7/19/10 - I went to the dentist yesterday. I dropped my keys next to my car on the way out. They landed next to this outstanding specimen of horse nettle, which is not bothered in the least by the need to grow through the parking lot's asphalt surface.
When I first bumped into horse nettle back in Virginny, I sought information about it from Virginia Tech, which has adopted the gloomy perspective that (for control of horse nettle in corn fields) "...few postemergence corn herbicides provide acceptable short- or long-term control of horsenettle."
A technique that seems to work is to pounce immediately on any horse nettle plants I see. I dig them up with my loyal soil knife and then squirt Weed-B-Gon down the hole, the better to poison the root that I know lurks below the surface. Then I examine the perimeter around the plant I just eliminated, knowing that additional culprits are probably nearby. If I find them, they get the same treatment--dig them up and spray Weed-B-Gon down the hole.
So far so good. I've been able to rid the horse paddock of the stuff. It has required three years of repeated treatment at the moment, but each year reveals fewer plants. One patch hasn't returned so far, but I won't let down my guard.