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Extension by Brian Mealor

by Wyoming Livestock Roundup

Environmental Stress May Affect Weed Control
By Brian Mealor, UW Extension Weed Specialist

    It seems that one of the major headlines over the last few weeks has been the warm, dry weather and how it will affect management decisions this year. Reduced water for irrigation, limited forage production, livestock watering difficulties and planning when to ship livestock have all been important and informative topics of discussion. Unfortunately, heat and water stress may also play an important role in controlling noxious weeds this summer.
    Intuitively, we may think that drought affects all plants equally because all plants need sufficient water to grow and reproduce. On a general level this is true, but differences among species in growth characteristics – things like rooting depth, leaf structure, cuticle thickness, etc. – lead to variation in drought tolerance. Unfortunately, many of our most problematic noxious weeds are well-adapted to drought conditions.
    Deep-rooted perennial plants, such as leafy spurge, Russian knapweed, perennial pepperweed, Canada thistle and others are able to use water from deep within the soil profile where it is not available for many of our perennial grasses with their fibrous roots.
    Hot, dry conditions not only alter short-term competitive relationships between weeds and desirable species, they may also impact how effective herbicides are on targeted weeds. Plants grown under drought stress often develop thicker cuticles than under full-moisture conditions. The cuticle is a waxy coating on leaf surfaces that helps keep plants from losing water as quickly as they would otherwise. This response helps plants survive in dry climates or during periods of drought, but thick cuticles may impede the movement of herbicides into leaf tissues. Foliar-applied systemic herbicides, which move within plants to sites where they are active, must penetrate the cuticle to move into the vascular system of the plant. Use of high-quality surfactants as a part of a herbicide program can improve herbicide uptake and overcome some of the potential problems associated with thick cuticles.
    Many herbicides are most active when plants are actively growing, but many plants do not actively grow during hot, dry weather – lack of available moisture in the soil and low relative humidity in the atmosphere causes plants to close their stomata to further conserve moisture. Stomata serve as air intake systems for plants, and take in carbon dioxide, which is one of the primary ingredients in the photosynthesis process. The lack of raw materials slows the biochemical processes needed for growth, so plants can nearly shut down during periods of environmental stress. As plant growth slows down, it may induce two different reactions in plants: 1) herbicide activity may be reduced in susceptible weeds or 2) herbicide activity may increase in non-target plants or crops if they are unable to metabolize herbicides quickly enough to prevent them from reaching toxic levels. For more detailed information on potential impacts of drought stress on weeds and non-target plants, refer to the label of the specific herbicide with which you are working.
    How might we optimize the effectiveness of our weed control program during unusually dry conditions? First, be sure to know the specific characteristics of the herbicides you will be using. The best way to familiarize yourself with these characteristics is to read and understand the labeling information. Next, use appropriate adjuvants to improve herbicide uptake, and be aware of herbicides that may volatilize. Volatility is usually increased in hot weather, so beware of non-target injury.
    Lastly, timing of herbicide application can play a crucial role in efficacy. Early bud to early bloom applications are recommended for many perennial weeds. Another timing that yields excellent results for some of the same weeds is a fall timing, when weeds sometimes display regrowth. Another potential benefit to later-season timings is a hypothesized movement of herbicides toward the roots of weeds.
    Finally, if herbicide efficacy is reduced during moisture stress, then integrating multiple approaches such as mechanical or biological control methods makes even more sense. Above all, commitment to long-term management, creativity and persistence are keys to controlling perennial weeds. Dry years make for tough decisions, so be wise in your weed management choices and you will improve your success.
    Brian A. Mealor is an assistant professor and Weed Extension Specialist at the University of Wyoming and can be reached at

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