This chaste tree does better here in our zone 7 humid summer conditions and of course further South as well. In zones 5 and 6 it is often used as summer shrub, much like Buddleia. Diane has pruned our larger trees much like a crape myrtle into attractive 'see through' understory trees with multiple trunks. The blue spiked flowers attract many insects, and butterflies. The leaves are highly aromatic. The plants bloom pretty much all summer.
 
Zone 5-9.