Weeds: Wait for post-harvest regrowth

Fall is a good time to control perennial and winter annual weeds. The message this week includes two reminders: Wait for post-harvest regrowth. And know the best timing for the weeds present.

Wait for regrowth. Before spraying, make sure weeds are actively growing with new supple leaf area to target. Weeds cut off at harvest need time to accumulate new leaf tissue that will act as suitable surfaces for absorption of herbicides applied post-harvest. Even with the recommendation to wait for 4 to 6 weeks of regrowth, leaf surface area is still only a fraction of what it was prior to harvest. Therefore glyphosate rates may need to increase by 2 to 3 times to get the same concentration of glyphosate in the plant. If frost is predicted or has occurred, avoid application until leaf condition of the target weeds can be evaluated.

Timing for perennials vs. winter annuals. Ideal timing for a fall herbicide application should depend on the weeds present in the field. Perennials are best controlled in September, especially when using glyphosate. You need to wait about a month after harvest for adequate leaf area regrowth so the herbicide has a target, but at the same time, the later you get into fall the higher the risk of losing healthy leaf tissue to frost. Without healthy leaf tissue, the herbicide can’t get translocation to the weed’s crown and storage roots where the killing can occur. Even with good regrowth, the leaf area to target is not as great as prior to harvest, so glyphosate rates need to be proportionally higher to get the same amount into the plant.

October is the best time to control winter annuals such as narrow-leaved hawk’s-beard, stork’s-bill, annual sow thistle (common and spiny) and cleavers. That way you get all that have emerged. With winter annuals, the goal is to break winter dormancy (which you can do with a Group 4 herbicide) or killing new buds being laid down for next year (which you can do with glyphosate).

Further reading:
Fall weed control on fields planned for canola in 2015
Fall weed control and frost
Foxtail barley and fall weed control