Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityAugust, 1995
Why Herbicides 'Fail' - and Factors affecting Herbicide Performance
Keywords: herbicides, weed control, landscape, nursery, weather
L ast week, watching feathery Canada thistle seeds dance in the air, it occurred to me to write an article on herbicide 'failure'.
It's a very frustrating experience for a landscape or nursery manager to go through the time and expense of herbicide application, only to discover a few weeks later fields or landscapes chock-full of weeds. Human nature leads us to blame the chemical first, our actions second. Unfortunately there are many reasons for weed control failure, and most of the time, it's not the chemical at fault.
Postemergence herbicides - climatic factors. Rainfall or irrigation too soon after applying postemergence herbicides can destroy control. Most postemergence herbicides available for use in ornamentals are absorbed through the leaves, and thus require some rain-free period for this process. The period varies, e.g., Roundup requires a minimum of six hours whereas Fusilade and Vantage require only an hour. Intact weeds translocate herbicides best, so avoid cultivating or mowing for at least several days after application. Once a translocated herbicide is adequately absorbed, weed control is promoted by the same climatic factors which promote growth and translocation. Translocated herbicides may take effect more slowly if a cool, cloudy weather pattern follows their use.
Postemergence herbicides - Rate of application. Determine whether the actual rate applied matched the intended rate. Mistakes in calculations, or sprayer calibration and operation can lead to failure with any chemical application. Consult the following OSU Extension Publications for technical information: Bulletin 704, "Equipment for Application of Granular Pesticides," Bulletin 705, "Using Low Pressure Sprayers," Bulletin 816, "Reducing Spray Drift," and Fact Sheet AEX 520, "Boom Sprayer Calibration. These are available through your county extension agents.
Postemergence herbicides - weed characteristics. The rate, as well as timing of herbicide application has to be matched to the weed control problem. Herbicide labels usually specify a range of application rates because annuals and small weeds usually may be controlled by lower rates than perennials and large weeds. Roundup generally kills annual weeds at any stage of growth, whereas perennials are most effectively killed close to the flowering stage. However, don't wait so long that weed seeds set and disperse. Some herbicides are less effective on mature weeds; e.g., - use Vantage and Fusilade on grasses under six inches or less in height.
Postemergence herbicides - spray volume and coverage. Be sure to apply postemergence herbicides in adequate volumes of water per acre to ensure coverage of weed foliage. Roundup, Vantage, and Fusilade may be used in low volume applications (10-20 GPA), whereas Finale and Paraquat require higher volumes (20-60 GPA) since coverage is more critical for control. Failure to include a nonionic surfactant (e.g., - Fusilade) when specified on the label also may result in inadequate coverage, wetting, and penetration of weed foliage by the herbicide.
The rate of application also interacts with coverage and volume. For example, wipe-on applications of Roundup can effectively control weeds, but a much higher concentration (30-50%) must be used compared to a spray application (1-2%) because much less foliage is covered.
Preemergence herbicides, - timing and preparation for herbicide applications. Probably the most common reasons for preemergence herbicide 'failure' is that the chemicals are applied to fields that haven't been cleaned up, or are applied too late - after weeds have germinated. While a few preemergence herbicides have some effect on emerged weeds, (e.g., Goal, Ronstar, Casoron, Kerb) most don't, and must be applied to weed-free soil in order to
be effective. If your application is late, first eliminate weeds prior to application or include a postemergence herbicide in the tank. Problems also occur in early spring when applying herbicide to what appears to be a weed-free field. Most preemergence herbicides only control weeds from seed and have little effect on the emergence of quackgrass, Canada thistle, or nutsedge from below-ground structures.
Preemergence herbicides - climatic factors. Drought following the application of preemergence herbicides can result in failure, since all preemergence herbicides require some moisture for activation. Many preemergence herbicides can 'wait' for rainfall a week or more; however, until the chemical is activated, it cannot control weed germination. A worst-case scenario for preemergence herbicides occurs when sufficient moisture prior to application allows weeds to germinate, but lack of rainfall after application prevents the herbicide from controlling the weeds.
