Lawn Care Cultivator

Saying “Hasta la Vista” to Unwanted Turf Terrors
By Dr. Van Cline

Landscape professionals know one of the most important ways to keep a lawn looking its best is through proper and regular mowing. But there’s much more that goes into creating -- and maintaining -- perfect turf. Cultivating can help cure a lawn’s suffering from some of the most common maladies, including rampant weed problems, heavy thatch, acute soil compaction and critical nutrient deficiencies

What is dethatching and why do it?

Thatch is a dense layer of under-composed 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. 

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, and dethatching in spring opens the canopy for annual weed germination that usually results in more significant weed populations.

The easiest way to remove thatch from a lawn that is more than 3,000 sq ft is with a power rake or vertical mower. For smaller lawns or lawns with thin, 1/2- to 1-in. 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 in. into the soil and 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.

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. It also encourages more vigorous and healthy rooting in general. Core aerification is an effective method of controlling thatch when combined with other prudent turf practices such as the judicious use of fertilizers and water.

Usually done once in late summer or early fall, it’s a good idea to 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. 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 make sure to keep the lawn watered during the period of healing.

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 – 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, strapping a sprayer to your ATV or UTV to spread the appropriate insecticide is the best solution.

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’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 Creeping Charlie, 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, which increases their effectiveness.

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 pre-emergent 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.

FERTILIZING FACTS

How often should a lawn be fertilized?

Frequency of fertilization depends on several factors, including turf species, soil conditions, intensity of management, intensity of lawn use and type of fertilizer. Some species, such as the fine fescues, require less fertilization. Sandy soils require more than naturally fertile loam or clay soils. Higher intensities of management and use require more fertilization for healthy growth. Slow-release fertilizers can be applied in fewer, but heavier, applications than soluble or quick-release fertilizers. However, as a general rule of thumb, most lawns should be fertilized two to three times a year. 

Sixteen essential nutrients are required for healthy plant growth. Most of these nutrients are supplied naturally through the soil system. Lawn fertilizers typically supply the three nutrients consumed in the largest amounts: nitrogen, phosphorus and potassium. Nitrogen is the single most important nutrient, and as a result, fertilizer applications are usually referred to in terms of pounds of nitrogen applied per 1,000 sq ft of turf. Most turfgrasses require 1/2 to 1 lb of nitrogen per 1,000 sq ft of lawn per month of active growth. 2 to 4 lbs of nitrogen in Northern or Midwestern climates is sufficient for an irrigated lawn. Southern climates with longer growing seasons will need more.

Fertilizer applications on cool season lawns should be made in the spring and fall during active growth and avoided during the heat of summer, when turf is stressed.  One scenario is to apply 1/3 of the total seasonal requirement in the spring in one application of a high-quality, slow-release fertilizer and the remaining 2/3 in the late summer and fall in two applications. Late fall applications should include more soluble or quick-release nitrogen. Warm season grasses should be fertilized more in the mid-summer period, when the plants are most active.

Fertilizers should be applied responsibly, since they can be a significant source of pollutants in surface and ground water. Phosphorus-containing fertilizers actually have been banned in some states and municipalities, except for the establishment of new turf or in phosphorus-deficient soils. Label instructions for fertilizer applications should always be followed. Applying more than the suggested amount does not necessarily produce a healthier lawn.

As a matter of fact, over-fertilizing can reduce the overall health and vigor of a turf and pollute the environment at the same time. Fertilizers should never be thrown onto hard surfaces, where they can end up in storm sewers. Over watering is detrimental, as well, because it flushes nutrients out of the soil system and into surface or ground waters.

Collecting and disposing of grass clippings also is a potential source of pollutants and is not necessary to maintain a healthy lawn. In fact, collecting clippings removes up to 30 percent of the nutrients that you apply as fertilizer. The best advice is to apply only what you need to maintain your desired level of quality and to do it responsibly.

What do the three numbers represent on a bag of fertilizer?

Most lawn fertilizers contain a combination of nitrogen (N), phosphorus (P) and potassium (K) from different chemical sources. The three numbers on a fertilizer bag represent the concentration of each nutrient in that order (N-P-K) as a percent. For example, a 50-lb bag of fertilizer labeled as 28-3-3 contains 28 percent nitrogen (or 14 lbs of the 50 total), 3 percent phosphorus (1.5 lbs of the total) and 3 percent potassium (1.5 lbs). If you know the square footage of a lawn, you easily can calculate the amount of a specific fertilizer product that you need to apply based on the desired application rate (guidelines above), the amount of nitrogen in the bag and the area of the lawn.

What’s the best way to apply fertilizer?

In general…

• Use only the amount of fertilizer called for based on turf type and a lawn’s square footage.
• Use a rotary spreader, which applies fertilizer more evenly, allows for a quicker application and helps avoid a striped fertilizer pattern in the grass.
• Spread the fertilizer in at least two directions for each application (i.e. half the amount in one direction and half in the opposing direction).
• Immediately water the turf well after fertilizing.
• Sweep up any fertilizer spilled on paved areas and save for later use.
• Use slow-release fertilizers whenever possible, since they last longer and don’t have to be applied as frequently as quick-release varieties.

Is there such a thing as over-fertilizing a lawn?

A healthy lawn is supported by a healthy, deep root system. Applying too much nitrogen fertilizer stimulates excessive leaf growth at the expense of root growth. Since a grass plant has a limited amount of energy and resources to sustain itself, too much available nitrogen causes it to partition or shunt more energy to shoot growth which robs the roots of resources. The result can be an inadequate root system that, over time, results in a less vigorous and lower-quality turf. Excessive fertilization also causes what’s called soft or succulent leaf tissue, which is more susceptible to fungal diseases and is less tolerant of traffic and wear.

Dr. Van Cline is an agronomist for The Toro Co. and manager of turf research in Toro’s Center for Advanced Turf Technology, which studies ways to improve turf health through better care. Prior to joining Toro 18 years ago, he was a landscape architecture faculty member at the University of Minnesota. Along with a bachelor of science degree in forestry and a master’s degree in landscape architecture, he holds a Ph.D. in horticultural science, with a specialty in turfgrass science.