Finale Update

The Finale weed killer works as related to me; I've been dripping in a couple of drops of the concentrate to each individual cut-off-top plant, and it kills them quite completely. There's no spread that I can see - I have healthy, happy horsetail less than an inch away from dead, dead, DEAD horsetail, which is, of course, the better kind.

So if anybody was waiting for that followup, yep: it works.