Direct seed sunflowers in spring after the last frost. Plant seeds 1 inch deep and thin to 30-60 cm when the first true leaves appear. Sunflowers will often self-seed giving you years of flowers from a single sowing. To harvest flowers, cut when just beginning to open. To harvest seeds allow heads to dry on stalks then cut heads and remove seeds. It helps to cover the heads with row fabric while they are drying to keep birds and squirrels from getting all your seed.