ive found that nuking the place with roundup in the later part of the growing season is a good way to control it. when u do it in the spring or early summer, the plant isnt really moving roundup into the subterranean colony (the insidious part of can. thistle). the idea is that u have to move as much of the roundup into that root system as u can before top killing it. but for how long and what is left in the wake of this distruction is in question. ur left with dead open soil and the seeds of that insidious monster still lurking. oy the agony! sure u can call up PN and get a ton more but that stuff is kinda expensive to keep throwing on ur mound, only to nuke it with expensive roundup.

but i have a better solution if u are game...

24d and/or other broad leaf herbicides only kill broad leaves and prairies are 2/3 grass anyhow. get urself some pretty grass seeds (little bluestem maybe?) and dump a few extra on that place whilst u nuke it with broad leaf herbicide until u have nuthin but grass and the decaying matter of what once was a horrid colony of botany gone wild.

i use something called triplet and i add 24D to the mix, this is a powerful mixture. (if u are lost, please! please! go to ur local co-op with a copy of this and they will give u what u need.)

one more comment is that im not sure what a mass of fiberous roots will do to ur mound. my guess is that its no worse than the prairie u planted but dont look me up when prairie clogs ur pipes... but alas i have a motto; 'dont mow it, grow it.' and when im in doubt, i refer back to that.

froggy

Have you called prairie nursery and asked them about your thistle problem? I would start there. They should be able to tell you the best way to eliminate the thistle. Or call Prairie Moon and see what they suggest.

if the thistles took off early, you may have some original natives in that area that didnt germinate - that may still come back once the competition has been removed... unfortunately anything native that had germinated in that area and had been hit by round up is now dead :-( ...

but if you have already killed everything on top - then you probably will want to reseed... if the rest of the mound is growing well - i would probably collect seeds from your existing plants and sprinkle them into the areas where you have killed everything else... i would do this as the seeds mature - i would probably save a couple handfuls of each kind for trading, and another handful for cold stratifying. some should be mature now and ready for sprinkling or collecting. if you are collecting and trading - you may be familiar with cold stratifying - i seem to have best luck with that. if you dont cold stratify - then spreading in the fall is going to be the best time... i like to try to think about what mother nature does, and then try to imitate her... purchased seeds have usually already been prepared by the nursery... the seeds that i collect and exchange seem to do better if i cold stratify them over the winter and then sprinkle in the spring... i dont think it matters whether you spread the seeds before or after a spring burn - i would probably spread before because the heat from the fire can help germinate them...but i dont think either way would do any harm...

you may want to consider alternating your burn times between fall and spring - it helps control a greater variety weeds that way ... most people prefer a spring burn because the plants left standing over the winter provide food and shelter for animals and winter interest - however a fall burn can help control later growing fall weeds...