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, sow thistle 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).
Here are your fall weed control options for fields planned for canola in 2014.