This theory begs a practical test. I can send you all the "Canada" thistle (Cirsium arvense) you want. I recommend you plant it in every corner of your property. Enjoy!

Some weeds are extremely invasive and impossible to kill without herbicides. They will choke out crops and gardens quite happily. I really don't like using toxic goop, but this is war.

The thistle colony in my potato patch thinks of 2,4,D as "gin and tonic" and glyphosate (Roundup) as "a slice of lime in a nice refreshing Corona."

http://www.agriculture.gov.sk.ca/Canada_Thistle_Control

http://en.wikipedia.org/wiki/Cirsium_arvense