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Understanding thatch and how to prevent it from damaging your lawn

In the Garden

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Most homeowners want healthy, vigorous, and attractive lawns throughout the growing season. However, as lawns age, excessive thatch may accumulate and reduce those characteristics.

Thatch is an accumulation of dead and partly decomposed leaves, stems, and roots above the soil surface. The majority of thatch accumulation results from stems. Leaf clippings usually break down before they can accumulate as thatch. The most vigorously growing grasses tend to produce the most thatch. When thatch is more than half an inch deep, most of the grassroots (and underground stems, called rhizomes, in some grass species) grow in the thatch and not in the soil. This can become a problem because thatch has little water-holding capacity and the grass will be more susceptible to drought.

Thatch offers roots and grass crowns little protection from temperature extremes, so summer heat and winter freezing are more likely to kill grasses in thatchy lawns. In addition, an increase in thatch will promote the incidence of diseases such as Pythium blight and the overwintering of certain insects such as sod webworm and adult billbugs. Finally, the effectiveness of certain pesticides and the efficiency of fertilizers are greatly hampered when thatch is excessive.

Thatch accumulates when the rate of shoot-leaf production exceeds the rate of shoot-leaf decomposition by microbes. Grass species and even specific cultivars differ in the rate of thatch formation. Some grass species and cultivars produce thatch faster than others because of their vigorous growth habits. Others develop thatch faster because their plant tissue resists decomposition.

Thatch may develop over a period of several years before noticeable damage occurs. Good cultural practices should begin with a new lawn. These good practices will not prevent thatch indefinitely but can slow its formation.

Make moderate, well-timed fertilizer applications to maintain adequate vigor without excessive growth. Excessive fertilization, especially with nitrogen fertilizers at the wrong time of year, such as early spring, may cause excessive shoot growth. Three to four pounds of actual nitrogen per 1,000 square feet of lawn per year is adequate for a Kentucky bluegrass. If you leave clippings on the lawn, your turf will need approximately 25 percent less nitrogen per year.

Thatch buildup in turf | Photo: K Mathias, University of Maryland

Besides nitrogen, your lawn may need three other fertilizer elements: phosphorus, potassium,
and sulfur. Apply fertilizer in small amounts of one-half to 1-pound of nitrogen per 1,000 square feet, rather than in larger amounts less frequently. Three-fourths of the total fertilizer amount should be applied in the fall (September to early November) and the balance during late spring (mid-May to mid-June) after the flush of spring growth.

Excessive irrigation can promote thatch by creating waterlogged conditions that inhibit the breakdown of the thatch by microorganisms. Regardless of the season, the amount of water to apply at each irrigation will depend on the water-holding capacity of the soil at the grasses’ rooting depth.

During the warmer part of the summer, a lawn requires an average of about one-fourth of an inch of water per day. But, we obviously aren’t going to be watering every day, so that is just the average. The amount to apply per watering and the frequency of watering depends on the soil type and rooting depth.

On deep loam, clay loam, and clay soils, apply two inches of water at each irrigation and irrigate weekly.

Sandy-type soils would need about one inch of water every three or four days. Shallow soils and coarse sandy or gravely soils hold less water, so you would apply less water at more frequent intervals.

In spring and fall, the daily water needs of turf are much less than in summer, so adjust your irrigations accordingly.

Another aspect of thatch buildup is soil compaction, which is caused by foot and equipment traffic. Compaction destroys the soil structure by pressing soil particles closer together and adversely affects oxygen levels and water movement. As a consequence, turf grasses soon become susceptible to drought, disease, and insect damage. In addition, weed invasion often occurs because some weeds, such as knotweeds for example, survive better in compacted soils than grasses.

To remedy this problem, implement a lawn aeration schedule to help break up the thatch.

Core aeration involves using a machine that punches a hollow tine into the soil, removing one-fourth to half an inch diameter soil cores that are approximately two to four inches in length. The soil cores are deposited on the surface of the turf and are pulverized and distributed by the next mowing. Aerating should be done in the late summer to fall. It should not be done in the heat of summer as it is hard on the lawn due to extra dehydration and the damage it does to roots.

The last technique is power raking, which is a method of removing thatch buildup on your lawn. Power raking is damaging to the crowns of the grass plants and should not be done when it is hot. Lawns can be power raked in the spring, however, competition from annual grasses such as crabgrass will be much greater while the turfgrass is recovering.

Lawns with a serious thatch problem may require a power raking each year until thatch depth is less than half an inch. When a power raking treatment is necessary, at least three to four weeks of good growing weather should follow in order for the lawn to recover. Avoid hot weather in July and August.

For further questions, contact Lance at (208) 624-3102.

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