Straight Talk on Turf

Part 2 of 3

Curbing Crabgrass?EUR??,,????'???Plus Other Cultivating Tips

By Dr. Van Cline, Center for Advanced Turf Technology, The Toro Co.

Core aerification ?EUR??,,????'??? the process of removing plugs of soil from turf ?EUR??,,????'??? is an effective method of controlling thatch when combined with judicious fertilizing and watering.

Landscape professionals know one of the most important ways to keep a lawn looking its best is through proper and regular mowing. But there?EUR??,,????'???s much more that goes into creating?EUR??,,????'??+and maintaining?EUR??,,????'??+perfect turf. The following cultivating tips can help cure lawns suffering from some of the most common maladies, including rampant weed problems, heavy thatch and acute soil compaction.

What is dethatching, and why do it?

Thatch is a dense layer of undecomposed organic material made up mostly of grass roots and stems that accumulates between the green canopy and the surface of the soil. When it becomes too thick, new grass roots will grow within the thatch instead of reaching deeper into the soil, resulting in a turf that is less resistant to stress.

Thatch can become hydrophobic, which impedes water infiltration at the surface, reducing the effectiveness of irrigation. Thatch also can tie up nutrients applied as fertilizers and can provide an incubation zone for turf diseases. Contrary to popular belief, grass clippings do not contribute significantly to thatch, so collecting clippings after mowing does not prevent thatch build-up. Over fertilizing, over watering and compacted soils are the leading causes of thatch. As a result, thatch is usually an indication of other turf problems that should be addressed.

Aerators work best when the soil is moist and cores are removed intact; the cores can be left on the lawn and gradually will disintegrate over time.

The best time to dethatch is late summer or early fall, when a lawn is thriving and when annual weed seed germination has stopped. Dethatching should not be done during the stressful heat of summer, when cool season grasses are stressed. Dethatching in spring opens the canopy for annual weed germination, which usually results in more significant weed populations. The easiest way to remove thatch from a lawn that is over 3,000 square feet is with a power rake, or vertical mower. For smaller lawns or lawns with thin, 1/2- to 1-inch layers of thatch, a manual thatching rake will do a satisfactory job.

When using a vertical mower to remove average amounts of thatch, set the blades to cut 1/8 to 1/4 inch into the soil. Make several test passes on an inconspicuous area of the lawn to judge how much thatch (and turf) will be removed. If too much or too little is removed, raise or lower the blades accordingly.

Over-fertilizing, over-watering, and compacted soils are the leading causes of thatch?EUR??,,????'???a dense layer of undecomposed organic material between the green canopy of grass and the surface of the soil. The old belief that lawn clippings contribute to thatch is inaccurate; clippings actually decompose quickly under normal conditions.

How often does a lawn need to be aerated?

Core aerification is the process of removing plugs of soil from the turf, which relieves compaction, increases oxygen and water penetration into the root zone, and encourages more vigorous and healthy rooting in general.

When thatch becomes too thick, new grass roots will grow within the thatch instead of reaching deeper into the soil, resulting in turf that is less resistant to stress. Dethatching in late summer or early fall is recommended.

Core aerification also is an effective method of controlling thatch when combined with other prudent turf practices, such as the judicious use of fertilizers and water. Core aerification done once in late summer or early fall is usually adequate.

Avoid aerifying cool season grasses during the stressful summer months when it is difficult for turf to recover. Aerifying in the spring usually results in an increase in annual weed populations for the same reasons as those cited for dethatching. Aerators work best when the soil is moist and cores are removed intact.

Contrary to popular belief, grass clippings do not contribute significantly to thatch, so collecting clippings after mowing does not prevent thatch build-up.

Cores can be left on the lawn and will gradually disintegrate over time. Turf will be more susceptible to drying following aerification due to the holes that extend into the root zone, so keep the lawn watered during the period of healing.

Grubs feed on the roots of grass plants, which results in dying patches of turf. A secondary problem often results when birds and rodents forage for the grubs, causing even greater damage to the turf.

What happens if leaves are left on a lawn and not picked up?

Leaves can damage a lawn if not picked up or mulched. A dense layer of leaves left on the lawn in the fall will deprive the turf of light and will restrict air movement, keeping the surface wet and resulting in the growth of various fungi. The best bet is to continue mowing the lawn even after it has stopped growing in the fall?EUR??,,????'???just so you can mulch or bag accumulating leaves.

How do I rid a lawn of grubs?

Grubs are larval forms of several different species of beetles. The adult beetle lays eggs in turf, which hatch and develop into white worm-like larvae just below the soil surface. These grubs eat the roots of grass plants, which results in patches of dying turf.

A secondary problem often results when birds and rodents forage for the grubs, causing even greater damage to the turf. If you suspect grubs as the cause of dead or dying turf, look for them by pulling up clumps of the damaged grass to expose the larvae just below the surface. If you find them, treatment by spraying with an appropriate insecticide is the best solution.

Creeping Charlie, or ground ivy, is an aggressive perennial weed that can be treated with herbicides at any time of the year. Because of its persistent nature, repeated herbicide applications may be required.

Insecticides can target either the adult beetle or the larval grubs. Since insecticides are potent chemicals that should not be used unless they are necessary, it?EUR??,,????'???s best to first consult with your local university extension service or a reliable lawn and garden center that can verify the problem and prescribe a treatment.

What is the most effective way to kill ground ivy?

Ground ivy, or ?EUR??,,????'??creeping charlie,?EUR??,,????'?? is a persistent and aggressive perennial weed that is difficult to control. Because it is a creeper, it can cover large areas, quickly choking out turf in the process. Ground ivy can be controlled with several common broadleaf herbicides, although repeated treatments may be necessary for complete control. Make sure ground ivy is listed on the label when selecting an herbicide. Ground ivy has an interestingly low tolerance for the element boron, which is a micronutrient for plants. Research has shown that detergents containing sodium borate, such as Twenty Mule Team Borax, mixed at a concentration of about one tablespoon per gallon of water and applied as a spray also will control ground ivy. Because of its persistence, repeated applications again may be required.

Perennial weeds such as ground ivy or dandelions can be treated with herbicides at any time of the growing season, although fall applications often are the most effective. During the late summer and fall, these weeds are storing energy for the upcoming winter and following spring. During this fall growth period, perennial plants readily move carbohydrates produced in the leaves to the stems and roots. Herbicides applied in the fall are moved to the roots more rapidly, as well, which increases their effectiveness.

Because crabgrass is an annual weed, it reappears every year only through the germination of new seed. If this seeding cycle can be broken for several consecutive years, the crabgrass population will disappear.

What should I do if a lawn is overrun with crabgrass?

Crabgrass is a warm season annual weed that hits its stride in lawns in the heat of summer, when cool season grasses are suffering. Until recently, the only effective controls were the use of preemergent herbicides in the early spring or hand-weeding. There now are post-emergent herbicides selective for crabgrass that increase the opportunity for conquering this pesky lawn invader. Because crabgrass is an annual weed, it reappears every year only through the germination of new seed. As a result, if this cycle can be broken for several consecutive years, the population of crabgrass in a lawn will disappear. Of course, the best strategy for controlling most weeds in a lawn is to maintain the healthiest and most dense turf possible through sound fertilization, watering and mowing practices, including cutting at higher heights to shade out sun- loving annual weeds. Healthy turf will naturally beat out most weeds.

Until recently, the only effective controls for crabgrass were spring applications of pre-emergent herbicides or hand weeding. Now, there are post-emergent herbicides selective for crabgrass that increase the chance of conquering this invasive weed that hits its stride in late summer.