Frequently Asked Questions - Fall Weed Control

When is it too late to spray herbicides to control perennials in the fall?
What is the cheapest way to control winter annual weeds for next season?
Are there restrictions on the crops I can plant after using herbicides for winter annual weed control?

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When is it too late to spray herbicides to control perennials in the fall?

As long as the leaf tissue of the perennial plant is still green and relatively pliable, fall applications can be made. If the plant has experienced some frost previously, there may be some tissue damage to the leaf, which can reduce herbicide uptake. Check the leaves carefully prior to application and if more than 60% of the original leaf tissue remains, control can still be obtained. Rates may need to increase to the higher end of recommended ranges as tissue loss approaches 40%. Green leaves that are brittle do not make good targets and should be considered damaged.

Following damage from frost or harvest, perennial plants may produce new green growth that will make a good target. From four to six new leaves are required and herbicide rates will need to be at the upper levels of the recommended range. Glyphosate is the most commonly used herbicide for perennial control, but Amitrole may also be used on dandelions at the same rates as prior to seeding.

What is the cheapest way to control winter annual weeds for next season?

Winter annual weeds are weeds that germinate in the fall and remain alive under the snow through the winter, then resume their life cycle early the following spring to produce seed and die. Some common weeds that fit this description are stinkweed (M), flixweed (M), shepherd’s-purse (M), yellow or wood whitlow-grass (M), pygmyflower, narrow leaved hawk’s-beard, and over mild winters cleavers and black medick can act as winter annuals. Plants in the mustard family = M.

The optimal time for winter annual control is just prior to freeze-up in the fall or early spring as soon as plants green up in spring. Plants in the mustard family (M) are easily and cheaply controlled using the lowest recommended rates of 2,4-D at this time, but applications made in early fall and late spring are less effective. Early fall applications will miss weeds germinating after application and late spring applications will require higher rates or the use of glyphosate. Glyphosate is somewhat more expensive than 2,4-D and higher rates of glyphosate are required as weeds grow.

Glyphosate is also required for more difficult to control winter annuals like narrow-leaved hawk’s-beard, cleavers and black medick. The 0.5 L/acre rate (356 or 360 g/L forms) of glyphosate products is usually sufficient to control these plants before they begin to bolt or elongate in the spring, but higher rates may be needed as these plants age.

Refer to the Guide to Crop Protection for product rates.

Are there restrictions on the crops I can plant after using herbicides for winter annual weed control?

Yes. Research at the University of Saskatchewan indicates that the lowest rates of 2,4-D can be used in the fall before planting beans, peas, lentils and chickpeas, but that spring applications prior to planting these crops are unsafe. Neither fall nor spring applications of 2,4-D should be used prior to planting flax or canola (all varieties). Cereals have excellent tolerance to 2,4-D, and most other broadleaf herbicides for in-crop use when applied before planting. When in doubt, use glyphosate products before planting since they are safe before all crops.