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Chapter 18 Other turfgrass problems.

OBJECTIVES

After studying this chapter, the student should be able to

* Discuss the unfavorable growing conditions that occur in the shade

* Explain why certain shade-tolerant turfgrass species and cultivars are able to adapt to moderate or partial shade

* List the maintenance practices that help turfgrass plants survive on shaded sites

* Discuss the causes of soil compaction

* Describe the problems resulting from soil compaction

* List methods of preventing and alleviating compaction

* Explain why a thick layer of thatch is undesirable

* Describe the reasons for thatch buildup

* Explain how thatch is reduced and controlled

Introduction

Most of the serious problems encountered by turfgrass have been explained in previous chapters. Three important problems remain to be discussed--shade, compaction, and thatch. Shade is one of the major reasons for turfgrass decline. It is difficult to maintain satisfactory turf on a shaded site. Grass plants growing in the shade may be unable to survive because of low light intensities and other stress factors. Compaction occurs when soil particles are pressed close together and results in a decrease of larger pore spaces. Air, water, and fertilizer cannot readily enter a compacted soil. This unfavorable soil condition is caused by intense traffic on the turfgrass. Thatch is defined as the layer of partially decomposed or undecomposed organic matter formed above the soil surface. A thick layer of thatch exhibits many undesirable characteristics and reduces turf quality. These three problems and their solutions will be discussed in this chapter.

Shade

Shade is one of the most common reasons for the deterioration of turf. It is estimated that as much as 25 percent of the turfgrass grown in the United States is shaded to some extent by trees. Grass and trees are highly desirable in a landscape, and it is inevitable that both are grown together. Unfortunately, they are somewhat incompatible. It is often difficult to maintain turf under trees.

Problems in the Shade

Tree leaves block sunlight and prevent it from reaching turf. Grass growing beneath tree species with dense foliage, such as maples, beeches, oaks, lindens, and hemlocks, may receive only 5 percent of the amount of sunlight that grass in nonshaded locations intercepts (Figure 18-1). The light that does filter through the tree canopy is of poorer quality for photosynthesis.

Low light intensities restrict the rate of photosynthesis. A typical turf that is heavily shaded is weak and sparse because photosynthesis is severely limited (Figure 18-2). Shoot and root growth are reduced, and the plants are unable to store adequate amounts of carbohydrate reserves. The likelihood of injury from stresses such as cold, heat, and drought increases when the plants have insufficient supplies of stored food. Grass growing in the shade is also easily damaged because it has succulent, tender foliage and shallow roots.

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Light exclusion is not the only problem caused by trees. The roots of shallow-rooted tree species such as elms, willows, maples, beeches, and cottonwoods may interfere with grass development. Trees compete with the turf for nutrients and water. Grass plants may suffer from moisture and nutrient deficiencies because trees require large amounts of water and the same fertilizer elements that the turfgrass needs. The blocking of the sun's rays by tree foliage results in cooler air and soil temperatures beneath the tree. Turfgrass growth is slower at these lower temperatures. The roots of certain tree species may even exude toxic substances that can injure turfgrass.

Environmental conditions in the shade are ideal for the development of disease organisms. Pathogens are a serious threat when the surfaces of plants are wet. Turf growing under a tree is more likely to be covered with moisture than turf located on sunny sites. This is because the relative humidity is higher in the shade. Air movement and the heat of the sun have a drying effect. Trees hinder drying by reducing air circulation and sunlight.

This damp microclimate is not the only reason for increased disease problems in the shade. The pathogen that causes powdery mildew disease is inhibited by sunlight and is a greater problem at low light intensities. Grass plants situated under trees are more disease susceptible because of their weakened condition. They are less able to resist an attack by a disease than healthy, vigorous plants. This disease susceptibility is also a result of the succulent leaf tissue produced by grass in the shade. The thinner cell walls are more easily penetrated by fungi. In the winter and early spring, turf shaded by evergreens is slow to warm up and thaw out. This increases the likelihood of winter injury. For example, ice can accumulate and kill the grass.

Growing Turfgrass in the Shade

The denser the shade, the more difficult it is for turfgrass to survive beneath trees. How dense the shade is depends on the tree species, the number of trees, and the distance between them. Conifers such as pines, spruces, and firs individually cause less shading than most deciduous trees. This is because most pines have an open canopy, whereas spruces and firs have a narrow canopy. However, when planted in groups conifers will cause a shading problem (Figure 18-3). Some deciduous species that produce a light shading are poplar, locust, ash, birch, ginkgo, silk tree, Kentucky coffee tree, and the Japanese pagoda.

It is difficult to grow quality turf on heavily shaded sites where the grass receives less than four hours of full sunlight. When tree species with a dense canopy are planted close together it may be impossible to maintain a decent stand of turfgrass. Adequate amounts of sunlight may reach the grass if the trees are spaced far enough apart. People who are planning a landscape must consider the shading effect of trees 10 or 20 years after planting when the trees become large.

