_Canadian_ thistles have a lot longer life, so you certainly want to spray them in fall, their root mass keeps building, and they feed their roots in fall, they feed the bloom/seed in tthe first half of the year. Sap runs up in spring/summer, down in fall. You won't kill a _Canadian_ thistle in spring, only wound the top growth. The root mass will not be affected.
So, you want to spray _Canadian_ thistles now in fall, you will get the best kill this time of year as the plants are flowing sap to the large root mass.
They act like other thistles in that they are weak the first year they grow, and it's the 2nd year you really notice them. But _Canadian_ tthistles continue on as a dense thicket of a weed patch, regrowing from seed, root, and leafy cuttings, always creating topgrowth and seeds in the 1st half of the year, and feeding their root mass in the fall of the year.
_Canadian_ thistles are a perannual.
What you are describing are the bull, or Russian, or several other thistles, which are bi-annual plants.
_Candain_ thistles are a horrid thing, very very hard to kill. One needs to hit them right, and keep after them. Organically that are _very_ hard to get rid of, need to keep their patches black for a couple of years, so no vegitation sees the light of day - that huge root mass will keep trying to send up runners for a couple of years once a patch is established. Cut off any plants, and the green stem will send out rootlets and try to regrow itself.
They are a very hardy & tough weed.
The best time to spray them is after they set seed - unlike most other plants that go dormant or dead after they set seed - _Canadian_ thistles go into root-feeding mode and continue to grow through light freezes, feeding themselves for next spring. They appear to just be sitting there, but are actually very actively growing - seending all nutrition to their growing root mass.
They are a _very_ special weed for those of us in their prime growing areas who remember before Roundup..... Very special memories....