Aster x frikartii 'Monch':
Soft lavender blue flowers in late summer. Very early flowering and shorter than many asters at about 80cm. Easy and reliable.
Echinops ritro (Globe Thistle):
Blue globes held above stout grey green foliage. Attractive to bees. Likes well drained soil and plenty of sun. Hardy. Up to 1m.
[Collect Only]
Salvia uliginosa (Bog Sage):
Branched stems of clear blue flowers and lance-shaped leaves. Needs a sunny spot but with moist soil. Late flowering. To 2m.