Some turfgrasses are more shade tolerant than others. Certain species and varieties can perform satisfactorily in partial or moderate shade, although no grass grows well in heavy shade. Several factors contribute to shade tolerance. These grasses are able to adapt to low light intensities because they use light more efficiently or require less light than intolerant species and varieties. They have to be resistant to diseases that are common in the shade, such as powdery mildew. Drought tolerance can be important because of the competition with tree roots for soil moisture.

St. Augustinegrass, zoysiagrass, centipedegrass, and fine fescues exhibit varying degrees of shade tolerance. Certain Kentucky bluegrass cultivars such as Glade, Eclipse, Ram I, and Chateau can adapt to semishaded conditions. However, the majority of the Kentucky bluegrass varieties respond poorly to low light intensities. Rough bluegrass (Poa trivialis) can tolerate wet, shady sites. Tall fescue has good shade tolerance when grown in the transition zone and warmer areas of the cool season zone. These species and cultivars are the best choices when turf is to be established beneath trees.

Even shade-tolerant grasses perform best when grown in sunny locations. They have a better chance of adapting to shaded conditions than other grasses, but special care is necessary to ensure their survival in the shade. Certain maintenance practices are recommended to reduce the stress experienced by turf situated under trees.

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Soil testing is necessary to determine if soil pH and fertility levels are correct. The competition for plant food between tree and grass roots may result in nutrient deficiencies. Unfortunately, attempting to satisfy the nutritional needs of both trees and grass by applying a heavy rate of fertilizer to the turf can have disastrous results. As the nitrogen moves down through the soil it will encounter the grass roots first. Large amounts of nitrogen will be taken up by the turfgrass, and the plants become lush and succulent. The thinner cell walls cause the grass to be more disease susceptible and less wear resistant. The nitrogen also stimulates a greater rate of vegetative growth. This is a problem because rapid growth results in a depletion of carbohydrate reserves and the need for increased photosynthesis to replace the stored food that is used. Grass exposed to low light intensities may not be able to photosynthesize enough to replenish its carbohydrate supplies.

Turfgrass located in the shade, because of its slower growth rate, requires less fertilizer than grass receiving full sunlight. Light rates of fertilizer are usually sufficient (Table 18-1). Trees should be fertilized separately if the application occurs when the turf is actively growing. This is accomplished by inserting a tree root feeder into the soil beneath the majority of the turfgrass roots (Figure 18-4). Surface application with a fertilizer spreader is an easy and effective method of fertilizing trees. However, when turf covers the tree roots, surface applications should occur in the late fall or winter, when the grass is not growing and the soil is not frozen. The turf should be watered heavily enough to move the fertilizer off the surface and into the soil. The tree can be fertilized without injuring the grass.

Shaded turfgrass should be cut to a taller height than nonshaded grass. As a general rule, the mower blade should be raised 1 inch (2.5 centimeters) in shaded areas. A mowing height of 2.5 to 3 inches (6.4-7.6 centimeters) or more is best, if the species can be cut this high. Allowing grass beneath trees to grow taller is advantageous for three reasons.

Grass plants exposed to low light intensities tend to grow very erect, as if they were reaching for the sunlight. This upright growth habit requires a higher than normal cutting height to avoid removing too much leaf tissue per mowing. Plants with longer leaves can also photosynthesize more because they have greater leaf surface area and more chlorophyll than shorter grass. For each 0.125 inch (3.2 millimeters) the cutting height is raised, the photosynthotic area of the grass can be increased by as much as 300 [ft.sup.2] (28 m2) per 1,000 [ft.sup.2] (93 m2) of turf. This increased photosynthetic area helps to compensate for the reduced amount of sunlight intercepted by grass growing in the shade. A third advantage is that a higher cutting height results in a deeper root system, which allows the grass to compete better with tree roots for nutrients and water. Stopping to raise the mower blade when cutting shaded areas is only a minor inconvenience and has a very beneficial effect on the turf.

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Some studies have shown that the application of plant growth regulators improves shade tolerance. The energy that would have been used for excessive shoot growth is available for chlorophyll production or root growth or can be stored.

Moisture stress may occur because of the dual use of water by trees and grass. Deep watering once or twice a week is usually best. Traffic should be kept off shady sites. The tender, shallow-rooted turf exhibits poor wear resistance and is easily damaged. Fungicide applications may be necessary to protect some grass species and varieties from diseases such as powdery mildew.

Turfgrass that is struggling in dense shade may be saved by trimming all the branches off a tree up to a height of 10 feet (3 meters) or higher. Removing the lower branches will allow a greater amount of sunlight to reach the turf (Figure 18-5). Selective pruning in the crown of a tree will open up the canopy and increase light penetration. A tree that is pruned properly will be healthier and have a more attractive appearance. Branches that are dead, diseased, crossing, or growing close together are logical choices for removal. When many branches must be trimmed to decrease light blockage, all of the limbs should not be cut off immediately. Gradual removal over a period of a few years is preferable to one severe pruning, which could harm the tree. Thinning should be done by someone with arboriculture training.

