Sow March to May thinly, 12mm (0.5in) deep in drills 30cm (12in) apart in well cultivated soil raked to a fine tilth. Germination usually takes around 21 days.
When large enough to handle, thin seedlings to 30cm (12in) apart. Alternatively, you can sow indoors in late winter/early spring for earlier flowering. Sow individually into 7.5cm (3in) pots at 20-30C (68-86F). When well grown, gradually acclimatise to outdoor conditions before planting out after all risk of frost.