The following question was sent to the P&PDL diagnosticians here at Purdue University:
Question: How do I eradicate canada thistle? (I believe that is what it is - will get to 5' +, no matter how deep I dig can't seem to get whole root. It has a prickly stem with serrated leaves.) I've been digging it out for 2 1/2 years, and my housemate dug for 12 + years before me. HELP PLEASE!
Answer: Canada thistle is a perennial weed that grows from underground creeping roots. It is very difficult to control by digging, since any segment of root remaining in the soil will most likely have a bud that will produce a shoot. Best to try and control with a translocated herbicide when plants are at flower bud stage of growth. Repeated applications will be necessary, especially on fall regrowth. These herbicides will kill most anything that is green, so be careful around desirable plants.