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Applicators must always be acutely aware of weather conditions when they spray herbicides. But making applications during a drought presents special challenges for vegetation managers. Dr. Shep Zedaker, professor of forestry at Virginia Tech University, offers the following information to help herbicide applicators who are dealing with drought conditions.

Herbicide Applications During Periods of Drought

Applicators must always be acutely aware of weather conditions when they spray herbicides. But making applications during a drought presents special challenges for vegetation managers. Dr. Shep Zedaker, professor of forestry at Virginia Tech University, offers the following information to help herbicide applicators who are dealing with drought conditions.


Effects of drought on plant growth


 Cupping of the leaves.Some of the short-term effects of moisture stress are simple things, like closing of the stomata on the leaves because they are losing too much water. When plants close their stomata, they can’t take up any carbon dioxide, so they can’t make food. If the moisture stress becomes severe, the leaves begin to wilt and curl over. CO2 uptake is also reduced, meaning plants can’t grow because they use CO2 to make the building blocks of plants. So everything — including leaves, roots and annual rings on forest trees — gets reduced in size. But plants have some remarkable mechanisms for dealing with water stress.

Long term, plants do things like decrease their leaf size so they don’t lose as much water. They increase their cuticle thickness, reduce their stomatal density and suberize roots. These are all things plants do to protect themselves against drought, but they actually work against the efficacy of herbicides.


Competition for nutrients between desirables and invasives

Invasive plants have properties that make them very aggressive in taking in water, nutrients, root growth and sunlight. They are much better adapted to continue to grow in drought conditions than a crop plant or native species. So drought generally favors weeds because they are so aggressive. They also have a lower-than-average permanent wilting point, meaning they can extract water from the soil at much lower water potentials and not wilt, while most desirable plants will begin to wilt at the same level of soil moisture.


Why herbicides do not work as well during periods of drought

There’s a set of steps that has to occur for herbicides to work. For foliar-applied herbicides, the first step is that the herbicide has to be intercepted by a leaf, and leaves of plants are held more horizontally when they’re well hydrated. When they’re wilted, they droop. And, obviously, if a leaf is held vertically, it’s going to intercept less of the spray solution.

The second step is that the herbicide has to stick on the leaf. If leaves are hanging down and curled, the herbicide can just roll off.


Moisture stress discoloration and growth inhibiting.
Photos courtesy of Dow AgroSciences

The third step is entry into the plant. One major route for herbicide entry is directly into stomata cavities, and that can’t happen if they’re closed due to lack of moisture. Also, if plants have had prolonged drought, they deposit more wax on their surface, making it much more difficult for herbicide to get into the cytoplasm of the leaf.

The fourth step is movement in the plant. The herbicide comes into the plant, and often applicators want the herbicide to translocate from the leaf into the root system, so it can kill the entire plant. If there’s no translocation of sugars throughout the plant, which basically stops if a plant is wilted, then the herbicide can’t move throughout the plant either.


Herbicide degradation during a drought

If the main pathway of herbicide degradation is through biological activity in the soil and it’s very dry, organisms are not active because it’s too dry for them to expend the energy. So you could apply the herbicide on the surface of the soil, and it would just sit there for a long time. Another breakdown pathway for herbicides is through hydrolysis, and it can’t occur if there’s no moisture.

Many herbicides that are soil-active have to be moved into the soil by subsequent rainfall to reach the root system and be taken into the plants. In a drought, herbicides will sit on the top of the soil. They can be broken down by photodegradation, or broken down from the light of the sun, before they get a chance to get into the soil and work. The way a herbicide is broken down can affect the potential for herbicide carryover and, depending on the herbicide, could have the potential to damage desirable crops or plants if they are near the treated area.


How specific herbicide formulations influence efficacy

Formulations that are more readily taken into the plant will give better control. For example, oil-soluble herbicides have more rapid uptake into plant cuticles because they solubilize the cuticular waxes to a certain extent and are, therefore, less affected by drought than water-soluble herbicides. Each herbicide, whether it’s an amine or an ester formulation, reacts differently to the whole system of drought effects on plants.


Ways to overcome the effects of drought

To overcome the effects of drought, applicators could increase the carrier rate, either oil or water, which may hydrate the whole system and would generally increase interception of spray droplets on the plant leaves. Using better surfactants, or using surfactants where they hadn’t been used before, can help because they increase retention of herbicides on the leaf and increase uptake. Making sure you have the right concentration and the right type of surfactant in your spray solution can greatly increase efficacy during a drought. If the label allows, increasing the rate of herbicide application to control the same plants can help.

One of the best options, if an application can wait, is to wait for rain. Also, early morning applications are generally better under drought conditions because at night, when temperature is low and relative humidity is high, plants have a chance to rehydrate themselves from soil moisture even during drought periods.


What not to do during a drought

If you’ve gotten to the point of a prolonged drought where there is leaf necrosis or if plants are visibly wilted, it’s probably better to wait for a period where plants are rehydrated. If it’s dry enough for foliage to start dying, herbicide efficacy is going to be poor and the material used will be virtually wasted.


Article provided by Dow AgroSciences LLC and Vistas, a publication of Dow AgroSciences. Reprinted by permission. For more information, visit www.vegetationmgmt.com

Always read and follow label directions.



Herbicide Safety Tips

Hot weather usually means lounging by the pool and wearing shorts, t-shirts and sandals. But, for their own safety, herbicide applicators should avoid the temptation to wear traditional summer clothing when on the job. There’s enough time for tanning another day.

Always follow the label directions when it comes to protective equipment. Long-sleeved shirts, long pants and gloves are the basics. Wear chemical-resistant gloves according to label directions or with prolonged/frequent contact with herbicide. Avoid leather, cloth or paper gloves because they absorb liquids. In addition, tuck gloves inside of shirt sleeves to help avoid any seepage. Also, wear the proper chemical-resistant footwear and socks. Tennis shoes should be avoided.

If the label recommends it, wear a hat or helmet to protect hair and scalp, and let it serve a double purpose to shield the sun. Trade in the sunglasses for goggles or face shields, especially when the label calls for such or when herbicide concentrate could accidentally get in the eyes.

When handling on-site, always follow safety precautions.

Mixing products:

Keep containers below eye level when opening, and stand with your head above the fill hole of the spray tank.
Keep fill hoses above water level in spray tanks to prevent back-siphoning.
Check wind direction before pouring.
Never use your hands to stir or retrieve anything in the tank.
Use closed-loop handling systems with returnable, refillable containers <dash> requiring no mixing.

Applying products:

Walk in and spray out to avoid marching through mist and treated vegetation.
Direct spray solutions away from people.
Minimize overhead spraying.
Determine wind direction to avoid drift and exposure.
Check equipment for loose connections and cracked hoses.
Test equipment before use.
Don’t overpump during backpack applications to avoid blowing a seal.


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