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When leaves drop to the ground in autumn, prompt removal is essential (Figure 18-6). Leaves smother the grass and exclude light. Turf covered with leaves remains constantly damp, and this encourages disease problems. The amount of light available to the grass increases substantially once a tree loses all of its leaves. If the fallen leaves are collected immediately, these higher light intensities will reach the turf and stimulate a greater rate of photosynthesis. The grass will then be able to build up its food reserves before winter arrives. Mulching mowers may be used to chop up the leaves fine enough that the small pieces can be left in the lawn.

Practicing these cultural techniques enables turfgrass to be more adaptable to shaded conditions. Careful and expert management is necessary to grow good-quality turf beneath trees. On heavily shaded sites, however, even skillful management may not be enough. The decline of properly maintained shade-tolerant grasses indicates that the shade is too dense for turf to survive. When light exclusion is this severe, there is no point in replanting grass unless the trees are cut down. Other alternatives are available. The area under the trees can be covered with materials such as wood chips, bark mulch, bricks, or marble chips. If plants are preferred, a number of vegetative ground covers can grow at very low light intensities (Figure 18-7). These landscape plants can survive on extremely shady sites (Figure 18-8).

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Moss and Algae. Moss and algae, when they invade a turf area, are usually found in the shade. These simple plants are unable to compete successfully with turfgrass if the growing conditions are satisfactory for turfgrass. Their presence is an indication of unfavorable environmental conditions and improper turf maintenance. Shade, poor drainage, overwatering, and soil acidity, infertility, and compaction all contribute to the decline of the turf and favor the development of moss and algae (Figure 18-9).

Any remedy that improves the health and competitiveness of the turfgrass helps to control moss and algae. Correct management of turf in the shade is very important. Low fertility is a common reason for their appearance. Adequate fertilization often solves the problem.

Hand-raking is the simplest method of ridding a turf of moss and algae. They can also be partially controlled with chemicals such as ferrous ammonium sulfate, copper sulfate, or chlorothalonil, but these unwanted plants will return unless the conditions that weakened the grass are corrected.

Moss problems on golf course greens are discussed in Chapter 20.

Soil Compaction

A compacted soil is a soil in which the mineral particles have been pressed close together. Compaction is usually a result of excessive traffic. The particles are forced together because of the mechanical pressure exerted by foot traffic or the tires of machinery and vehicles. Compaction generally occurs in the top 2 to 3 inches (5-7.6 centimeters) of soil.

An easy method of diagnosing compaction is to stick a knife into the soil. It is difficult to push the blade into a compacted soil. A soil sample can be examined. When the surface soil is compacted it looks and feels hard and dense. There are instruments available, called penetrometers, to measure compaction.

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Soil compaction is a serious problem. As the mineral particles are crushed together, porosity decreases. There is a significant decrease in the large pore spaces--the macropores--and this decrease near the soil surface restricts the movement of air, water, fertilizer, lime, and pesticides into the soil. A soil with a good structure may be composed of 40 percent air on a volume basis. The air content of a severely compacted soil may be as little as 5 percent.

The result of this loss of pore space is a substantial decline in turf quality. Roots may be unable to penetrate the soil because the particles are tightly packed in a solid mass. Reduced aeration is also a problem because roots need oxygen to respire and produce energy. The barrier formed by the compacted layer can trap carbon dioxide and other gases in the soil. These gases are released by roots and soil organisms and may become toxic to grass roots as their concentrations increase.

A major problem is poor water infiltration and percolation. The soil turns into mud after rain or an irrigation. Puddles often form on the surface, and the turf becomes unusable. The soil drains slowly and remains extremely wet during periods of rainy weather. When the soil is wet it is more prone to further compaction and the turf is less wear resistant.

Studies have shown that sports injuries increase significantly when athletes play on compacted fields. Athletes are more likely to be hurt when they fall on a hard, compacted surface. Injuries are less common on a softer, noncompacted field that has a thick cover of turfgrass, which serves as a cushion.

Factors Contributing to Compaction

The major reason for soil compaction is intense traffic. Compaction is usually associated with athletic fields, golf greens, and other heavily used recreational turf. Areas at the edges of driveways, highways, and sidewalks are often compacted. The problem occurs on lawns located at industrial parks, shopping centers, schools, colleges, and other complexes where there is a large amount of foot traffic (Figure 18-10). A home lawn seldom becomes seriously compacted unless the neighborhood children use it as a playground.

How many times people walk, run, or drive equipment over a turf has the greatest effect on whether or not compaction develops. Other factors have an important influence as well. For example, severe compaction is much more likely on a fine-textured, clayey soil than on a coarser, sandy soil. The smaller, platelike clay particles can be crushed closer together. Clay soils also hold more water and drain slower than sandy soils. A film of water surrounding soil particles acts as a lubricant. When a wet soil is subjected to mechanical pressure the particles readily slide together.

A dense stand of turf helps to protect the soil from compaction. The leaves, stems, and thatch act like a cushion or shock absorber. Thin or bare spots are prone to compaction because a foot or tire exerts its mechanical pressure directly against the soil. Crumb rubber, a product made from old tires, can be spread on top of the soil. It absorbs pressure and helps to protect the soil.

