Sow indoors from March to April at a temperature of 15-25C (59-77F). After sowing, keep the surface of the compost moist but not waterlogged and do not exclude light as this helps germination. Germination usually takes 7-14 days. When seedlings are large enough to handle, transplant and grow them on in cooler conditions. When zinnia plants are well grown and all risk of frost has passed, gradually acclimatise them to outdoor conditions for 10-15 days before planting outdoors.