You need leaf area to absorb the glyphosate. I sprayed during a dry spell in January and it is working nicely, some 4 or 5 weeks later. I would strim off now, and wait for some good re-growth before spraying. It will work faster the, so you won't loose too much time. Better to get it right, even if it's a bit later than you wanted, otherwise you'll be halfway through the season if you need to do it again.