If the stems are hollow, either inject concentrated Roundup into the stems or cut a main stem two to three inches above the ground and fill the hollow an inch to most of the way with Roundup. If the stems are solid, cut as mentioned and then -- with a drill or other implement -- drill a hole two or three inches deep and fill an inch to most of the way. *NOTE: This is best done during warm (temps above 70F generally speaking), sunny, drier weather as this is when the plants will be in more active growth and thus will have a greater tendency to absorb the herbicide.