Spray weeds after a temperature shock

Fall is a good time to clean up weeds, especially winter annuals and perennials. Cold temperatures help trigger winter annuals and perennials to start moving food reserves down into below ground tissues, so waiting until after the forecast cooler weather later this week may improve weed control.

Tips for success:

1. Make sure the plants are actively growing with new supple leaf area to target. Weeds cut off at harvest need time to accumulate new leaf tissue. This is essential for herbicide uptake and efficacy.

2. Frost damaged leaves may look green but they’re not healthy and will not take up herbicide. If frost has occurred, avoid application until leaf condition of the target weeds can be evaluated. Leaf tissues that are blackened, browned or dark green yet brittle are all symptoms of cold temperature damage. Leaves should be vibrant green, shiny and pliable.

3. Spray in the afternoons when temperatures are warmer and heavy dew is off the plant. Ideally, you want to apply when temperatures are above 10 C and rising on days with predicted highs above 15 C but preferably higher. If applying on days where temperatures are at the minimum, bright sunshine is critical. Bright sunny conditions are ideal for moving herbicides to the roots where they will have the most impact next year. Check product labels for specific recommendations as temperature requirements may vary.

4. Use registered products on fields planned for canola in 2013. Glyphosate is effective and can be economically applied at the higher rates needed for some larger weeds. Tribenuron (Express SG) can be tank mixed with glyphosate if needed to effectively control the spectrum and size (or stage) of weeds present. Check the guide to crop protection for the list of weeds this tank mix controls. Click your province for a link to your guide: Alberta Saskatchewan Manitoba. With tribenuron in particular, the label says to wait 2 months before seeding canola, and these should be non-frozen months. DuPont’s recommendation is to spray before Oct. 15 on fields planned for canola. With dry soil conditions, breakdown of herbicide may be slowed. Check with a DuPont rep and describe your soil moisture situation before applying.

5. Are they perennials, winter annuals or annuals? It helps to know whether the weeds present in the fall will last the winter. Canola volunteers, for example, that emerge in the fall are unlikely to last the winter and do not need to be sprayed. In fields where crops were harvested prior to mid-August, annual weeds growing back from below the cut may have a good chance of producing significant additional seed prior to a killing frost. These annual weeds may benefit from mowing or spraying. Beware glyphosate resistant kochia. If using herbicides to manage kochia growing back after harvest, be sure to mix a Group 4 herbicide like dicamba with your glyphosate to prevent selection for glyphosate-resistant kochia. Note that mixing with Group 2 herbicides will not prevent the selection of glyphosate resistant biotypes since nearly all kochia is resistant to Group 2 herbicides already.