I have a new landscape client who has a couple of patches of Zebra Grass she wants eradicated. zone 5. She says they have cut it down, sprayed it with Roundup, and dug up the roots. She says it keeps coming back strong as ever. I have very little experience with this particular grass. Is she, maybe, not mixing the Roundup correctly because frankly, I have never found an ornamental that Roundup wouldn't kill in 2 apps or less?