Our puncture vine (aka, goatheads, tackweed) is back. Last year i solarized this useless patch of gravel between our yard and the road, then dumped a toxic preemergent over it. This spring we switched to a non-toxic preemergent (corn gluten), and in the absence of a better plan, treated the area this weekend with a propane torch. Today i'm walking out in the yard again and see 100s of puncture vine seedlings, oblivious to the effect of the torch and the corn gluten.

I need to settle this once and for all. I'm against dropping roundup on there every month, or any other toxic preemergent. The environmentally sound methods i've tried have at best amused this nasty weed. This land is also technically county property, so i don't want to put in $100s in weed paper and rock mulch, or paving. Thoughts, suggestions?