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July 22, 2026 - The Fall Frontier Against Weeds

Late season weed control – when is it time to reconsider for the next season?

Controlling weeds might normally be considered a summertime activity, but truthfully some species of weeds are more affected by “late season” controls. It is never too late to have a positive impact on the war against weeds.

Thankfully, fall could be the perfect time to start implementing some late-season weed control. Because some invasive species are biennial or perennial, they begin translocating energy and nutrients to their roots late in the growing season. Management plans can take advantage of this by using herbicide during this time, because it is taken up by the roots more effectively. It is also important to note that at this point hand picking the invasive plants will become less effective, especially if most of the plant’s roots cannot be removed. If the roots are not removed, biennial and perennial plants will be able to regrow from those roots next season. 

One example of an invasive that is more affected by herbicide late in the season is Canada Thistle. Do not let the name deceive you, Canada Thistle is not a native species, it was introduced from Northern Europe and renamed “Canada Thistle” from “Creeping Thistle”. 

Although there are very few sites of Leafy Spurge within Clearwater County, it is designated as a noxious weed under the Alberta Weed Control Act, and it is also more affected by late season controls. Mechanical cultivation in the fall is most effective when regrowth of Leafy Spurge is about three to six inches. While Leafy Spurge is an example of a plant that responds to later season control, landowners do not need to worry as Leafy Spurge is covered under the Clearwater County Eradicable Weeds program.  

Late fall and early spring tillage will control rosettes of Scentless Chamomile. However, it is important to assert the distinction between the invasive Scentless Chamomile and the plant used for chamomile tea. They are not the same, and the invasive species cannot be used for making tea.  

Late season tillage can also kill new rosettes of Absinthe Wormwood. Absinthe Wormwood is not designated under the Alberta Weed Control Act, but since there is very little Absinthe Wormwood, it is good to be familiar with control strategies for this plant to keep numbers low.  

For most other invasive plants in Clearwater County, late season controls have decreased effectiveness in comparison to methods taken earlier in the growing season. Landowners should instead focus on creating management plans for the next growing season. Useful resources available to landowners in Clearwater County are the Custom Spray program and the weed control contractor list. More information regarding these resources can be found on the Clearwater County website or by calling the Agriculture and Community Services office. Landowners can request to be put on the custom spray list a year in advance of the growing season they want the spraying done, a great solution for landowners who missed the best window for weed controls this season.

Available in the different areas of the County are community pasture sprayers. These sprayers are housed on different farms and are available at no charge. If those get booked up, a surplus of rental equipment is available for rent through the Agriculture and Community Services Office. For a comprehensive list of our services and equipment, call the Agriculture and Community Services office at 403-845-4444 or visit the website at www.clearwatercounty.ca/p/services. 

Landowners should always consider multi-year management plans for future growing seasons which can make a positive impact towards Clearwater County’s war on weeds.