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Preventing and Alleviating Compaction

The most effective method of preventing compaction is to reduce the amount of traffic on a turf area. Hedges, trees, flower beds, shrubs, fences, and stone walls can be used as barriers to keep people off grass. They also guide the flow of traffic in the desired direction (Figure 18-11). Correct placement of sidewalks and pathways is important. Their location should allow pedestrians to walk directly and quickly to their destinations. If walkways are positioned properly, people will be less tempted to take shortcuts across the grass. The walkways should be wide enough to accommodate groups of people.

Curbing stops drivers from parking on the grass. However, a curb can restrict the access of persons confined to wheelchairs. Signs that remind people to stay off the grass are helpful. Golf cart paths on golf courses are marked with signs to prevent carts from being driven on greens, tees, aprons, and collars. On sports fields and other recreational areas it may be impossible to divert traffic to the extent that no compaction occurs. However, even some traffic reduction may decrease the severity of compaction. For example, the field that is used for football games on Saturdays should not be used at other times. During the week the team should scrimmage on a practice field. The two fields will recover more rapidly if they are not played on at all in the spring.

It is extremely important to keep traffic off turf when the soil is wet. Maintenance operations requiring the user of heavier equipment should be postponed until the soil is drier. The turfgrass manager should not irrigate a sports field immediately before a game.

Any vehicle or machine driven on turf should be equipped with special turf tires (Figure 18-12). These wide tires are designed to reduce compaction. The amount of pressure exerted by an object is determined by dividing its weight by the surface area that is in contact with the soil. As the amount of surface area in contact with the soil increases, the mechanical pressure exerted decreases. Wide turf tires distribute the weight over a larger area than regular tires; therefore, less compaction results.

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Whenever possible, turf areas that receive intense traffic should be established on a sandy soil to minimize compaction potential. A coarse-textured soil that is 80 percent or more sand is recommended for heavy-use sites such as athletic fields, putting greens, tees, or turfed pathways. A 12-inch (0.3-meter) layer is ideal. If this extremely sandy media is used, organic material such as peat moss or compost is incorporated into the top 3 to 4 inches (7.6-10.2 centimeters) to increase the nutrient-holding ability. An irrigation system is required because the sand will drain rapidly. Frequent irrigation may be necessary. Replacing a clayey soil with sand is an excellent solution to the compaction problem, but it is very expensive.

Mechanical cultivation helps to alleviate compaction for up to three or four weeks after it has occurred. Coring or core cultivation is a widely used maintenance practice. It is also referred to as aerating or aerifying. Hollow metal tubes called spoons or open tines are forced into the soil. When they are withdrawn cores or plugs of soil are removed, leaving holes (Figure 18-13). The distance between the holes and their diameter and depth depend on the type of machine used. The holes are spaced 1 to 6 inches (2.5-15 centimeters) apart and are normally 0.25 to 0.75 inch (6.4-19.1 millimeters) in diameter. A machine with 0.50-inch diameter tines making holes 2 inches apart will remove 5 percent of the surface soil in one pass. The deeper the holes, the more effective the core cultivation.

Removing plugs or cores relieves soil compaction. The holes allow better penetration of water, air, fertilizer, lime, and pesticides into the root zone. The result of this increased infiltration is deeper rooted, healthier, more vigorous turfgrass. Toxic gases are able to escape from the soil through the openings made by the core cultivator. Improved surface drainage helps to dry out the soil and prevents the formation of puddles. There are a number of benefits derived from core cultivation.

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One type of cultivator has hollow tines mounted on a crankshaft (Figure 18-14). The tines move up and down vertically and allow deep penetration without damaging or disrupting the turf. These machines are popular for golf greens because they cause minimal disturbance to the putting surface. Vertical motion aerators have a relatively slow operating speed. Though most aerators penetrate only 2 to 4 inches (5.1-10.2 centimeters), some, called deep-tine aerifiers, can make holes 6 to 16 inches (15-41 centimeters) deep. Conventional aerators can create a compacted area, called a cultivation pan, which occurs just beneath the point of their tines' deepest penetration. A deep-tine aerifier can break this up and can also penetrate through different soil layers that may impede water movement and root development.

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Deep-tine aerifiers use either solid or hollow tines. Solid tines penetrate soils that are too hard for hollow tines to enter. The solid tines usually penetrate deeper and are faster than hollow tines. There is also no cleanup because soil is not removed. Solid tines are also called shatter-core tines because the impact of the soil tine shatters or fractures the soil (Figure 18-15). Areas aerated with hollow tines, because soil has been removed, resist compaction better. The benefits of using solid tines are more temporary.

Deep-tine machines are significantly more expensive than conventional aerators. Rather than buying a machine, most turf managers contract a company to do the deep aeration for them.

A second type of core cultivator has open tines or spoons mounted on a drum or metal wheels (Figure 18-16). The drum or wheels turn in a circular motion and force the tines or spoons into the soil. These rotating units core cultivate turf more quickly than the vertical type and are preferred for aeration of larger areas. However, they cause more disruption to the turf surface and do not penetrate as deeply as the vertical motion cultivators. The penetration depth can be increased by placing weights on the aerator.

