We got Canadian thistle under control glyphosate (studies that do not have friends of Monsanto involved or other conflicts of interest show it as 'carcinogenic' or 'probably carcinogenic' - it just wasn't worth the risk to my loved ones and neighbours). We moved into our house 5 years ago - 100' x 50', so the thistle seemed to take over our little yard. It was a nightmare. They grew in hardpan here, so pulling up WITH their roots is literally impossible without heavy equipment (the roots supposedly can grow up to 20' down). Here's what we tried: 1) On advice from experts we tried cutting them back when they were still young. They kept growing back, and more - probably from the roots getting stronger. 2) On advice from more experts we tried cutting them down just before flowering - the theory being that all their energy goes into flowering and they will eventually die this way. They kept growing back. Options 1 and 2 were very time-consuming and only a band-aid to the problem, so I tried number 3: 3) I did extensive research and found that the USDA did a study in 2002 and found that 20% vinegar (acetic acid) killed 80-100% of Canada thistle. So we sprayed them - young/small and old/large. I even tracked some that I sprayed to see if they regrew from the same roots, and those did not. For the really large ones (3-ish ft tall), I cut them back to 6" or less, then sprayed. Needed to hit them a few times because their root systems are probably very strong. I even sprayed a 3' tall one through the fence, in the neighbour's yard, and it worked - that one surprised me! For the first 2 years, we had to spray all summer because there were so many. Now we only need to spray weedlings occasionally - mostly in the hottest part of summer. Most of our neighbours still have a problem with them - they either don't spray with acetic vinegar or only spray after they have a lot of mature thistles. Canada thistle is now my least problematic weed. Still haven't gotten those dandelions under control, and oh, the horsetail weed! I know this is an old thread, but hope this helps anyone who finds it in search of help for this tenacious weed.

Jay, you've recently been posting a lot of plants without flowers. Many of them can be identified to the species level if you wait for anthesis. The tentative Cirsium ID will be easier with a shot of the bottom side of the leaf, a close side shot of the head, and an overall plant photo to show relative head size and leaf shapes. Cirsium muticum is a possibility, but young plants of C. discolor can have a similar appearance. Those two easily separate in flower, though, because muticum has phyllaries with spiny tips up to 0.5 mm long, and those of discolor are 3-9 mm long. Flowering shots should also help to eliminate other local species, such as C. hillii and C. flodmanii.

A history is of both types being considered examples of Cynara cardunculus. With there in fact having been non-artichoke named forms such as Cristopher Lloyd's Giant and 'Florist Cardy'. Both of which have been offered by Oregon vendors in the past. And the artichoke cultivars having been differentiated by being placed within the aforementioned Scolymus Group. As in C. cardunculus Scolymus Group 'Green Globe'. The way the kind shown and asked about here has the same leaf size from top to bottom is not like cardoon plants I usually see. So it may in fact be a specific named form - perhaps even a different species in the same genus, if any of those have the same general appearance at all. For instance C. baetica ssp. maroccana - I have no idea what this looks like but it has been offered by an Oregon retailer in the past.

One of the Knapweeds of which there are many in N. America. To help further people need to know the geographical location and to have clear close up shots of flowers and leaves.