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Myth busters II
Killing weeds can be a tricky business. Dispelling a few common misconceptions about one of the most important tools in a superintendent’s arsenal.
Editor’s note: The January 2007 issue of GCM featured
“Myth Busters,” a story authored by David Spak, Ph.D., the technical
and development manager, fungicides, for Bayer Environmental Science, that tackled several common myths about fungicides. The story received such a positive response from readers that we’re doing it again, focusing this edition on herbicides.
Myth No. 1: All pre-emergent herbicides harm turfgrass roots.
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Not necessarily. Whether a herbicide harms turfgrass roots depends on how the herbicide affects the biology of the plant and the vigor of the desirable turf. Some pre-emergent herbicides, such as oxadiazon, among others, work by affecting the shoot of emerging weed seedlings. Used before the weeds emerge, oxadiazon will control weeds before they sprout and will not harm turfgrass roots.
On the other hand, dinitroaniline herbicides (also called DNA herbicides) are highly effective, but they work by disrupting root development. In a few situations DNA herbicides can damage the root system of the desired turf. Usually the damage is minimal and you’ll observe no negative effects to the turf. If the turfgrass plant has a weakened root system or if the plant is in the process of regenerating its roots due to damage from disease or nematodes, desirable turf may be damaged.
Myth No. 2: You should always use an adjuvant when applying herbicides.
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Adjuvants are used to wet leaves and facilitate coverage by the herbicide product, allowing better penetration into the leaf cuticle. Because many herbicides already contain an adjuvant, it’s not always necessary to add one. Pre-emergent herbicides never need an adjuvant, but most post-emergent sulfonylurea herbicides require one. One exception is foramsulfuron, which already contains an adjuvant. Be sure to check the label before adding an adjuvant on your own.
Myth No. 3: There is a damaging buildup of herbicides after three or four years of use in turf.
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Environmentally friendly herbicides break down and do not build up in the soil after they have been applied and have completed their job of controlling weeds. Newly registered herbicides meet rigorous standards to assure that accumulation in the environment will not be a problem.
Herbicides applied in the spring and summer are usually gone within a few weeks, or at least by the end of the season. A herbicide applied in the fall might carry over to the next growing season but will be gone by the end of that season. Herbicides vary in their residual activity but they degrade by different pathways. Those pathways include: photo degradation (by sunlight), microbial degradation, chemical degradation or hydrolysis (released by water).
Myth No. 4: Environmental factors do not matter in regard to herbicide performance.
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Yes, environmental conditions definitely affect herbicide performance. If post-emergent herbicides are applied to drought-stressed weeds, they may not perform effectively. Or, if a heavy rainfall follows a post-emergent herbicide application, the product may be washed away before it has had an opportunity to control weeds.
Conversely, if a pre-emergent herbicide is applied during dry conditions and the applicator does not irrigate to activate the product, the product may not perform effectively. Rainfall or irrigation are generally needed for activation.
In addition, the severity of the winter may affect the performance of a herbicide applied in the fall. An especially cold winter may put turf under additional stress and affect its tolerance to the herbicide. Stressed weeds may be more sensitive to the herbicide.
How fast, how well and whether a herbicide works all depend on environmental conditions.
Myth No. 5: Non-selective herbicidesare toxic to all plants. Selective herbicides are safe on all plants.
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The terms “selective” and “nonselective” are commonly used to describe herbicides, but their meaning is not absolute. The terms are best thought of as guidelines.
Nonselective herbicides (for example, glyphosate) will control the majority of weeds, but not every weed. They will damage most turf or ornamentals if applied directly. These herbicides, however, can often be used with care around ornamentals, provided they do not contact desirable plants.
Selective herbicides have a significant degree of safety when applied to particular groups of plants, but they can’t be used on every turf type or ornamental. For example, some selective herbicides control broadleaf plants but not grasses. Other selective herbicides can be used on warm-season grasses but not cool-season grasses. Degree of tolerance is a critical factor in deciding whether a herbicide can be used on a particular ornamental or turf type.
Myth No. 6: Once crabgrass germinates, it is impossible to control.
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Difficult, but not impossible. Crabgrass is a difficult weed to control once it has emerged.
That’s why most turf professionals use pre-emergent herbicides to prevent the weed seed from germinating. But some herbicides have effective early post-emergence activity and can be used on small crabgrass seedlings.
Other post-emergent herbicides, like fenoxaprop, are designed specifically for controlling established crabgrass and are well-tolerated by desirable cool-season turf. Applied on a post-emergence basis, compounds such as fenoxaprop and dithiopyr will control crabgrass weeds up to the five-tiller stage. Another herbicide, quinclorac, has been reported to provide good control on more mature crabgrass.
The bottom line: You can gain control of ugly crabgrass plants, even after they’ve established a foothold in your turf.
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