessive water evaporation, and to stabilize the soil and protect it against compaction.

Too little thatch when establishing new turf slows the establishment of a lawn. The best way to produce up to one half inch of thatch as soon as possible is by using extra nitrogen and water.

But once a turf is established, thatch control should be built into your management practices. Thatch accumulations are undesirable for many reasons.

Thatch restricts water and air movement into the soil. Dry thatch repels water and wet thatch enhances disease problems.

A thick layer of thatch makes mowing a lawn more difficult. The turf becomes spongy, allowing the mower to sink into the turf and scalp the lawn. This results in an uneven green and brown appearance.

Thatch provides an ideal habitat for insects and disease. It is associated with an increased incidence of brown patch, dollar spot, and leaf spot diseases.

A heavy thatch layer elevates the growing points of the grass, crowns, rhizomes and stolens, and roots above the soil surface. This results in more winter injury because the elevated plant parts are exposed to greater extremes in temperature. Winter kill of lawns is often associated with thick thatch layers.

Thatch can interrupt and restrict the movement of pesticides and fertilizers into the soil. This makes pest control difficult and produces a non-uniform response to fertilization.

Thatch build-up can be attributed to many factors. When vegetative production exceeds decay thatch can accumulate. Grasses depend upon constant regeneration for survival, and new growth of creeping grasses covers the old, causing residue accumulation. St. Augustine may have this thatch accumulation at a higher rate than bahia grass.

Improper management practices can also result in thatch accumulation. Over fertilizing, over watering, infrequent mowing or mowing too short are the biggest contributors to the problem. Also, failure to keep the soil environment favorable for bacterial and fungal growth, using pH control, adequate moisture, and aeration, decreases the rate of decomposition of thatch because these organisms are responsible for decay of organic matter.

Not removing clippings after mowing has been cited as a cause of thatch build-up. However, research does not support this concept. If properly mowed, clippings decompose readily and do not cause thatch build-ups. However, if the lawn is not is not properly maintained, these clippings could add to the thatch layer, but less significantly than will residues from stems and roots.

Cultural practices: Apply fertilizer only as necessary for proper growth and health. Avoid excessive growth, which will increase thatch.

A soil pH of 6.5 to 7.0 is ideal for maximum microbial activity and decomposition. Liming of acid soils may aid the decomposition of thatch and retard build-up. Proper watering so the soil is not too wet or dry also helps. Mowing practices can also help control the thatch buildup. Lawns should be mowed at the recommended height as frequently as recommended. If not mowed frequently enough the excess clippings should be removed with a grass catcher on the mower or by raking. If mowed too short the grass plant will produce excessive stolons in an attempt to grow more grass blades. These will creep over each other causing a buildup of thatch.

Mechanical thatch removal. Close mowing or scalping is a procedure where the turf is mowed to a much shorter height than normal. Scalping can remove some thatch, but it is not a substitute for vertical mowing.

Vertical mowing is the most common method of thatch removal using a heavy duty vertical mower. This specialized piece of equipment has evenly spaced, knife-like blades, revolving perpendicularly to the turf, that slice into thatch and mechanically remove it. This removes both thatch and mat layer, and simultaneously cultivates the soil and top dresses the undisturbed thatch. Top dressing, adding soil over the top of the lawn, increases decomposition by bringing soil microbes and moisture into contact with the thatch. Vertical mowing is an effective means of removing thatch, but if not done properly the grass can be so severely damaged that it may not survive. Experience with the method and equipment are essential. Only a reputable company should be considered to remove thatch.

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