I think you'll be fine. I sprayed around some trees and it downpoured afterwords. The stuff that sat about 2 hours died completely, the 2' tall grass that only had maybe an hour was hurting, but only showed about 60% kill. It was generic 41% gly mixed at 2 oz/gal.
As long as the plants were actively growing you should be good to go, check it in a week as roundup is slow.