Ageratum houstonianum / Eupatorium coelestinum
Ageratum houstonianum / Conoclinium coelestrium / Eupatorium coelestinum
The perennial or hardy ageratum seems to be named as any one of the above. Regardless of the name, the plant they are all describing is an easy to grow perennial. Ageratum houstonianum provides a spectacular display of tall, soft blue fluffy flower heads, about 90 cm high, through summer and early autumn.
It will not tolerate severe winters, but if planted in a protected area it normally survives and may just need a cutback, before shooting away again from the base and framework of the stems. It is a risk that I think is well worth it for the beautiful soft blue colouring it provides. It is also an attractor to bees and butterflies.
They prefer a good, well-drained but moist garden soil in full sun or partial shade and like a bit of shade in places with hot summers.