If they flowered- be nice to cut the flowers off & bag & dump em.
Yes- 2,4-d for sure. High rate & check back if new flush of em later. Repeat if necessary.
When u say “large” are u meaning they are the large thistle- as in musk or bull? Musk or bull have the huge flower and large plant - easier to deal with. Canadian are smaller with smaller flowers- harder to deal with.
Yes, smoke em!!! Gly is fine as well on top of 2,4-d. I can fire off a few more herbicides but really not needed. U probably will need to keep on these for a few years. If the flowering is too late (or even if not) still takes a couple years to settle the seed bank of them & they will peter out after a couple years of frying them. Another option is to run a year of beans or corn. Or do a year of beans - then a year of corn - your thistle problem will be solved. Lot of ways to handle. Just start the process & nuke em & next year- hit em earlier & often until no issue.
If thistles in clover itself ….. I’ve gotten rid of them with very hot rate on butyrac & multiple sprayings … keep on em in clover and they do peter out & vanish. Repeat sprayings is pry to be expected.
