Controlling Weeds In Turf Grass

In the turf business, spring brings a new growing season and a concern with weed control. Now is an excellent time to consider a weed control program and decide which approach is the most economical and effective for the conditions and results desired. This article will discuss weed control products and strategies and provide some new ideas and approaches for turfgrass weed control.

HOW IT WORKS. In order to use herbicides properly, one must understand the principles that control their effective use. For preemergence herbicides, the principles surrounding their use are relatively simple. First, the herbicide must be applied prior to weed seed germination. Second, the herbicide must be applied at the correct rate and uniformly so that the herbicide forms a barrier to weed seed establishment.

The only factor determining the effectiveness of a preemergence herbicide is the concentration of the herbicide around the germinating seedling. If that concentration is sufficient to kill that seedling, then death of the weed will occur. However, if improper overlap or poor distribution of granules containing the herbicide results in an insufficient concentration of the herbicide around the weed seedling, then that seedling is more likely to establish and require postemergence treatment for control.

The controlling factor in the effectiveness of a preemergence herbicide is the herbicide’s concentration at the soil surface. Many manufacturers and formulators have gone to lower active ingredient loadings on their granular products. By reducing the loading from 2.5 percent G to a 0.5 percent G, the producer has increased the amount of product applied by five times. If the particle size stays the same, this would increase the number of granules applied by five times as well, dramatically increasing the likelihood of uniform application.

Postemergence applications are more complicated because the herbicide must not only reach the target, but must be absorbed by the target weed in sufficient concentrations to achieve control. Therefore, a variety of factors may influence the effectiveness of postemergence herbicide applications.

For effective postemergence weed control, three principles determine the effectiveness of the applications. First, younger weeds are easier to control than mature weeds. Second, the weeds must be actively growing at the time the herbicide is applied. It is always easier to control weeds when they are small than when they have matured with tougher leaves and storage organs from which they can regenerate new growth. Third, the herbicide must reach the site of action in the plant in sufficient quantity to achieve control.

This third principle is the most important because the application of the herbicide is only the first step in achieving satisfactory results. The herbicide must then reach the active site(s) within the plant where it can exert its effect.

If the plant is drought stressed, the herbicide will have difficulty reaching the active site for two reasons. First, the cuticle on the leaf surface, a waxy barrier, thickens to pre-vent moisture loss and is difficult for herbicides to penetrate. Secondly, during drought periods, plant growth slows and even stops. If a plant is not growing, it is not producing food and more importantly is not translocating that food nor the herbicide through the plant.

How do you make sure the herbicide reaches the active site? First, make sure the weeds are actively growing at the time of application. Second, always follow the label directions for the rate of application, paying special attention to any recommendations on tank mixing and spray additives. Third, apply the material properly and uniformly. For best postemergence weed control, the spray volume should be optimized to thoroughly coat the leaf surface of the weed without excessive run-off.

PREEMERGENCE WEED CONTROL. One of the rites of spring for homeowners and professional turf managers alike, is the application of a preemergence annual grass herbicide. Target weeds for this application are crabgrass – the principle annual grass problem – goosegrass in the transition zone, and, to a lesser extent, foxtails and barnyardgrass. These herbicides target all grass weed seeds, not just annuals. However, many perennial grass weeds resume growth from vegetative plant parts and are unaffected by a preemergence herbicide application. While preemergence herbicides will control germinating quackgrass and annual bluegrass, they have no impact on the established plants of these species.

Most preemergence herbicides are broad-spectrum products that control many different weed problems. Preemergence herbicides currently labeled for use in turf are shown in Table 1.

Many of the herbicides used for controlling annual grasses work by inhibiting a phase of cell division (mitosis) in developing root tips. These herbicides include benefin, trifluralin, dithiopyr, oryzalin, pendimethalin, and prodiamine. Other products, such as oxadia-zon, siduron, atrazine and simazine, function by inhibiting photosynthesis.

A characteristic of many, but not all preemergence herbicides, is their very low water solubility and high soil absorption, as measured by the Koc values in Table 1. It is nearly a requirement of preemergence herbicides that they have these general properties or they will not be very effective herbicides. Because of their low water solubility and high soil absorption, these products tend to stick to the soil/thatch surface and not move downward with flowing water. This keeps herbicides in place to intercept germinating weed seedlings.

