6 Tips for Spraying in Warm Weather

We’ve heard a lot of questions over the past few days on applying herbicides during all the warm weather we’ve been seeing. Crops are currently battling with summer annuals like Waterhemp, which emerge, grow, and thrive in hot conditions and are naturally drought tolerant.

The optimum temperature for photosynthesis in waterhemp ranges from 68 -104°F, so these weeds are thriving even with these warm temperatures. Here are some quick tips on making sure your applications are still effective in the heat.


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1. Use higher labeled rates of Roundup® brand herbicides to help offset thicker cuticles and lower translocation rates.

Applying translocated herbicides, including Roundup® brand herbicides, in hot, dry conditions can result in delayed or reduced weed control. That’s because plants conserve water through changes in both the composition and thickness of the cuticle on the leaf surface when weather is hot and dry, which can create a barrier to herbicide absorption. Plus, herbicide movement within the plant may be reduced due to a slower rate of translocation and metabolism.

2. Use the full rate of ammonium sulfate (AMS) at 17lbs per 100 gallons of spray solution.

Using the full rate is particularly important when hard water is a concern. The AMS will aid in maximizing the availability of Roundup® brand herbicides for uptake by target weeds.

3. Consider adding 0.25% v/v non-ionic surfactant for weeds like lambsquarters.

Adding a surfactant, as allowed per label directions with Roundup® brand herbicides, can be a good way to counter the effects of warm weather, particularly when dealing with thick-cuticle weeds like lambsquarters.

4. Beware of crop damage when taking advantage of increased effectiveness of contact herbicides.

The activity of contact herbicides is increased on all plants at higher temperatures, so the weeds will be more susceptible to those treatments when it’s hot. Increased activity may provide improved weed control, but can also result in greater crop injury potential. 

5. Consider postponing PPO-inhibiting herbicide applications.

Contact herbicides that contain PPO-inhibiting herbicides need to be used with caution as temperatures increase to 85 degrees and above. If the temperature is over 90°F and weed size allows, consider postponing PPO-inhibiting applications to reduce the risk of crop injury. PPO-inhibiting herbicides are most effective when applied on weeds that are 4” tall or less.

6. Use a surfactant with PPO-inhibiting herbicides to increase crop response.

Not all PPO-inhibiting herbicides cause the same amount of crop response, so if crop response is a concern, consider adding a surfactant like Methylated Seed Oil (MSO) and/or Crop Oil Concentrate (COC) to your tank-mix when spraying in warm weather.


7. Reduce your chances of spray drift.

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