We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember (Andrew
Gabriel) saying something like:

Roundup or one its clones.

Isn't that glyphosphate based?
If so, it will only kill what's currently growing.
By design, it is deactivated when it hits the soil,
so it won't stop new plants growing afterwards.

Well, that's what it was supposed to do, but a 5% solution of one of its
clones caused a non-growing wasteland that lasted for a year before
anything grew back at all and even then only sparsely, and it was the
only thing that killed off the briars.