I understand that you can grow Asters in containers in a remote part of the
garden and move them into the border as late as 1st September where they
will flower perfectly happily. But that exercise only has value if you have
a plant to do the early shift.

Any suggestions for perennials the would flower early and, most important,
will not mind being moved into a pot whilst the Aster takes its place and
does its stuff. A Lupin would be ideal as it becomes an eyesore by late
summer- but it does not like being moved - does it? And any other
suggestions of perennial pairs to do a double shifts. The key must be
plants happy to be moved.