Sowing: Direct sow Sweet William seed after the last spring frost, planting it just below the surface and compacting the soil slightly. To start the seed indoors 6-8 weeks before the last frost of spring, sow just below the surface of a flat; compress the soil slightly and keep it consistently moist. Keep the temperature at 60 degrees F until germination, which should take place within 14-20 days. Thin or transplant the seedlings.