I think I'd say the best time has passed. If by "plugs" you mean 2.5" nursery pots, or smaller, even if you potted them up today, they probably wouldn't mature enough to survive the first winter in an above-the-ground unheated hoop house. In another thread you mentioned you are in a recently upgraded zone 6. You would have to have well established plants, hardy to zone 3, to survive a zone 5 winter, even with the protection of the hoop house.

Your best bet is to buy your plugs next spring, and pot them up to pot-in-socket in ground installations, with plans to tend them for at least a full year before selling them. Pot-in-socket, inside a hoop house or cold frame would work for most ornamental grasses.

Actually, your best bet would be to provide at least some heat to the hoop house. Electric cables or warm water plumbing in the ground would do a world of good.

Thank you. This is just the kind of advice I needed. Much appreciated!