The offending Wisteria’s main stem is in the no-mans-land between me and my neighbor, though I suspect it is mine. I want to kill the plants. There are two. Do I have to cut the stem before applying Roundup or can I simply spray the stem where it emerges from the ground?
Roundup probably will kill it either way. However, why don't you dig up the plant, once it is dormant (usually after the first frost) and give it to someone who would like it?