People grow sunflowers for a variety of reasons, among them, because they are attractive plants that produce an abundance of large, nutritious seeds. If you grow them to feed birds, stick with the black oil seed varieties like those found in birdseed mixtures. All sunflowers are edible, but if you have a craving for a bountiful crop of seeds for your eating enjoyment, choose large head varieties that produce plentiful big seeds. "Mammoth," "California Greystripe" and "Sunzilla" are all good choices for large flower heads and seeds.
Dig or loosen the soil with a spade or garden fork in a 2-foot circle to a depth of about 2 feet for each plant you wish to grow. Choose a sunny area of the garden with well-drained soil. Sunflowers may be spaced 6 to 12 inches apart, but for growing large heads, wider spacing is better.
Work a couple of shovels full of good quality compost and aged manure into the soil in each hole. The exact amount of amendment needed depends upon the soil fertility in your garden, as confirmed by soil tests, but this amount is usually sufficient for even the poorest soils. If necessary, you can obtain a soil test kit from your local soil and water department or county extension office.
Rake the area smooth. Push five or six seeds into the soil a few inches apart and about 1/2 inch deep within the circle. Cover the seeds with soil and firm the surface to ensure good soil-to-seed contact.
Water the spot well to settle the soil and make good contact with the seeds. Keep soil moist until the seeds germinate and seedlings emerge from the soil. Do not over water. You want soil that is moist but not soggy.
Select the three most vigorous seedlings to keep when seedlings are a few inches tall, and cut--do not pull--the rest at ground level to avoid disturbing the soil around the others.
Cull all but the strongest and healthiest plant in the circle when all are 1 to 2 feet tall, and allow that single plant to grow to maturity.
Water plants regularly and well, since roots may reach 4 feet below the surface.
Fertilize weekly with a dilute feeding solution. According to Renee's Garden, sunflower breeder Dr. Tom Heaton suggests digging a shallow “moat” around each plant about 18 inches in diameter for feeding. Pour liquid fertilizer in the moat rather than at the base of the plant, where excess water in the feeding solution may cause the stem to rot.
Cover flower heads with cheesecloth or a section of pantyhose when the petals begin to shrivel and fall, to prevent birds carrying off your harvest. Seeds are ready for harvest when the flower heads are completely dry and seeds are loose enough to shake or pull easily from the head.