Warm weather has some weeds showing and growing again. If weeds are green, leaf tissue is still relatively pliable and temperatures are relatively warm, growers may still have an opportunity to control perennial and winter annual weeds.
When scouting to determine the value of late-fall weed control, consider…
—Are there enough weeds to warrant a spray?
—Are they winter annuals or perennials? Fall is a good time to hit perennials, biennials and winter annuals. On annuals: Spraying annuals in the fall can be worthwhile if it looks like they’ll produce mature seeds before freeze up. If annuals have already set seed, it may be better to save the burnoff for next spring when those seeds emerge. (NOTE: Research out of southern Alberta suggests that kochia regrowing after a mid-August harvest will not produce seed before a killing frost in most years. If using glyphosate to control those annual weeds, you may simply be adding additional selection pressure for the development of glyphosate resistance.)
—Are weeds actively growing?
—Is frost damage or senescence (yellowing) is less than 40% of the leaf tissue?
If these conditions exist, then a late fall herbicide treatment may still provide a benefit.
Tip: Even though cool weather may trigger perennials to start moving sugars downward, you still want to apply glyphosate and Group 2 herbicides on sunny and warm days for best results.