Part of Weed management

Winter annual and perennial weed control

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What are winter annuals and when is the best time to control them?

Winter annual weeds germinate when the temperature begins to cool in late summer and early fall. Winter annuals can continue to germinate and grow after the first fall frost and usually continue to grow into early winter beneath the snow in soil that remains unfrozen. Winter annuals survive over the winter, resuming growth in early spring when they can compete with spring crops for moisture and nutrients.

Winter annual weeds can be controlled in the spring or in the fall. The recommended time to control them in the fall is from mid-October to freeze-up, preventing additional emergence after applying the herbicide. Spring control is best done early when these weeds turn green and are actively growing. Check herbicide labels for herbicides and rates that do not damage crops intended to be grown and to realize if spring applications are safe for crops.

Some common winter annuals include flixweed, narrow-leaved hawk's beard, stinkweed, shepherds purse and all members of the mustard family.

When is the best time to control perennial weeds?
Perennial weeds are best controlled in early fall when they are still actively growing, which is generally just prior to harvest. Post-harvest glyphosate has had inconsistent results with perennials such as Canada thistle, as there is usually poor re-growth after they have been cut off at harvest. 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. However, dandelion is different. Dandelion weeds are more actively growing in the cool fall conditions than in hot dry conditions and research indicates that the ideal time to control dandelion is from September 15 to 30.

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

Can I control winter annuals and perennials in the same application?
Some winter annual weeds may be present at the time that perennials are generally controlled; however, several more will germinate after this time. Similarly, spraying later to control winter annuals is likely spraying past the time that the herbicide will be effective on perennial weeds. Walk your fields to realize which weeds need to be controlled more. Remember that winter annuals may also be controlled in the spring.

What are the effects of frost on controlling these weeds and how long do I have to wait after a frost to spray?
Winter annuals can tolerate light frosts and perennials harden off upon experiencing several nights of low temperature in the fall. Various other factors can reduce frost damage including precipitation, dew and cloud cover conditions when the frost occurs. However, weeds need to be actively growing for herbicide efficacy. A harder frost, or early fall frost, can stress the weeds enough to shut them down. Check weeds a day or two after a frost to see how much damage occurred. If they are still green with little damage, apply the herbicide when the plants begin actively growing again approximately three to five days after the frost. If the plants are a brown to black colour and appear to be seriously damaged, wait for new growth to apply the herbicide.

What are the effects of dry conditions on weed control?
Drought stresses the plant and reduces control by reducing herbicide uptake. Under drought conditions a plant may produce a waxy barrier on its leaves designed to keep moisture in, which can also keep the herbicide out.