Posted  8 Oct, 2019 
In: Articles

Originally published in the October 7, 2019 edition of Agri-News

The later stage of the growing season is an effective time for controlling perennial weeds such as Canada thistle and quackgrass.

“In annual cropping systems, one option for perennial weed control in the fall includes a post-harvest glyphosate application,” says Mark Cutts, crop specialist at the Alberta Ag-Info Centre.

“A fall application of a glyphosate product results in the herbicide being moved down into the root system and generally provides very good control of these weeds.”

He says that producers need to evaluate a number of factors to achieve perennial weed control in the fall.

“An important component of post-harvest weed control is ensuring there is sufficient leaf area present on the target weeds. Weeds that grow up into the crop canopy, such as Canada thistle and quackgrass, will be removed during harvest, leaving minimal plant material. As a result, a re-growth period is required so enough leaf material is present.”

He notes that Canada thistle requires 3 to 4 new leaves that are several inches in length while quackgrass requires a plant height of 6 to 8 inches and a minimum of 3 to 4 new leaves.

“For dandelions, the removal of plant material during harvest typically benefits control. The removal of the crop canopy allows for improved access to dandelions and better herbicide coverage will occur.”

Another factor to consider is the impact of frost on the growth of the target perennial weeds. Frosts can terminate the growth of perennial plants, so it is important to evaluate the plants after a frost.

“Assessing the impact of frost is best achieved 1 to 2 days after a frost event,” he adds. “If the majority of the perennial weeds consist of leaf tissue that is green – greater than 60% – a glyphosate product can be applied.”

He adds that perennial weeds vary in their ability to tolerate frost and knowing their frost tolerance helps with their control. Typically, dandelions are the most tolerant to frost, quackgrass has moderate frost tolerance and Canada thistle is considered the most sensitive.

Harvest management can also affect the success of a post-harvest glyphosate application.

“Leaving higher stubble heights will increase the leaf area remaining after harvest and reduce the time required to reach the proper re-growth stage,” he explains. “Removal of straw or the even spread of straw and chaff will allow for consistent re-growth of perennial weeds throughout the field and also allow for good spray coverage of target weeds.”

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