The plugs must be picked up when a closely mowed playing surface is aerated. These soil cores would interfere with putting and be an eyesore if left on a golf green. Traditionally the plugs are raked up or shoveled off the green. Core harvesters that attach to turf vehicles can be used to pick up the cores (Figure 18-17). Some aerators have the tines mounted on a hollow drum. The plugs are pushed into the drum and collected.

The plugs do not need to be collected on higher cut areas such as lawns or fairways. A metal drag mat or piece of chainlink fence can be pulled over the surface to help break up the soil cores.

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Core cultivation should not be performed when the soil is too dry or wet. A very dry soil is hard and difficult to penetrate. Damage to the turf is more likely if the soil is wet. Opening up the soil can cause root desiccation during hot, dry weather. Irrigation is necessary after core cultivation. Core cultivation is generally avoided in midsummer. The best time to perform this practice on warm season turf is in late spring or early summer. The ideal time on cool season turf is late summer or early fall.

A deep-drill aerifier is another approach. Drill bits bore holes up to 12 inches (30 centimeters) deep. The machine is slow, but there is very little surface disruption. It is primarily used on putting greens. Some of the machines refill the drilled hole with sand.

The newest innovation is high-pressure (5,000 psi) water injection. Jets of water at speeds in excess of 600 mph make holes in the soil 6 to 16 inches (15-41 centimeters) deep (Figure 18-18). The holes are less than 0.125 inch (3.2 millimeters) wide. There is no surface disruption. High-pressure water injection can be performed during the summer.

Spiking and slicing are two other practices that help to relieve compaction. They provide temporary relief and are not as effective as core cultivation. However, they offer two advantages. First, plugs are not extracted, and no cleanup is necessary after the operation. Second, the turf is only minimally disturbed during these operations.

A spiker uses solid tines to punch shallow holes by forcing the soil downward and laterally. These holes are extremely compacted at the bottom and along the sides because the hole is made by crushing the soil instead of removing it. A slicer has thin, V-shaped knives that cut into the soil. Narrow slits 2 to 4 inches (5-10.2 centimeters) deep are sliced in the turf.

Aeration is not a permanent solution to a compaction problem. The soil gradually expands and the holes fill in again. Heavily trafficked areas may require cultivation several times each year. Some sports fields are aerified twice a month. The aerifier can be run across the turf in more than one direction if a soil is severely compacted. This produces a greater number of holes. It is common for a golf course superintendent to use two or three different aerification techniques each year.

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Rolling

Rolling is a maintenance practice used to push grass roots back into contact with the soil. Grass plants may be heaved up as the result of soil freezing and thawing in the winter. The spikes and cleats worn by athletes on sports fields contribute to uprooting. Grass will wilt and desiccate when its roots are removed from the soil. Raised plants are also likely to be damaged by mowers.

The weight of a roller presses down on the turf and forces roots into contact with the soil surface. The roots can then grow and reenter the soil. Water ballast rollers can be filled with water to increase their weight. A 20-gallon (76-liter) roller, for example, will hold approximately 170 pounds (77 kilograms) of water.

Rolling can cause compaction if a heavy roller is used on a wet soil. The turf manager should avoid rolling unless raised, uprooted sod is a problem. A light roller is usually sufficient to press plants into the soil. A soil that is too wet should not be rolled, or compaction will occur.

A roller is a valuable tool for smoothing and firming a seedbed. Tilling loosens up and expands a soil. The soil will eventually settle, but a light rolling will pack it down immediately. People can then walk on the seedbed without sinking in when they are seeding and mulching. Another light rolling after the seed is spread ensures good seed-soil contact, which results in better germination. Newly installed sod is often rolled.

Thatch

Thatch is a layer of partially decomposed and undecomposed plant tissue. It accumulates above the soil surface and is composed of dead and living roots and stems (tillers, rhizomes, and stolons). Thatch is brown in color and contains little or no soil (Figure 18-19). The mat, directly beneath the thatch, is a soil layer with organic matter mixed in it.

Thatch buildup occurs when the production of plant tissue is greater than the decomposition rate. Plant debris is primarily decomposed by soil microorganisms such as fungi, bacteria, and actinomycetes. Earthworms also feed on organic matter and help to break it down. Decomposed organic material is called humus and has many characteristics that are desirable for plant growth. Undecayed organic material, thatch, has many undesirable characteristics.

Lignin is a compound in plant tissue that makes cell walls strong, hard, and rigid. It is resistant to decomposition. Ligneous tissue is decomposed at a very slow rate by microorganisms. The parts of a grass plant that contain the greatest amounts of lignin are the major components of the thatch layer. Roots are highest in lignin content, but stems, rhizomes, stolons, and leaf sheaths also contain substantial quantities. Leaf tissue, which is relatively low in lignin, does not contribute significantly to thatch accumulation because it is decayed rapidly.

A thin layer of thatch is beneficial. It acts as a cushion and reduces sports injuries and compaction. The undecomposed organic matter serves as a mulch and protects the soil surface from drying. It insulates the crown of the plant from sudden temperature changes. Thatch can become quite dry, and this prevents weed seed germination.

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For most turf areas a thatch layer less than 0.5 inch (1.3 centimeters) thick is desirable. Problems begin to develop when the thatch accumulation becomes greater than 0.5 inch. The layer should not exceed 0.3 inch (0.76 centimeter) on a putting green.

