I sprayed an area with Roundup in preparation of planting hellebores but the Moonseed vines just laughed at me...how can I kill it but not poison the soil? Help!
Spray the foliage?
Probably need to get out the shovel and get that rhizome out of the ground. Any missed pieces that sprout next spring you can slice and douse with a small amount of weed killer.
This is probably not what you want to hear, but from what I've read, by the time you notice it, the time has passed to eradicate it entirely (especially if its root system has intermingled with those of other ornamentals, which of course they DO.
I dig up as much of the roots as possible and spray what remains in the ground with herbicide. This helps control it, but it always returns.
BTW, have you ever noticed how the most invasive vines always situate themselves where they can least likely be removed without harming your favorite plants?
Vine tips can be inserted in a jar/can/florist spike of herbicide, to 'drink' it. Then only what is needed is used, and it's not put on the soil.