UW Extension Education: Fall is a Good Time to Push Back Against Perennial Weed Problems

Fall is here. Summer is spent running from one crisis to another, sweating under the relentless sunshine. Now, the roads are filled with beet trucks, and the sun has gotten lazy – coming out after the kids are at school and retiring before football practice has ended.  

This is the best time of year to push back against perennial weed issues.  

Canada thistle

A major perennial problem for all of Wyoming is Canada thistle or Cirsium arvense. 

Canada thistle is a deep-rooted perennial weed distributed throughout Wyoming. It flowers in late spring to early summer and puts on a relatively small seed head which generally has between 1,000 to 1,500 seeds.  

The seeds have a feathery pappus to catch the wind and spread and serve as the plant’s long range dispersal system. 

However, it is the vegetative buds in the soil that make the plant such a persistent problem. 

Canada thistle puts most of its reproductive resources into its vegetative propagation. The root system establishes quickly and spreads out around the plant. A single plant can colonize an area around itself from three to six feet in diameter. 

Tillage of this area will cut off parts of the plants root system and drag it along. These cut-off root sections can be as small as one-quarter inch long and still have enough energy to produce a shoot and turn into a new plant. 

Over time, these single plants will turn into colonies of plants and expand, outcompeting with both crops in farmland and grass in good pasture areas. 

Keeping a productive field with a lot of competition is always the best way to push back against weeds. 

While Canada thistle is a real issue once it gets going, the seeds it makes are much smaller and slower than other types of weed seeds, and a well-planted or established field will shade out a new seedling. 

This leaves field edges or other disturbed areas around a farm as the most likely place for new weeds to establish. 

It is always best to make note of any thistle plants or patches one comes across this summer. 

Fall control

It is this time of year the plant is starting to pull nutrients back from the above-ground portion which will soon be frozen and store it below ground in the root system.  

Canada thistle patches which have had a light frost – while the plants leaves are still green and healthy – are primed for control. 

A nice dosage of glyphosate, clopyralid or dicamba will be more effective for long-term control now than at any other time of year. 

When choosing a herbicide, always keep in mind the surrounding area, future land use and, especially, the label directions. There is not one hammer for every nail.

Unfortunately, it’s not normally a one-and-done process.  

After hitting the patches this fall, individuals should follow up with additional treatments in the spring. Wait for spring plants to grow to a rosette with adequate leaf surface and treat again.

Producers who grow hay should consider attending WESTI Ag Days on Feb. 12 in Worland. This conference will be an excellent opportunity to obtain more information about hay growing in Wyoming. Topics will include weevil control, cut timing, weed control and more. 

For more information, contact a local weed and pest district or reach out if you would like to chat. I would be happy to hear from you. 

Dan VanderPloeg is the University of Wyoming Extension agricultural and natural resources educator serving Washakie County and Northwest Wyoming. He can be reached at dvanderp@uwyo.edu or 307-347-3431.