The reverse situation - excessive rainfall, is much less a problem since the solubility of most preemergence herbicides is extremely low. Most remain in a very thin layer on top of the soil, moving only a fraction of an inch from the surface.
Preemergence herbicides - temperature. Temperature has a critical effect on a few herbicides. Casoron and Kerb are examples of herbicides which perform better in the cool season and won't last long into the summer unless incorporated into the soil. Similarly, volatilization and loss of Treflan increases with warm temperatures. Very high temperatures conceivably may reduce the longevity of all preemergence herbicides because higher temperatures speed up the biological and physical processes that break down herbicides in the soil.
Longevity of herbicides in the soil is also affected by physical or chemical characteristics of the herbicide and its application rate. For example, Factor and Barricade have a long persistence because of the very low solubility of their active ingredient. In general, higher rates of application will provide longer control. Conversely, applying too little will result in losing control sooner than expected. Thus, ensuring that the rate applied matches the intended rate is just as critical for preemergence herbicides as described above for postemergence herbicides.
Preemergence herbicides - soil characteristics. Some herbicides require higher application rates in heavy clay or highly organic soils because the chemical is more strongly bound by the soil. Soil pH is factor in the effectiveness of the triazine class of herbicides (e.g., - Princep). Triazines are less effective in low-pH soils because they may be inactivated. The inactivation is reversible, and triazines may be reactivated by an increase in pH. Liming a field that has been treated with Princep for many years can result in crop injury when the accumulated chemical is reactivated.
Preemergence herbicides - mulch. Mulching over treated soil or physical incorporation can destroy the effectiveness of Goal and Ronstar. These chemicals are-light activated, and control is achieved by formation of an unbroken barrier on the soil surface. Mulching over most other preemergence herbicides can extend their life, but mulch layers exceeding 2" should be avoided since thick layers of mulch can provide a habitat for weeds to grow and 'escape' from the herbicide layer below.
Preemergence herbicides - weed species. Weed control by preemergence herbicides can appear to be a failure if we expect the herbicide to control all weeds. Most preemergence herbicides provide either superior control of grasses or broadleaves, and usually not both; thus for broad-spectrum control, it is desirable to combine a grass with a broad-leaf herbicide. Unfortunately this isn't possible in some sensitive crops for which we have good grass-, but not broadleaf herbicides available.
Finally, even when a strong combination of herbicides is used, there will be 'escapes,' - weeds which resist control by chemical means. It is important to watch for weeds that are resistant to control. It's possible to create a serious weed problem if a resistant weed is released from competition from other weeds through repeated herbicide use. Using another combination of herbicides that can control the problem weed and/or non-chemical means of control can help avoid this problem.
Some notes on fall weed management. I have observed that preemergence herbicides are applied in many nurseries in the late fall. I'd like to point out that applications this late do not control winter annuals unless herbicides with both pre- and postemergence activity are used (e.g., - Kerb or Casoron). Most winter annuals germinate earlier in the fall; in order to control them with preemergence herbicides, existing weeds need to be cleaned up and chemicals applied by late-August or mid-September.
Applying preemergence herbicides in late fall or winter to control spring-germinating weeds can be quite helpful since labor is tight all through the spring. This is made possible by using low-solubility preemergence herbicides that aren't appreciably lost during the winter. A range of application rates, rather than a single rate usually appear on herbicide labels. Select the higher rate when making late fall applications, this should provide good weed control through late spring. This way, reapplications of preemergence herbicides can be postponed until after the peak digging season.
Update: The new OSU Extension Bulletin 504, "Insect and Mite Control on Woody Ornamental and Herbaceous Perennials" is hot off the press and may be purchased from your local OSU extension agent.
