I believe the active ingredient in Roundup is glyphosate. It is best sprayed when the weeds are actively growing as it is systemic. It can be sprayed at any time, just it will take longer to work the less growth there is. I would suggest that now is a good time to spray as weeds are still growing and it should then die allowing you to get on with your winter digging. Don't be alarmed if nothing seems to be happeniing for a while as it takes at least two weeks to start taking affect at this time of year...the grass will yellow first followed by the deep rooted weeds