Lawn Care/Maintenance/Cultivation

Mechanical Cultivation in turf modifies the physical characteristics of the soil or turf. Most cultivation practices are designed to either reduce excessive thatch or alleviate soil compaction.

Thatch

Soil Compaction

Aeration

Rolling

Types of Cultivation

Thatch

Close-up of Thatch Mat Source: Texas A&M Turfgrass Program

Many turfgrasses accumulate thatch at the soil surface. Thatch is a layer of living and dead stems, leaves, and roots of grasses that can develop between the actively growing vegetation and the soil surface. An excessive thatch layer increases the disease susceptibility of the turf, reduces tolerance to drought, cold, and heat, and minimizes the movement of air, water, fertilizers, and some pesticides through the turf into the soil. Thatch also decreases the turf's capacity for growth, resulting in an all-round deterioration of lawn quality. Some buildup of thatch is desirable, however, because it provides a source of nutrients from the breakdown of organic matter. The buildup of thatch is directly related to the rate at which dead plant materials accumulate. If the production of dead material exceeds the ability of microorganisms to break down the material, thatch accumulates. Therefore, vigorously growing grasses, heavy fertilization, excessive soil acidity (which inhibits bacterial activity), poor soil aeration and drainage, cold soils, and some pesticides can all contribute to thatch buildup.

The depth of thatch can be determined by cutting a wedge of turf and examining the profile of green vegetation, thatch, and soil. If the compressed layer of dead materials at the soil surface is greater than 1/2 in. in thickness, the thatch should be controlled.

Mounds of Havested Thatch Source: Texas A&M Turfgrass Program

Two basic methods are used to control excessive thatch: (1) physical removal of the thatch by various types of machinery, and (2) modification of the environment to encourage more rapid bacterial activity, reduce the growth rate of the turfgrass, or both.

Physical removal: This is typically done with a rigid vertical mower or with a flexible power rake, used quite often on home lawns. Vertical mowing is the use of blades, rotated in a vertical plane, that penetrate the turf and bring organic matter and soil up to the surface. Since the physical removal of thatch can be destructive to living turf, removal should occur only during peak grass-growing periods. Deeper vertical mowing (or verticutting) is used on lawns to physically remove the accumulated thatch. This can be destructive to shallow-rooted turfgrasses, particularly during periods of stress, and can damage the site. The best time for vertical mowing is just prior to the peak growth periods.

Vertical Cutter Blades Source: Texas A&M Turfgrass Program

Bacterial activity: Modifying the turf environment to remove the thatch layer slowly is generally more effective for long-term control than physical removal. An accumulation of thatch tends to decrease the pH. Since acidic conditions minimizes the growth and reproduction of bacteria and other microorganisms, the addition of lime to raise the pH within the thatch promotes more rapid degradation. Thatch restricts the movement of air, water, and fertilizer into the soil surface which decreases the ability of microorganisms to multiply. Core aerification can increase microorganism activity in the top layer of soil by providing channels through the thatch for the air and water to reach the soil. Core aerification is a process of removing small cylinders of soil and turf in a uniform pattern through the turf. Several different types of equipment can be purchased or rented to remove cores over large areas in a relatively short time. The majority of the machines deposit the cylinder of turf and soil onto the surface, where they are then removed or broken up and left to disintegrate, allowing the soil to sift down into the thatch layer.

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Soil Compaction

Compacted soils due to foot and/or vehicle traffic have poor aeration and drainage, poor surface water infiltration, and shallow root growth. Turfgrasses in compacted soils grow slowly if at all and lack vigor. Soil compaction is more prominent on fine-textured soils subjected to concentrated foot or vehicle traffic. First, prevent future compaction due to foot and/or vehicle traffic by constructing walks or fences, or by planting shrubs. Because soil is more conducive to compaction when it is very moist or wet, it is crucial to minimize traffic on a turf after a heavy rainfall or irrigation. Moderately moist or dry soils are less prone to compaction than are wetter, heavier soils.

The effects of soil compaction can be reduced with machines that create openings extending into the underlying soil. Mechanical cultivation should be carried out only during periods of rapid turf growth. The two types of mechanical cultivators to alleviate compaction are core aerifiers and spikers or slicers. Core aerification machines remove small cores of soil, depositing them on the surface of the turf or collecting them for removal from the site. Spikers and slicers employ solid spines or knives, respectively, to create narrow openings in the turf. These machines are not as effective as core aerifiers, but are generally less disruptive to turf growth.

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Aeration

Aerifier with Hollow Tines Source: Texas A&M Turfgrass Program

Aerification is most beneficial when the largest tines available are used, penetration is 2 to 3 inches deep, and when 20 to 40 holes are punched per square foot. Aerifiers with reciprocating arms are the most effective, whereas the aerifiers that roll behind garden tractors are less effective because they do not penetrate deep enough nor punch enough holes per square foot to reach 20 to 40 holes/ft². Many units marketed to the homeowner are designed to slice or spike into the lawn, but remove no soil and are not considered aerification.

Close-up of Aeration Hole Source: Texas A&M Turfgrass Program

Aerification of Kentucky bluegrass, perennial ryegrass, fine fescue, and tall fescue should be done in the fall or spring when the turf is actively growing. Zoysiagrass should be aerified in early-to-mid summer. Core aerification is the practice of removing small cores of turf and soil with a hollow-tine. Tines generally come in sizes of 1/4"-3/4" and penetrate the soil in proportion to the bulk density and moisture content of the soil. While core aerification does alleviate soil compaction in the area where the cores are removed, it also adds a certain degree of compactor to both sides of the hole and to the spot underneath the hole.

The benefits of coring include:

1. Improved wetting of dry soils
2. Accelerated drying of persistently wet soils
3. Control of thatch (especially when cores are reincorporated into the lawn)
4. Improved turf response to fertilizer.

The disadvantages can include:

1. Temporary disruption of the turf surface
2. Increased potential for turf desiccation since the opening allows more moisture to evaporate.
3. Increased damage from insects that reside in the holes.

Pull-behind Aerification Unit Source: Texas A&M Turfgrass Program

Coring is best done during periods of the year when the turf is vigorously growing. Irrigating after coring can help to reduce any soil drying. The infiltration capacity of the soil should improve after aerification as a result of the increase in surface area following aerification. Coring can increase the surface area up to 2.5 times.

Fertilizers, particularly those that are relatively immobile such as lime and phosphorus, are more effective if applied after coring because they can move farther down into the soil profile.

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Rolling

Rolling with a weighted roller can damage existing soil by compacting it, so on all but very sandy soils, rolling should be used only during seedbed preparation.

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