Advantages of Fall Weed Control

Fall is a great time to target perennial weeds. Cooler temperatures as well as specific growth stages will trigger a perennial weed to begin movement of food reserves down to the root systems for over-wintering. This ultimately is a long-term survival tool used by many perennial weeds. Applying a herbicide that translocates or "moves" within the plant will ensure that more herbicide can find its way down to the root system. Therefore control is usually better than when the same perennial weed is targeted for control in the spring.

For instance, a study conducted at Ridgetown College¹ found that control of dandelion was significantly better when glyphosate was applied in the fall versus the spring. So when it comes to perennials, fall applications can give you more bang for the herbicide dollar!

Cooler Temperatures

The challenge with fall weed control is the mindset that "because temperatures are getting cooler, the herbicide application will have little impact". After all, active weed growth and good weed control are usually associated with warmer temperatures. However, just as some crops can grow under cooler conditions (i.e. winter wheat), so too will many perennial weed species. In fact, some species will require a light frost before they begin to send sugars down to their root system².

Post Harvest Weed Control

There are four herbicides registered for post harvest weed control in Ontario:

• 2,4-D,
• Amitrol 240,
• Banvel II and
• glyphosate.

Refer to the Guide to Weed Control (Publication 75) forspecific information regarding weed specific rates, use precautions and re-cropping restrictions. In general, glyphosate is the most widely used post-harvest herbicide as it is very effective, economical and provides the greatest level of re-cropping freedom for the following spring. Glyphosate is also the only product that can be applied pre-plant prior to a winter cereal.

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Appropriate Growth Stages

The key to successful long-term control of perennial species is targeting the herbicide application at the appropriate growth stage. Table 1 gives an overview of the best growth stage to apply an effective herbicide for control of the many common weed species.

It is important to note that the best growth stage for application doesn't always occur at a time that is practical for fall herbicide applications. For example, the best time to apply a herbicide for the control of field bindweed is at the full flower stage. Most of us can appreciate that Field Bindweed is usually at full flower in July, certainly not in September or October. So another approach for control of this species would be to plant a glyphosate tolerant crop and apply two in-crop applications. However, targeting field bindweed with a fall herbicide application after a winter cereal can still be quite effective provided there is re-growth of this weed.

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Cereal Stubble

With over one million acres of cereal stubble scattered throughout the province there is tremendous opportunity to clean up some of those troublesome perennial weed problems. Scout fields for perennial weeds that are still green and growing, make note of their growth stage, and if the conditions are right, take advantage!

References:

• ¹Jim Shaw and Rudy Brown, 1989. Fall vs. Spring application of burndown herbicides for alfalfa cover crop control in no-till corn and soybeans. ECW Research Report. 2-496-506.
• ² Wison, R.G. and A. Michiels. 2003. Fall herbicide treatments affect carbohydrate content in roots of Canada thistle (Cirsium arvense) and dandelion (Taraxacum officinale).Weed Sci. 51:299-304.

Related Links

• Guide to Weed Control, OMAFRA Publication 75

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