Canadian Thistle



Canadian Thistle/Canada Thistle (Circium arvense)


Also know as Creeping Thistle, this is a cool season perennial. All other thistles are biennials. These weeds are likely to be seen in the northern U.S. region and likes fertile, heavy soils.


This thistle starts as a small prickly rosette. Dig the rosette out ASAP before root system gets established. Once removed, let the roots completely dry out before throwing into compost.


While biennial thistles are beginning to die, after two years the Canadian thistle has already rooted itself many feet into the ground. This is where the term creeping thistle comes in. While creeping underground more budding occurs ultimately creating a very destructive group of perennials.


This large weed has prickly hair-like structures everywhere, including stems, leaves and flower heads. The leaves are green, serrated and spiny with a silvery color on the bottom side.


Branches off the stems at the top of weed holds multiple prickly spike-like purple flowers. Flower heads are about 3/4 inches wide and have seeds are attached to tufts of hair which carry them in the wind.

Photo: Koakidbob54

Spring into summer is when flowering occurs, by early summer germination has occurred. Fall is when the root structure begins to form. 

Keep in mind that Canadian thistle roots run very deep and this offers no guarantee of completely eliminating the weed.


The goal for removal is to starve the root. Easier said than done as never ending persistence is required. Consider leaving area the thistle unplanted for growing season allowing focus on weed eradication.


Start by cutting off thistle stalk as closely to ground as possible. Pulling is not very effective depending on how large weed and its roots are. Make sure to wear appropriate clothing. Don't use a shovel to dig the root out. Broken roots can produce more sprouts. Keep removing sprouts regularly when they surface.


Apply solarizing black plastic mulch over area. Most rooting systems can die after a few weeks with this method. When applying method keep this physical barrier on for about 6-8 weeks.


After removing plastic there are a few options for the next step.

  • One option is to apply a very thick layer of mulch. This can be pine bark nuggets to rubber mulch, depending on your needs. 

  • Depending on the location of the growing thistle, after removal place smother crop in its place. Smother crops can also be called a cover crop. A cover crop consists of certain plants that are grown over an area specifically to choke out weed growth with the dense roots. Cover crops usually are more effective with annuals than perennial, none the less they are effective.

Apply a foliar herbicide regularly. These foliarly herbicides move down through the rooting system ultimately damaging them. Purchase these herbicides by a retailer well versed on complete instructions for use.




FYI:

  • If your garden has ground covering Junipers, keep an eye out for this thistle wanting to sprout within ground cover.





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