Weather, Wind & Weeds: Keys To Herbicide Success

Now that seeding is wrapped up and we enter spraying season it is important to remember key factors for herbicide efficacy. When the weather is unpredictable, finding the proper conditions to spray in a timely matter can be a challenge. Temperature swings, strong winds, drought or excess moisture can all influence the performance of your spray application.

Temperature

Temperature influences herbicide effectiveness by affecting plant growth, absorption, translocation, and metabolic activation. Herbicides typically will be most effective when plants are actively growing. Most post emergent herbicides are most effective when applied at temperatures between 18 to 29 degrees Celsius. When temperatures drop below 10-15 degrees Celsius, plant metabolism slows, growth is reduced, and processes within the plant such as transpiration and photosynthesis decrease. Important to remember not all herbicides are affected the same. Group 1 (fops), 9, and 22 herbicides are more effective at lower temperatures. Group 1 (dims), atrazine (group 5), and glufosinate (group 10/Liberty) are more effective at higher temperatures. Contact herbicides perform better under high temperatures and systemic better under lower temperatures, however excessively high temperatures can stress plants, leading to heart stress and reduced efficacy.

Wind

When it comes to wind, optimal spraying conditions in the wind are anywhere from 5 to 15 kilometers an hour. Anything stronger than 20 kilometers an hour it is highly recommended to park the sprayer. Completely calm conditions are not ideal as they can create inversion risks that allow spray droplets to drift unpredictably. If you are in a pinch for time, adjusting speed or increasing the volume of water can help maintain droplet size and reduce the risk of herbicide drift.

Environment

Drought can affect plant growth which can directly influence the effectiveness of some herbicides. Drought influences plants (including weeds) to develop thicker, waxier leaves. This can reduce herbicide uptake for systemic products like glyphosate. If experiencing drought this season it is important to look at adjuvants or surfactants that can help with herbicide efficiency. On the other hand, when it comes to excess moisture, pre emerge herbicides lose efficacy when they don’t have adequate soil moisture to activate or to move into the roots zone where emerging weeds can absorb them.
~Derek Keddie