That's the key. Get desirable species growing densely and the thistles don't have a chance....UNTIL you open/disturb the soil...then you have to exhaust the thistle seed bank (or deal with the rhizome like roots of CT).
Funny, I hadn't seen this when I got that letter from the township - but I knew it already too...
Brooks, another option for this time of year would be fire. If the thistle is cooked to where the vessels burst it literally bleeds out just like scorched flesh would. The plant doesn't have anything to draw down to it's roots and it dies. No chemical residuals and it's kinda fun once you learn the exposure needed with your torch. It's like fire painting on the plants. You can see them change color as the damage has happened. No need to dry them and actually burn them off - just scorch them to where they leak out and they'll decay naturally.
I had a nasty CT infestation along the road which I crippled with 2 years of spot flame treatment over maybe 4 times total. Replaced them all with wonderful 6-7 foot tall shade tolerant grasses (in full sun :D ) and then the township came and sickle mowed it last month. I got a letter dated Friday that there's thistle all over and I'm supposed to remove it. I told them I had, and if it came back it was because some dumbass mowed my grass and restarted the seed so they can deal with it now. :mad: I didn't call them a dumbass, but it was really tempting.
So I'll second Stu's comment - if you want it gone, you just need to outcompete it early in the season next year and wipe out anything that's established before it goes dormant. It's not all that tough to kill, but the seed will remain for a long time and that's what you'll be fighting.
I've also read that repetitive tilling works, but that's hard on your soil. You'll need to keep churning every 3-4 months until all the seed has germinated and been ripped apart but before any of the plants make it to seed.