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Battling Thistles Organically at Cornercopia Student Organic Farm

Courtney Tchida at the Cornercopia organic farm describes how she and her students combated Canada thistle this season.

Thistles spread quickly in vegetable
fields via underground rhizomes.
Photo: Courtney Tchida.

Author: Courtney Tchida. Over the years, Canadian thistle is the number one enemy weed we try our best to control on our organic farm at the University of Minnesota’s St. Paul campus. We’ve spent a fair amount of time digging and pulling by hand, which is only so effective. We rotate chickens on a white dutch clover pasture through our fields every 8-10 years for a year or two. This does a great job building soil organic matter and fertility in the soil but it also tends to be when thistles will proliferate into a field.

Earlier this spring on a field that chickens had been on recently we noticed way more thistles than we had planned for sprouting. Our plans for that field quickly changed and rather than trying to grow a cash crop and fight thistles we decided to just fight thistles.

In 2017, on an edge of this field that had thistles, I had tried a solarization experiment. After the thistles popped up in the spring I got a 100 ft X 20ft sheet of plastic and laid it down on top of the thistles and buried all the edges. After 2 months all the thistles were dead and we had a crazy crop of purslane thriving under parts of the plastic. We removed the plastic and purslane and planted a fall broccoli crop. The thistles did not come back. Solarizing a patch that big was a fair amount of work burying the edges of the plastic worth it but intensive.

Heavy clear plastic can be used to
solarize and kill weeds.
Photo: Courtney Tchida.
This year I decided to try a couple other methods. First, I got a 48ft X 12ft UV treated Vinyl billboard, it was heavy enough that I just had to weigh down the corners of it and it stayed put. I left the billboard in place for two months and it did a great job killing the thistles. This time we planted a fall salad mix after we moved the billboard to a different spot.

The other two methods I tried were classic smothering cover crops. I used sorghum-sudangrass and buckwheat. Both were planted after the field had been mowed and tilled. Both had great germination and left no room for thistles. The buckwheat grew to be 4-5 feet tall and when I finally cut it back we were able to harvest quite a bit of seed from it, restocking my supply of buckwheat seed. The sorghum-sudangrass grew over 10 feet tall. We ended up carving a maze out of it for our fall open house. It too outcompeted the thistles. When it was mowed this fall there were no thistles to be seen.
Sorghum-sudangrass and buckwheat smother
crops at Cornercopia. Photo: Courtney Tchida

I’m curious to see if any come next year or if we will have outcompeted them in just one season.
Controlling Canada thistle is a challenge in organic systems. Previously I had only heard of planting Alfalfa and cutting it every time it blooms for 3 years as a viable option. It’s exciting to know that other options exist and can work much faster.

Author: Courtney Tchida, Manager of the U of MN Cornercopia Student Organic Farm

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