Winter Annual and Perennial Weed Control - Frequently Asked Questions

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 When is the best time to control winter annual weeds?
Winter annual weeds can be controlled in the spring or in the fall. Late September to mid October may be the best time to control them, as the plants are young, small and susceptible to herbicide. Winter annuals may continue to germinate and grow after the first fall frost so a late fall application of herbicide maybe more effective. Some common winter annuals include narrow-leaved hawk's-beard, stinkweed, and shepherds purse.

When is the best time to control perennial weeds?
Perennials are best controlled in early fall when they are still actively growing. This generally happens before harvest. Post-harvest glyphosate has had inconsistent results with perennials such as Canada thistle, as there is usually poor re-growth after harvest. However, dandelions may be more active in the cool fall conditions than in hot dry conditions. As a general rule of thumb, perennials are best controlled with pre-harvest herbicide, but can also be controlled with a post-harvest application if there is enough re-growth.

A few common perennial weeds include Canada thistle, quackgrass, dandelion, and foxtail barley.

Can I control winter annuals and perennials in the same application?
Winter annuals may be present at the time you are controlling perennials, but may still germinate after perennial weeds need to be controlled. Similarly, spraying late for winter annuals is spraying past the time that the chemical will be effective on the perennials.
Walk your field and see which weeds need to be controlled more. Remember that winter annuals may also be controlled in the spring with a pre-seed burn down.

What are the effects of frost on controlling these weeds and how long do I have to wait after a frost to spray?
Frost is a stress to the plant and can cause it to shut down. This means that the chemical will not be taken up by the plants. Check the plants a day or two after the frost to see how much they were damaged by the frost. If they are still green with little damage, wait about three days after the frost, until the plants start growing again, to apply any chemical. If the plants are brown to black and very damaged, you will have to wait for new growth to apply the chemical.

What are the effects of dry conditions on weed control?
Drought also stresses the plant, and reduces control by reducing uptake of the chemical. If plants are not actively growing they may produce a waxy barrier designed to keep moisture in, and may also keep the chemical out.

Prepared by Karla Bergstrom, Ag-Info Centre, Alberta Agriculture and Rural Development

For more information about the content of this document, contact the Ag-Info Centre.
This information published to the web on November 20, 2003.
Last Reviewed/Revised on May 29, 2014.