If you’re investing in a crop protection application of a fungicide, herbicide, miticide, or insecticide, you want to make sure you’re getting the best performance out of each dollar you spend.

Unfortunately, the weather conditions can make a big difference in how those products work in the field.

HERE’S A LOOK AT SOME CONDITIONS AND THE ADJUSTMENTS THAT CAN BE MADE TO ENSURE THE BEST OUTCOMES OF WHATEVER YOU’RE SPRAYING…

You’ve probably heard that if it’s hotter than 90 degrees Fahrenheit, you shouldn’t spray. Most of the time, that’s a fair statement. Hot temperatures often lead to plants switching into defensive mode. We see leaves roll up or droop down. Leaves develop a thicker waxy coating and close stomata to prevent moisture loss, but this also limits penetration of spray droplets. You should also take into account how evaporation losses occur with smaller spray droplets floating in the air that never make it to their target, as well as bigger droplets that sit on leaves and don’t soak in.

Of course, there are exceptions to this, such as in areas with zero moisture stress and plenty of humidity making it so plants are less likely to shut down in hot weather.

That said, if it’s super hot, you’re generally better off waiting to spray until a cooler time of day (preferably in the morning rather than the evening) or until the weather becomes more ideal after a few days.

If it’s too cold, that brings its own set of challenges. When temperatures drop below 32 degrees Fahrenheit (freezing), this is self-explanatory as plant growth has stopped and some leaves or even whole plants may die. However, if nighttime temperatures dip into the 40s and low 50s, it can impact performance by slowing plant growth.

Take Roundup, for example. In order for Roundup to work completely, it must translocate (move) through the plant to deliver a lethal dose to every single growing point. When plant growth slows, so does the movement of Roundup through a plant. This gives the plant more time to break down the Roundup, lessening the chance of a lethal dose making it to every single growing point throughout that entire plant. The result is significantly less weed control compared to spraying during a warmer week.

Here’s the other key point. If conditions have been less than ideal and plant growth has slowed or stopped, it will take several days of good weather to get the plants back to functioning in an ideal state. For example, if the nighttime temperature is below 50 degrees within two days before or after spraying Roundup, we recommend increasing the Roundup rate by 50 percent (if the label allows).

When conditions have been on the wet side, plants may be experiencing stress from water-logged soils. Again, if a plant isn’t actively growing, it’s typically not advisable to be treating it with crop protection products. However, let’s say it’s been raining very regularly but the soils and plants are handling it well. Plants with plentiful moisture are not on defense – at least not like they are when it’s too hot or too dry. Stomata are wide open, and the waxy coating on leaves is at a bare minimum. Chances are if you spray a “hot” product like Cobra, Buctril, or Huskie during these times, you could see more leaf burn than normal. This often results in excellent weed control. If you’re concerned about too much leaf burn, be careful and back down on some of the adjuvants like crop oil concentrate (COC) or methylated seed oil (MSO). Either use them at lower rates or switch to adjuvants with less burn like non-ionic surfactant (NIS).

One other big concern when it’s too wet is creating compaction with your sprayer. If you need spraying done and don’t want soil compaction, the easy answer is to call an aerial applicator or a drone application company. We did this earlier this season on some alfalfa fields that we needed to spray between rains. If you are going to use a ground rig, lower the air pressure in the tires once you’re in the field. Cutting the air pressure in the tires greatly increases the size of the footprint, effectively lowering the pounds per square inch of pressure on your soil. There are a variety of central tire inflation systems out there to raise and lower tire air pressure from your tractor cab in seconds.

Check them out!

The last four years have been pretty dry on our farm, so we are very sensitive to the low humidity and lack of moisture that comes with short-term or prolonged droughts. If you find yourself in this situation, once again the leaves of the plant will be on high defense. In order to penetrate through them, switch to spraying in the morning when it’s cooler and the humidity levels are higher. Also, use the oils as allowed on product labels (COC or MSO) to increase leaf penetration. Be careful not to spray too early in the morning or too late in the evening to avoid dew on the leaves, as well as air inversions that can move your spray off-target when using dicamba and old 2,4-D formulations.

Three last pieces of advice on adjusting your spray applications to weather conditions: