Fall is the most effective time to manage certain weed species. The recommendations seem clear-cut: winter annuals = fall herbicide application/tillage; annuals = no fall management. But figuring out the life cycle of the weeds in your field this fall is the catch……
Bromes, cleavers, chickweed, night-flowering catchfly, narrow-leaved hawk’s-beard, shepherd’s-purse and stinkweed are all facultative winter annuals, meaning that they can germinate in either the fall or the spring depending on environmental conditions. These weeds are often best managed in the fall, if populations warrant it. In general, waiting until about this time of year maximizes fall-germinating flushes of winter annuals. If using a herbicide, consider weed stage and the weather forecast, prior to application.
The problem is, given the right conditions – like the long falls and mild winters we’ve had the last few years –several of our annual weed species can also successfully overwinter:
Biennial wormwood – Despite its name, biennial wormwood behaves like an annual in agricultural fields. When scouting, estimate the average growth stage of biennial wormwood populations in a field. If the majority of the plants have already set seed, a fall herbicide application won’t help. An application may be worthwhile only if there is a large flush of biennial wormwood that haven’t set seed and are less than ~3 inches tall. Herbicide tank-mixes containing glyphosate + group 4 are more effective than glyphosate + group 2 on this weed.
Round-leaved mallow (RLM) – This annual weed can act as either a winter annual or a short-lived perennial, although it is more sensitive to freezing than our common winter annuals. Mild winters in 2015 & 2016 provided the right conditions for RLM to overwinter, allowing it to become (even more) problematic in certain fields over the last few growing seasons. Long range forecasters are predicting a harsh winter across the prairies this year, which should control RLM. However, if you have little faith in forecasts and decide to apply a herbicide, glyphosate mixed with either Distinct or DyVel DSp has activity on this weed.
Round-leaved mallow post-harvest
Stork’s bill – Like biennial wormwood, stork’s bill tends to be predominantly an annual in Manitoba. If this is a problem weed for you, scout affected fields to determine average weed stage. Again, if most of your stork’s bill has set seed you’re better off working on a plan for next year. Stork’s bill, especially larger plants, is relatively tolerant of many herbicides. If you decide to apply a herbicide because of stork’s bill this fall, glyphosate + group 2 or glyphosate + group 2 + group 4 on weeds up to the 4 to 6 leaf stage is probably your best bet.
Submitted by: Jeanette Gaultier, Provincial Weed Specialist, Manitoba Agriculture