Purple tansy (Phacelia tanacetifolia) is often grown as a green manure or cover crop, but its slightly strange, long-lasting blooms make excellent cut flowers. They’re also one of the best food sources for bees, as well as other beneficial insects. The striking flowers reach anywhere between 50cm and 150cm high and bloom in spring and summer for several weeks.
Phacelia is a fast-growing annual, flowering 6-8 weeks after germination. It likes full sun and free-draining soil. In fact, phacelia grows well in dry soil. I’ve sown the seeds and forgotten about them and they’ve still flowered, a few weeks later, in their very shallow seed tray (perhaps a little stunted).
To get around that, sow seeds directly after the last frosts.