Problems Associated with Thatch

A thick thatch provides a good environment for the survival and growth of some insect pests and disease organisms. Lawnmower wheels sink into thatch because it is soft and spongy. When the wheels sink the mower blade is lowered and scalping can occur.

Roots, rhizomes, stolons, and crowns are located in the thatch if the layer is thick. They are more likely to suffer cold, heat, and drought injury because the thatch is not as protective as soil. Shallow rooting is another consequence of heavy thatch accumulation. The roots become confined to the thatch layer. The turf is weakened and more susceptible to drought and other stresses because of the shallow root system.

Thatch is very porous and contains many macropores. It exhibits many of the same characteristics as sand. Water retention is poor because it drains rapidly. The layer is hydrophobic and tends to repel water. Once thatch dries out it is difficult to rewet. Dry thatch can significantly reduce the water infiltration rate. Thatch does not have any capillary activity, so water cannot move back into it from the soil below. However, when the thatch layer becomes wet, it remains wet, and water moves slowly down into the soil. Moisture levels in the thatch are often unsatisfactory. The layer is relatively infertile because of poor nutrient retention.

Thatch impedes the movement of pesticides into the soil. For example, some insecticides are tied up in the thatch. If they are absorbed, even a heavy watering after application may not move them through the layer. Grub control becomes more difficult when the downward movement of insecticides is significantly slowed by a thick layer of thatch.

The opposite problem occurs with preemergent herbicides. These pesticides form a chemical barrier at the soil surface which prevents the seedlings of crabgrass and other annual weeds from emerging. The herbicides kill these weeds by inhibiting cell division in their roots. The chemicals could also destroy the roots of the established, desirable turfgrasses. This does not normally happen because these herbicides are immobile in the soil and remain on the surface; they do not move down to where roots of mature, perennial grasses are located. Nevertheless, preemergent herbicides have a greater mobility in the thatch and will move around. Turf roots that are confined to a thick thatch layer can be injured.

Why Thatch Accumulates

Thatch accumulates when the rate of tissue production is greater than the rate of tissue decomposition. High-maintenance programs that stimulate rapid turfgrass growth can be partially responsible for this imbalance. The production of plant tissue is increased substantially when vigorous cultivars are fertilized and irrigated heavily on a regular basis. Excessive nitrogen fertilization is a major reason for thatch buildup.

Soil microorganisms decompose plant tissue. A large microorganism population is necessary to keep thatch under control because each individual microorganism is so small. These organisms are very prolific and have tremendous reproductive rates. If soil conditions are favorable for the development of these populations, the thatch decomposition rate will be high. When soil conditions are unfavorable, organic matter will accumulate because of the lower populations.

Soil problems that inhibit microbial activity should be corrected. Many of these organisms are pH sensitive. Optimal development occurs at a soil pH in the 6.0 to 7.0 range. Populations drop off sharply when the pH is too acid or alkaline. Low-maintenance programs that do not provide enough lime and fertilizer contribute to thatch buildup. Soils deficient in nitrogen cannot support adequate microbial activity. Moderate fertility is best. There should be enough nitrogen applied to the turf to encourage microorganisms, but not so much that excessive plant growth is stimulated.

Compacted, poorly drained, overly wet soils lack oxygen, which is required by most of these microbes. Moisture is also necessary for microbial activity to occur. Decomposition stops when the thatch and soil are dry. Soil temperature is very important, but it is a factor that the turf manager cannot control. Optimal temperatures for thatch-decomposing microorganisms are in the 95[degrees] to 100[degrees]F (35[degrees]-38[degrees]C) range. The warmer the temperature, the faster the decomposition rate.

Leaf tissue has little effect on thatch because the tissue is soft and easily decomposed, unless the clippings removed by the mower are quite long. Longer pieces of tissue do not break down as rapidly as short clippings. Frequent pesticide use may lead to thatchy turf. Soil insecticides that are lethal to earthworms will stop their decomposing activities. Fungi feed on organic matter and decay thatch. Fungicides used to control fungi that cause turf diseases also reduce populations of beneficial fungi.

Thatchy turf is often produced by turfgrasses that are vigorous growers, develop extensive root systems, and have higher concentrations of lignin in their tissue. Hybrid bermudagrass, St. Augustinegrass, zoysiagrass, bentgrass, and the fine fescues are very prone to thatching.

Controlling Thatch

A vertical mower removes thatch from a turf. It is also called an aeroblade, thatcher, verticutter, or dethatcher. The machine has a series of knives mounted on a horizontal shaft (Figure 18-20). As the shaft rotates at a high speed the blades slice into the thatch and rip it out of the turf (Figure 18-21). The blades are spaced 1 to 3 inches (2.5-7.6 centimeters) apart on the shaft.

