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  MG Manual Reference
Ch. 12, pp. 35 - 39

[Maintenance: irrigation | new lawn | fertilization | renovating | mowing | mowing heights
| dethatching | aeration | weed ]


Turfgrass dethatching and aeration are two distinct cultural management practices which are used to promote a healthier, more vigorous turf. "Turf" is actually the grass and soil which comprise the "lawn". It is important to manage the soil which indirectly affects the growth and well being of the grass.
Dethatching involves the mechanical removal of thatch. It consist of tightly intermingled layer of dead and decaying turfgrass tissue derived from leaves, stems, stolons and roots. Leaf clippings contribute very little to thatch accumulation. Dead and decaying roots, rhizomes, stolons, and shoots are major contributors to thatch since these structures resist decomposition. Located between the green vegetation and the soil surface, thatch accumulates when turfgrass organic matter production exceeds decomposition.
Thatch forms to a much greater extent with stolon and rhizome producing grasses. Therefore bermuda grass and zoysia would tend to produce more thatch than ryegrass and tall fescue which are bunch grasses. KBG and buffalograss are intermediate in thatch production.

A Comparison of Physical Properties of Thatch, Sand and Soil
  Thatch Sand Soil

Aeration Good Good Fair
Compaction Resistant Moderately Resistant Susceptible
Moisture Retention Poor Poor Good
Nutrient Retention Poor Poor Good

Small amounts (less than 1/2 inch) of thatch can be beneficial because it increase the turfs resiliency, improves its wear tolerance, and insulates it against soil temperature changes. When thatch layers exceed 1/2 inch, however, the disadvantages generally outweigh the advantages. The turf's tolerance to heat, cold and drought decreases with increasing thatch accumulation. Localized dry spots, scalping, disease and insect problems also increase. As thatch accumulates, there is a tendency for root and rhizome growth to occur primarily in the thatch layer rather than in the soil. This results in a weakened, poorly rooted turf that is prone to stress injury and requires frequent irrigation and intense management. Scalping occurs readily on heavily thatched turfs.
To determine thatch accumulation, cut a pie-shaped wedge of grass and soil from the turf. Remove it and measure the organic matter that has accumulated. Measure the accumulation from several areas in the turf since thatch is not uniformly distributed. If this layer exceeds 1/2 inch, steps need to be taken to reduce it.
Thatch Removal

Remove thatch during periods of active turfgrass growth because dethatching is an injurious process. Remove thatch when at least 45 days of favorable growing conditions are anticipated following the process. This will ensure turfgrass recovery and minimize weed competition, and potential stresses associated with dethatching.
Warm Season Grass Dethatching

Wait until the grass has greened up for 2-3 weeks and the night time air temperatures are above 60°F if you want to dethatch in the late spring. Early and midsummer dethatching is best! Do not dethatch heavily in the fall, since the warm season grass will be damaged and be weak for the winter. Dethatch zoysia in the early to mid-summer period only. Collect the thatch by raking or by mowing up debris. Follow with 1.0 lb. -N- per thousand square feet of either a balanced (N-P-K) or a fertilizer such as ammonium sulphate, ammonium phosphate, or ammonium nitrate.
Thatch can be removed by hand raking or with a power rake. Hand raking is laborious and is only practical for small areas. Power rakes can be rented from rental firms, or the service hired from a professional lawn care company. Power raking devices (also called a verticutter, or ren-o-thin machine) use rigid wire tines or steel blades to lift thatch debris and a small amount of soil to the lawn surface. The soil should be moist, not dry, for best results. Power raking during excessive soil moisture conditions tear and pulls the turf from the soil instead of slicing and lifting the thatch debris as desired. Remove clippings and thatch debris immediately. Always dethatch in at least two directions.
Heavy dethatching of bermuda grass should not be part of the fall overseeding. If done too late in the summer or early fall when temperatures are warm (above 90°F) dethatching may indirectly injure the turf. This is because the bermuda puts energy back into new growth, when existing growth should be normally slowing down and preparing for winter rest. If the bermuda grass does not "harden off" properly in the fall, it may be slow to complete during the spring transition, allowing the surviving ryegrass to linger on.
After thatching in early summer, apply a preemergence herbicide to prevent the potential encroachment of crabgrass. This application can be done in combination with fertilization (1.0 -1.5 lb. N/1000 sq. ft.). Make sure that the preemergence is applied at least 75-90 days before fall overseeds take place.
Cool Season Grass Dethatching

KBG should be dethatched in the early fall and not during summer. A second choice is early spring after green up. The idea is to allow enough time for regrowth during the cool weather. Follow with fertilization as described in the previous section. Remember, each time you fertilize you must irrigate afterward.
Tall fescue and perennial ryegrasses are slow to build up thatch. They are also slow to recuperate from dethatching, since they are bunch grasses, which produce no runners at all! If you must dethatch these two grasses, do so in the early fall. You may follow with reseeding if the damage is severe.
KBG varieties differ in their ability to produce thatch (as do bermudagrass varieties). The taller, upright, light-to-medium density cultivars produce less thatch than the lower growing, denser varieties. Low thatch producing types would include South Dakota Common, Delta, and Park KBG.
The more aggressive KBG types which would produce more thatch would include Mystic, Touchdown, Princeton 104, and Warrens A-34.
Heavy thatch producing KBG varieties are desirable from a wear tolerance standpoint and when managed properly are the varieties of choice for sports fields. The attached table includes variety descriptions and use factoring.
Minimizing Thatch Accumulation

Thatch accumulation can be minimized by using proper cultural practices. Proper mowing frequency and height are the principle cultural practices that can be used to reduce thatching tendency. Mowing frequency should be dictated by the turfgrass growth rate. No more than one-third of the leaf blade should be removed with any mowing. If proper mowing frequency is maintained clippings do not need to be removed. Turfgrass leaf clippings contribute little to thatch buildup. They break down rapidly and recycle nutrients when returned to the turf.
Apply fertilizers at rates and in programs that meet, but do not exceed, the nutritional needs of the turf. Excessive nitrogen applications may result in organic matter production rates that exceed breakdown and stimulate thatch accumulation. Avoid light, frequent irrigations. It is best to irrigate turfs deeply and infrequently, water when the turf shows moderate signs of moisture stress.
Use pesticides only as needed. Thatch accumulation can be minimized by avoiding unnecessary use of pesticides because pesticide application may affect desirable microorganisms and earthworm populations. Earth worms digest thatch, improve soil aeration and drainage and introduce soil microorganisms into thatch. They play an important role in thatch control and, where feasible, should be encouraged through proper management.

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