Fall is a good time to control perennial and winter annual weeds, but spraying immediately after harvest may not provide the best results.
Perennial weeds cut off at harvest need time to accumulate new leaf tissue to absorb herbicides. Four weeks is a minimum recommendation and six weeks is ideal. Even when waiting this long, leaf surface area is still just a fraction of what it was prior to harvest. Therefore fall glyphosate rates may need to increase by 2 to 3 times over pre-harvest rates to get the same concentration of glyphosate into the plant root.
Before spraying, identify the weeds present. Perennials such as thistles and dandelions are best controlled from mid September to early October. As noted, waiting at least a month after cutting will increase the target leaf area, but later dates increase 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.
If weeds are green and the leaf tissue is still relatively pliable after a frost, growers may still have an opportunity to control perennial weeds with glyphosate or another systemic herbicide. Control can still be obtained on warm sunny days shortly after a frost if no more than 40% of the original leaf tissue is damaged. If most of the weeds are dead, herbicide uptake will be minimal and waiting is recommended.
Warmer temperatures and bright sunshine improve herbicide activity. Apply glyphosate and other systemic herbicides during the heat of day when perennial weeds are actively growing and putting energy into their roots.
October until freeze up is a good 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. But check weed staging. Many of the post-harvest product labels have weed staging listed, and winter annuals can hit those stages before October. Apply herbicide when the majority of winter annuals have emerged and are at the right stage for control.