Some vertical mowers can penetrate as deep as 2 to 3 inches (5.1-7.6 centimeters). The machines can be adjusted to cut into the turf to various depths. The proper depth setting depends on the thickness of the thatch. The knives should penetrate to the bottom of the thatch layer, and some turf specialists recommend slicing into the surface soil beneath. Decomposition may be encouraged if the blades pull soil up into the thatch. Vertical mowing (verticutting) is beneficial because new growth is stimulated when the stolons and rhizomes are severed. This can result in increased turf density and is the major reason why vertical mowing is performed on putting greens.

The frequency of dethatching operations is determined by the rate of thatch accumulation. When the thatch layer becomes thicker than 0.3 inch (7.6 millimeters) on greens, vertical mowing should be considered. Managers should periodically cut into the turf with a knife and check the thatch depth. Turf areas that are slow to build up thatch may require verticutting once every several years, or never at all. Areas that are prone to thatching may need to be dethatched once or even several times per year. The organic material that is removed should be raked up or vacuumed.

The best time to vertical mow warm season grasses is in the late spring or early summer. Late summer or early fall is preferred for cool season grasses. The spinning knife blades injure the plants as they rip through the turf. This is less likely to be a problem if the grass experiences good growing weather after the operation. To recover quickly, turfgrass needs at least a few weeks of favorable growing conditions after it is verticut. Turfgrass injury is more extensive if the soil and thatch are too moist when vertical mowing is performed.

[FIGURE 18-20 OMITTED]

[FIGURE 18-21 OMITTED]

There is some question about the effectiveness of vertical mowing as a thatch reduction technique. Relatively small percentages of the total amount of thatch are removed after vertical mowing in just one direction. Slicing the area in several directions to remove greater quantities of thatch can result in serious turfgrass injury. This is especially true if the grass is shallow rooted. Some studies have shown an overall decrease in turf quality after vertical mowing. The best approach is to try to limit the development of the thatch layer so thatch removal does not become necessary.

When the thatch layer is allowed to become excessively thick it may be uncontrollable. The only solution in an extreme situation is to remove the grass and thatch with a sod cutter. Soil must then be added to level the area before it is reestablished. The turf manager can avoid this drastic solution by keeping the thatch under control.

Many greensmowers have interchangeable vertical mower units. A greensmower can be converted to a vertical mower in half an hour by installing these units in place of the grass-cutting blades. The grass catchers can be used to collect the thatch. Another piece of equipment that removes thatch is a power rake. The flexible wire tines, similar to those on a leaf rake, are able to pull small amounts of thatch from the surface of the turf. A power rake is ineffective if the thatch layer is deeper than 0.5 inch (1.3 centimeters).

Topdressing is a very effective method of reducing any negative effects of thatch (Figure 18-22). A thin layer of soil is spread over a turf area and then dragged or brushed into the thatch. Adding topdressing to the thatch layer is beneficial because the soil particles dilute the thatch and modify its adverse effects. It also creates a favorable environment for the development of microorganisms by improving water retention and other conditions. Topdressing results in increased microbial activity and increased thatch decomposition.

Topdressing is a more effective method of thatch control than vertical mowing. However, topdressing is expensive, and its use is limited primarily to golf courses and sports fields (Figure 18-23). The soil is applied with a topdresser. This machine can be calibrated to apply topdressing material at various thicknesses. A metal drag mat or large brush can be attached to the rear of the topdresser to work the soil into the thatch layer. The brush attachment on a greensmower can also be used to accomplish this. Another method is to brush the material in by hand with brooms. If the soil is dry, it can be applied with a fertilizer spreader.

[FIGURE 18-22 OMITTED]

[FIGURE 18-23 OMITTED]

The topdressing material should be identical to the topsoil beneath the thatch if the topsoil has favorable characteristics. The material commonly used contains 80 percent or more sand because most greens have a very sandy topsoil. Topdressing with pure sand has become popular because it results in a superior putting surface. The sand should be fine enough to be easily worked into the thatch, but not so fine that it can seal the surface and impede air and water movement.

The depth of the topdressing layer depends on the frequency of topdressing and the thickness of the thatch. A typical program for greens is to spread 0.03 to 0.06 inch (0.76-1.5 millimeters) of soil each application if the green is topdressed twelve or more times a season. When topdressing is performed three or four times annually, the rate is increased to 0.13 to 0.25 inch (3.2-6.4 millimeters) (Table 18-2). It is more effective to topdress at least eight times a season, but frequency may be limited by budget and labor restrictions.

Where intensive topdressing programs are used, the thatch is covered with topdressing material every two to three weeks.

The golf course superintendent should not topdress more frequently than necessary. If the thatch layer is completely diluted, the surface of the green becomes too firm. This hardness causes golf balls to bounce too much and not hold on the green.

Topdressing can be performed whenever the grass is actively growing. It often follows core cultivation. The sandy topdressing media will fill in the aeration holes if it is worked into the turf. These pockets of sand will increase air and water infiltration. This can be particularly helpful on native-soil greens, where drainage and aeration are inadequate. The plugs of undesirable soil should be removed from the surface of the green before topdressing. After a few years of incorporating sand into cultivation holes the characteristics of the growing media may improve significantly. However, if the topsoil has an undesirable texture, the best solution is to remove all of it and replace it with a sand that is appropriate for greens. This, of course, is expensive.

