Sow on the surface of a good seed compost in late winter. Make sure that the compost is moist but not wet and seal in a polythene bag until after germination which usually takes 12-21 days at 15-21C (65-70F). Do not exclude light which is beneficial to germination.
They can also be sown in mid summer in shallow drills overwintered in a coldframe and planted out in spring 60cm (24in) apart. This produces larger plants.
Transplant when large enough to handle into 8cm (3in) pots and grow on in cooler conditions. Gradually acclimatise to outdoor conditions for 10-15 days before planting out after all risk of frost 38cm (15in) apart.