Fall weeds. When is too late to spray?

frost on dandelion

Lower temperatures and shortened days in the fall trigger perennials such as Canada thistle, dandelion and quack grass to start moving sugars to below-ground tissues. Winter annuals and biennials are also doing this, but they don’t need a cool temperature trigger. Spraying these weeds in fall takes advantage of this downward flow, providing better control for next year.

Does herbicide work better after a frost? No. While cooler temperatures with daytime highs in the mid to high teens do increase movement of sugar, and with it glyphosate, to the roots of perennial weeds like Canada thistle, frost does not enhance that movement further. In fact, research reports that a light frost following a week or more of “hardening” at alternating 15°C and 5°C temperatures had no effect on movement of glyphosate or control of Canada thistle. However, while frost doesn’t improve control it also may not necessarily reduce control if enough healthy tissue remains to allow sufficient uptake of the herbicide.

Spraying after a fall frost. Killing frost for weeds is hard to define since it is influenced by so many factors. Different plants have different inherent tolerance to frost. For example a stinkweed plant can germinate in fall and survive the winter to grow the following year whereas a wild mustard plant will not. Weather preceding the frost may also affect frost tolerance. A Canada thistle plant that has little exposure to cooler temperatures may be susceptible to a frost of as little as -3-4°C, whereas a plant that has gradually acclimatized to cooler conditions and progressively deeper frosts may survive a frost of -10°C or more. This is why it is critical to continually check the condition of leaf tissues before filling the sprayer.

If weeds are green and the leaf tissue is still relatively pliable, growers may still have an opportunity to control the perennial weeds with glyphosate. To determine whether it is worth spraying after a frost, scout the field and check the amount of damage on the leaves. Control can still be obtained if no more than 40% of the original leaf tissue is damaged. Because the amount of leaf area after harvest, even with several weeks of regrowth, is only one third of what was there before harvest at best, using the same amount of glyphosate as in a pre-harvest application will not be effective. In reverse proportion to the leaf area, at least three times the rate used at the pre-harvest timing will be needed to get the same level of glyphosate into the plant.

Will it pay? When scouting to determine the value of post-harvest weed control:

—Assess if there is a significant population of weeds to warrant a spray, and whether they are annuals, winter annuals or perennials? Fall is a good time to hit perennials, biennials and winter annuals, and it can be worthwhile spraying annuals 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.

—Are weeds actively growing? Active growth is required to move herbicide through the plant and into target tissues.

—Is frost damage less than 40% of the leaf tissue? If yes, then there should be sufficient leaf area for herbicide uptake.

If these conditions exist, then a fall herbicide treatment may be of benefit. But note: Even though cool weather may trigger perennials to start moving sugars downward, you still want to apply herbicides on sunny and warm days for best results.

Further reading:
Fall weed control on fields planned for canola in 2015