Controlling uncontrolled weeds

Though we all would like it to be different, weed control attempts are rarely complete. In other words, on some level, they fail. You may achieve 95 percent control with a given product application, leaving 5 percent of the target weeds uncontrolled, but the fact remains, it’s not complete control.

Hydrophobic zone created by excessive thatch and fairy ring mycelium.

When the percentage of uncontrolled weeds is low, in the 5 to 10 percent range, most turf managers consider the application to be a success and direct their attention towards other issues. Unfortunately, that’s not always the way things go. Sometimes the percentage is higher; when it is, the uncontrolled weeds are considered to be escapes.

Common weed escapes

Annual grassy weeds emerging in lawns are one of the two most common weed escapes. Crabgrass, foxtail and goosegrass are often seen along the edges of turf, such as near driveways and narrow strips of turf between sidewalks and streets, the result of a preemergence application gone bad. Annual broadleaf weeds such as purslane and spurge fall into this same category.

Weed escapes can result from postemergence treatments aimed at control of broadleaf weeds including dandelion, plantain and white clover, or perennial grassy weeds such as tall fescue or bromegrass. There are many causes of these types of occurrences.

Causes of escapes

<0x2022> Skips, gaps and other application errors – The most common errors made are ones of equipment misuse. Clogged nozzles, gaps in coverage, etc., are likely to be the culprit. Make sure your sprayers and spreaders are up to snuff by performing simple calibration procedures on them. It’s amazing to see how often (even when using my own equipment) nozzles are not delivering the proper amount of formulation or the tire tracks are hard to locate. Spray marker indicators can be helpful in reducing coverage errors. Before newly hired employees begin making applications, provide them with thorough, supervised training in the classroom and in the field.

<0x2022> Weather – The fate of an herbicide application is heavily influenced by ambient weather conditions. The weather can be helpful or harmful, depending on the severity of the various components. The factors of rain, wind and temperature are the most important when it comes to herbicide applications. Unfortunately, these cannot be controlled by the turf manager. Instead, applications must be made based on the likelihood that they will significantly alter the outcome. After all, that’s what “managing” is all about.

Slope can cause weed escapes by accelerating movement of product away from turf.

Rain is usually thought to be problematic in terms of postemergence weed control, with good reason. If even a small amount of rain falls to the treated site soon after the application is made, the effect is one of dilution and reduced control. On the other hand, when a light rain falls on the site after a preemergence herbicide application, especially a granular one, the effect can be desirable, in that the particles will be moved off the leaf blades and into the rootzone, where the target pest lies.

Weed escapes related to preemergence herbicides can be affected by a failure to penetrate the thatch. This factor is most dramatically observed in the case of liquid applications for white grub control, where the insecticide usually dries on the leaf blades or is bound by the thatch before it can be moved into the target area. The problem is usually worsened when there is a severe slope, as the herbicide can be easily moved off the turf. Reduce this factor by selecting granular formulations, inform the customer about the importance of adequate irrigation before and after the application, and consider core aeration to assist with the downward movement of the formulation.

Wind can cause a drift of applied herbicides away from the target turfgrass area. Even though this occurs more often with liquid herbicides than granular ones, both types can be affected. The velocity of the wind directly affects the movement away from the desirable target. Spraying on days when the wind speed is above 10 mph is asking for trouble. In fact, endeavoring to apply product only when speeds are 5 mph or less is advisable.

Temperatures that are warmer or colder than the desirable range called for on the product label can cause a failure of the application and subsequent weed escapes. Just like wind speed, avoiding temperature conditions that are too cold or too hot will limit ineffective applications.

<0x2022> Calculation errors – This is a matter of taking the time to thoroughly read and follow label directions. Be sure you are using the effective rate and formulation for the target pest. Use liquids where a residual of herbicide on the grass blades is necessary for control. The critical overriding factor to all of these considerations is to deliver the active ingredient to the target zone at the proper time. Additionally, the pesticide label will indicate the desired pH of the water in the spray tank. In many parts of the country, the pH of the water supply is alkaline, sometimes in the range of 9 to 10. If a pesticide that performs well at 5.5 is placed into alkaline water, it’s residual or duration of activity can be reduced from several days to several hours. A simple water test can be a big help for this problem.

<0x2022> Wrong herbicide selection – Certain products work better for certain weeds, no doubt about it. Check with local university cooperative extension educators and specialists for the latest herbicide recommendations for turf weeds in your state. Resist the temptation to select a certain product because it is offered at a reduced price by your supplier. It may work quite well for weed A, but not for weed B.

<0x2022> Improper timing – Each weed has a stage of vulnerability, when it is most likely to be killed by an application. Insects in certain life stages can be virtually impossible to control. Similarly, certain weeds are much easier to control with a preemergence herbicide than a postemergence application.

Now what?

Once the escapes have been flagged and identified as to species and location, begin control by evaluating the extent of the escape. Compare the number of uncontrolled weeds to the expanses of turf where control has been achieved. This will help establish a threshold, which is the amount or number of a given pest above which their existence cannot be tolerated. Threshold is a relative term, in that the function of the site will determine how high or low it is set. For example, the owner of the largest bank in town is usually unwilling to accept many, if any, weeds in his/her front lawn, whereas a rural cemetery that only needs to look good on just a few days of the year – Memorial Day, Veterans Day and Flag Day – a larger number can be tolerated, and thus, the threshold is higher.

Next, consider the point in the season when the weed escapes occur. If they are noticeable in the spring, the need for follow-up measures are more necessary than if they exist in summer or fall, when there may not be enough growing season left to worry about. This is most certainly a judgment factor, with threshold, budget and labor availability playing a big role in determining how to proceed.

Rescue treatments are justified when the weed presence is objectionable enough and the threshold of tolerance has been surpassed. Consider a rescue treatment based on the pattern of weed escapes in the turf. If weeds appear throughout the turf sward, spot-spraying is in order, as the whole stand of turf is not covered. In extreme cases, it may be more feasible to simply reapply as if it were the initial application, although these instances are rare.

John Fech is an extension educator specializing in turf and ornamentals at the University of Nebraska-Lincoln.