These products rarely leach and loss of activity generally only results from microbial degradation. Periods of heavy rains following application don’t result in leaching, but often may stimulate microbial degradation instead.

WHAT TO USE? Selection of a preemergence herbicide for annual grass control should involve several factors in the decision. Most important is performance. Performance includes length of control and potential for turfgrass injury. Length of control is the most important performance factor because many of these herbicides can be viewed as working on an “all or nothing” basis.

While this is overly simplified, if the product is applied uniformly and at the correct rate, control should be complete, i.e. 100 percent, until enough dissipation occurs to allow some germinating seedlings to develop. Length of control is governed by rate of application and rate of dissipation from the soil. Consult your local cooperative extension specialist or state turfgrass specialist for data on herbicide performance in your area.

Cost is also a very important factor in determining what herbicide to use. Cost should be computed as dollars/1,000 square feet. Any other comparison is not accurate. Cost per bag or other unit pricing is not a valid method to compare prices. Other factors that may be important are ease of handling and other weeds a product controls.

Note in Table 1, there are many preemergence grass herbicides but only one preemergence broadleaf herbicide. Isoxaben is a unique chemistry that provides preemergence control of a wide range of broadleaf weeds. The biggest advantage offered by isoxaben is its preventive action. Weeds never become part of the landscape because they are killed at germination.

POSTEMERGENCE OPTIONS. Postemergence herbicides are used to control broadleaves and grasses after they have emerged in the turf. These herbicides are more difficult to use and control varies depending upon environmental conditions.

When is the best time of the season to control broadleaf weeds? The answer depends on the weeds to be controlled. If there are a significant number of summer annual broadleaf weeds, then spring is the best time to control them based upon the principles mentioned. “Spring” is defined as the time when summer annual weeds are actively growing and young.

However, if a preemergence grass herbicide is used, choose one that will also control some summer annual broadleaves.The preemergence grass application will do double duty and pick up many of the common summer annual weed problems such as spurge and oxalis.

The best time to control the common perennial broadleaves often found in turf, such as dandelion, white clover and plantains, is immediately after germination when weeds are young and actively growing. However, there are situations where these weeds are mature perennial plants. The best time to control these weeds is in the fall. Why? Fall is the time when many perennial weeds are storing food reserves for the winter and early spring growth. The storage organ is usually the root system, and fall herbicide applications will be translocated from the shoot tissue to the root tissue, killing the entire plant.

In addition, fall is the only time of year when ester formulations of postemergence herbicides can be used safely. Every broadleaf herbicide listed in Table 2, with the exception of bromoxynil, is an organic acid and is formulated as either an amine salt or an ester. Most have never applied 2,4-D, but instead, a derivative of 2,4-D in either an ester or amine form. Esters of 2,4-D and other organic acid herbicides are better herbicides because they penetrate the leaf surface and get into the plant more readily than do amine formulations of the same active ingredient.

However, amines are the formulation of choice for most turf managers because amines are non-volatile, whereas esters are slightly volatile. Volatility – the process of the applied herbicide evaporating from the plant or soil surface and being transported as a gas – from a broadleaf herbicide can cause extensive damage to non-target plants. If a sensitive plant species takes in air containing the volatile ester herbicide, injury or death will result. Therefore, applications of broadleaf herbicides made in the spring or summer should be made using an amine formulation of a broadleaf herbicide because volatility, which increases with rising air temperatures, is likely.

Non-target plants, such as flower and vegetable gardens or tree and shrubs near the turf, are very sensitive to these herbicides. However, in the fall, most plants are going dormant for the winter and temperatures are down, reducing the chance of volatility and the risk of volatility injury.

Contractors trying to control perennial broadleaves in the spring may encounter the opposite situation from that seen in the fall. Broadleaves in the spring are resuming growth and calling upon stored food reserves to initiate that growth. Thus, food is being translocated from the roots to the shoots and little of the applied herbicide may get to the root of the plant. The foliage may be burned off but the plant can regrow from stored reserves.