Fall weeds: Snow and frost and timing

Snow and fall weeds. When freezing temperatures stop fall weed control plans, snow is likely more of a help than a hindrance with respect to overall weed condition. The snow layer is likely to insulate the weed leaf material from the colder conditions that follow it. That could mean you’re back spraying earlier than you would be with frost alone. Spray decisions — when to spray or whether to spray at all — will depend on leaf condition after the snow is gone. Note that it could take a couple of days to properly assess frost damage, as green leaf material can appear quite healthy immediately after a frost. Read more.

Frost and fall weeds. When scouting to determine the value of post harvest 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 less than 40% of the leaf tissue?

If these conditions exist, then a fall herbicide treatment may be of benefit. But this is important: You still want to apply glyphosate and Group 2 herbicides on sunny and warm days for best results. Read more.

Best timing depends on the weeds present. Fall is a good time to control perennial and winter annual weeds, but spraying immediately after harvest may not provide the best results. Before spraying, identify the weeds present. Are they perennials? Winter annuals? Annuals? Clubroot hosts? This article has tips for each.

Herbicides for fall application ahead of canola