Sow the seed where it is to grow in autumn or late spring. Alternatively, sow in trays placed in a cold frame in spring and transplant when large enough to handle. The seeds do require a cold period before they germinate, so spring-sown seed in trays should be watered, then kept in the bottom of the fridge for 4 and 12 weeks. Germination can be slow, but they are worth waiting for, and once they start to grow they are easy and undemanding plants, which if happy, will self-seed and form generous swathes. For this reason, it is currently illegal to plant, or cause it to grow, in the wild.