I'd wait until the first week in September if possible (you can still put them on your plant list and take dibs/requests so you know how many clumps). Then they'll be starting to go dormant rather than blooming/seeding full throttle. You could probably just bag them (grocery bags around the root clump with handles tied around the stems so the leaves stick up works great) rather than pot... they are tough and will survive in bags for a couple weeks as long as they don't totally dry out. If you tie up the clumps like that you shouldn't have to trim to save space... the plants will probably lose a few leaves on their own.
