klc said: I'm assuming she is using roundup or something but I'm not sure. I've never seen her using anything but there is a definite line of dead grass under the fence and my butterfly bush is technically right on the property line which is within a few inches of her fence. I'm going to have to find a place to move it to.
RoundUp in its typical basic form shouldn't affect via roots that grow under her property because it is has solely glyphosate as an active ingredient, which isn't soil active. As Frillylily said, though, there are RoundUp products that contain other active ingredients. There are also other herbicide products besides the RoundUp brand that could contain soil active ingredients.
That's why it would really help to know exactly what product is being applied so as to determine if it can be soil active. If the butterfly bush is right on the property line and the foliage received some spray or accidental drift then it could be affected by regular RoundUp (glyphosate) or certain other herbicides that are not soil-active. If the problem is aerial drift rather than through the roots, then a plastic barrier for the roots won't help, but a solid fence should limit aerial drift if it's only from a hand-held sprayer.
