| | Fall is always busy time and, as is often the case, producers may not have time for fall weed control. While many producers use glyphosate for pre-harvest weed control and crop dry down, pre-harvest isn’t the best time to control winter annual weeds. These weeds excel by germinating late in the year and over-wintering, to bloom very early the following spring and going to seed before the pre-seed burn-off. This fall, with an early harvest in the works, may be the perfect time to nip this problem in the bud.
“Winter annuals are a group of weeds that germinate in the fall, form a rosette, survive the winter, and then quickly flower and set seed once spring comes,” says Harry Brook, crop specialist with Alberta Agriculture, Food and Rural Development, Stettler. “These are the weeds that are a problem early in the spring, as they are often too mature to be killed with pre-seed burnoff. Some notorious winter annuals include dandelion, narrow-leaved hawk’sbeard, stinkweed, shepherd’s-purse, flixweed, and yellow whitlow-grass. They germinate well in the fall and continue to germinate until freeze-up. Some perennial weeds are best controlled in early fall, but unfortunately are usually best controlled before harvest. For effective fall, post-harvest perennial weed control, significant regrowth after harvest is needed. Canada thistle, quackgrass and perennial sow thistle fall in this category.”
Timing is always crucial for successful weed control. In all cases, green growing plants are needed to absorb the herbicide when sprayed. Plants damaged by frost or drought with injured leaves will not take up herbicide. Spraying early enough to get perennial weeds may mean that the will be a lot of winter annuals germinating after spraying. Producers have to decide what their most serious weed problems are, and then treat at the best control time.
Recent research from Kristin Hacault and Rene VanAcker on dandelion found the best control was achieved with a late fall application of herbicide. Glyphosate worked well, especially when mixed with florasulam (contained in Frontline) or tribenuron (Express). Some of the common herbicides used in fall weed control include 2,4-D, dicamba and glyphosate. Carryover is not a big problem as these chemicals break down over the winter. However, a bioassay may be necessary prior to seeding sensitive crops the next spring.
Most winter annuals are small and vegetative in the fall, having recently germinated, and are very susceptible to herbicide at this stage. Spring pre-seed herbicide application is often too late for the rapidly maturing winter annuals and too early for other spring germinating weeds. Perennial weeds, such as Canada thistle and sow thistle, are better controlled in the fall as energy flow in the plant is down to the roots. In the spring the flow is upward to the shoots.
“The ideal opportunity for weed control is when harvest is finished and the weeds are still green and growing. This is a perfect time to get the jump on some of those problem spring weeds,” says Brook. “This year Alberta producers can take advantage of this one-in-five to one-in-ten year opportunity for weed control as soil moisture is right, daytime temperatures are right and these weeds are still germinating and growing. Now is the time they are most susceptible to chemical control and effective control can be achieved using relatively inexpensive herbicides.”
Contact: Harry Brook (403) 742-7901 |
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