A good dose of roundup sometimes won't kill for 7-10 days.
A repeat application at 5 days after the initial is always a good idea.
If that doesn't do it, try this:
Fertilize it.
Something cheap with a good dose of N(itrogen) to fire up the vegetative growth.
Give it enough water.
Stay with me here....
About a week after the new, light green growth shows up, preferably on a hot sunny day, do a dry application - i.e. granular - of a broad spectrum herbicide. Water well. Wait about an hour, to let the leaves dry, then do a foliar application of glyphosphate (Roundup, etc).
Use the crossbow(2,4-d), roundup, sticker mix you've still got in the can, concentrating on the new growth.
Adjust the nozzle to the finest mist, and spray until it runs off the leaves (blades).
This drill causes the vigorously growing plant to suck the systemic herbicides up through the bottom and absorb it in through the tops.
And can work really well on stubborn woodies and herbaceous perennials.
edited -2,4-D is not effective on grape hyacinth, but will not impair the effectiveness of glyphosphate in a mix.
