Hi, welcome to GoY, you can sow the seed now, in a tray with a clear vented cover, and put in a cold frame or cold greenhouse, or you can sow the seed direct where you want them after the last frost, Derek.

They are easy to grow. Follow the advice given by Derek.
welcome to go too.

Be aware this can be an invasive species, almost impossible to get rid of!

This is one of the clump forming, upright perennials so non invasive. I grew it easily from seed several years ago and have a small area for it as it's good for the bees.
It's been a beautiful addition to the garden.