Hello Alan
I think it may be possible although without seeing how much couch and thistle there is, I can’t be sure. Certainly better for the aparagus than moving plants. If you have compost/manure, put on four inches or so now, then when asp. spears are visible you can put cardboard down between plants – or do it now if confident about where the crowns are!
Through the year, keep pulling couch leaves and thistle stems, by year’s end most thistle should be much weakened and remaining couch roots nearer the surface to pull or loosen with a trowel. Good luck.