Selecting a topdressing material requires careful consideration. Choosing a topdressing source that is not physically compatible with the present media can lead to disaster. For example, placing a fine-textured layer over coarser particles will create a perched water table at the surface of the green. Reduced water movement and other serious problems will result.

Some people prefer straight sand topdressing; others like to add peat or a small amount of soil to the sand. There are also great differences between the size and shape of sand particles. The turf manager must send samples of prospective topdressing materials to a soil testing laboratory to determine which one is most compatible with the growing media already present.

There is another benefit from topdressing besides thatch decomposition. When a green is matted after topdressing, the sand or soil is not spread equally over it. A greater amount of topdressing tends to be pulled into low spots, which reduces surface irregularities and levels the green. A smooth surface enables a golfer to putt accurately--the ball runs truer. Topdressing also occurs on athletic fields for leveling purposes. As much as 1 to 2 inches (2.5-5.1 centimeters) of soil or sand is spread and then drag-matted to smooth the surface. Large topdressing machines are manufactured for sports field use.

Core cultivation has a topdressing effect if the plugs are broken up with a mat or verticutter and the soil is mixed into the thatch layer. Working the soil particles from the cores into the thatch is a more effective method of thatch reduction than vertical mowing. One type of core cultivator grinds the plugs into smaller particles and spreads this soil as it aerates. Some thatch is also removed when the plugs are extracted. This amount is significant when the holes are large and spaced close together. The percentage of thatch removed increases even more if the site is core cultivated in several directions. Core cultivation also produces soil conditions that are more favorable for thatch-decomposing organisms.

Lime will stimulate thatch decomposition if the pH of the thatch is too acid for microorganisms. Normally lime is applied to a soil only every few years. However, frequent, light lime applications may be most helpful because calcium and magnesium move through the thatch layer quickly. The pH of topdressing material should be in the 6.0 to 7.0 range.

Research has shown that the use of wetting agents can improve water infiltration and movement in thatchy soils.

SELF-EVALUATION

1. A common turfgrass disease in the shade is --.

2. Some turfgrass species and varieties can adapt to -- shade.

3. As a general rule, the mower blade should be raised -- inch(es) when cutting grass growing in shaded areas.

4. -- or -- are often found growing on wet, shady sites.

5. Soil compaction is the result of intense --.

6. When soil is compacted there is a reduction in the number of --.

7. The machine that removes plugs of soil is called a --.

8. A -- is used to press uprooted grass plants back into contact with the soil.

9. A layer of undecomposed or partially decomposed organic matter is called --.

10. The compound in plant tissue that is very resistant to decay is --.

11. Microorganisms and -- help to decompose plant debris in the soil.

12. Optimal soil pH range for thatch decomposition is --.

13. Perhaps the major reason for thatch accumulation is overapplication of --.

14. The knife blades on a -- rip thatch out of the turf.

15. Spreading a thin layer of soil on a turf area is called --.

16. Discuss why thatch accumulates.

17. What techniques can be used to improve the quality of turf growing in the shade?

18. How would you construct an athletic complex to minimize compaction problems?
Table 18-1 Maximum Annual Nitrogen Requirements of
Turfgrass Growing in Shade

                            NITROGEN PER
SPECIES                 1,000 [FT.sup.2](LB)

St. Augustinegrass              2-3
Centipedegrass                  1-2
Zoysiagrass                   2.5-3.5
Tall fescue                     2-3
Fine fescue                   1.0-2.0
Kentucky bluegrass              2-3
Poa trivialis                   2-3
Annual bluegrass              2.5-3.5
Bentgrass                     2.5-3.5

Table 18-2 Soil Volumes Required to Topdress 1,000
[Ft.sup.2] (93 [M.sup.2]) to Various Depths

         DEPTH                     SOIL VOLUME

     IN         MM    [FT.sup.3]   [YD.sup.3]   [M.sup.3]

1/32 (0.03)     0.8       2.6          0.1          0.07
1/16 (0.06)     1.6       5.2          0.2          0.15
1/8 (0.125)     3.2       10.4         0.4          0.3
1/4 (0.25)      6.4       21           0.8          0.6
1/2 (0.5)      12.7       42           1.5          1.2

Figure 18-7
Examples of vegetative ground covers that are
adapted to the shade.

COMMON NAME                      SCIENTIFIC NAME

Bugleweed or carpet bugle        Ajuga reptans
Wild ginger                      Asarum spp.
Leather crassifolia              Bergenia crassifolia
Lily-of-the-valley               Convallaria majalis
Epimedium or bishop's hat        Epimedium macrothum
Wintercreeper                    Euonymus fortunei
English ivy                      Hedera helix
Plantain lily                    Hosta decorata
Creeping lily                    Liriope spicata
Moneywort or creeping jenny      Lysimachia nummularia
Creeping mahonia                 Mahonia repens
Pachysandra or Japanese spurge   Pachysandra terminalis
Star jasmine                     Trachelospermum jasminoides
Periwinkle or creeping myrtle    Vinca minor
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Author:Emmons, Robert D.
Publication:Turfgrass Science and Management, 4th ed.
Date:Jan 1, 2008
Words:8031
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