Post-harvest weed control helps minimize spring problems.

Fall is a busy time for farmers, but it’s no time to forget about weeds. In fact, post-harvest fall weed control might be the ideal time to eliminate your most troublesome weed species.

“Certain residual herbicides applied in the fall can provide weed control in the early spring, allowing growers to start with clean fields,” said Greg Dahl, Weed Science Society of America (WSSA) president. “This can be especially important if the spring becomes wet and planting is delayed.”

Fall weed management can especially help control winter annual weeds, such as marestail, which are easier to control while they are still small. If left untreated after harvest, most annual weeds that successfully overwinter often will be more difficult to manage in the spring.

However, winter annual weeds do not always emerge immediately after harvest, so scouting and weed mapping are key first steps. Avoid targeting summer annuals with fall applications of residual herbicides, weed scientists warn, because many of those species have prolonged emergence patterns that are better handled in spring.

Still, compared to spring herbicide applications, fall applications of herbicides on most perennial weeds can translocate more effectively to the roots for better control. In addition, fall weed control can be advantageous in cases where higher herbicide application rates are possible due to fewer crop damage concerns compared to spring applications. On the other hand, fall residual herbicide applications can limit which crops can be sown in the spring and should be planned carefully.

Temperature is another key factor in weed control. Avoid spraying foliar herbicides such as glyphosate and synthetic auxins when temperatures are below 50 degrees. Warmer temperatures are necessary for proper herbicide uptake and translocation.

Cover crops are an additional management tactic to consider, particularly to obtain weed control benefits. Assuming enough biomass accumulation before termination, cover crop mulches can suppress weed seed germination, emergence, and growth.

Your local MFA agronomists can provide more guidance on fall weed management, or you can find a variety of detailed information online at wssa.net/weed/wssa-fact-sheets/.

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