How to grow the perfect Sunflower
Planting: If possible, choose a site in full sun on the north side of the garden, so the tall plants won’t shade your other vegetables. Sunflowers aren’t fussy about soil.
Seedlings are cold-resistant, so short-season growers may want to get a head start by planting several weeks before the last frost. In most areas, though, it’s best to wait until the soil is warmer, around the last frost date. Sow seeds 1″ deep and 6″ apart. Thin large types to 1 1/2′ apart and dwarf or medium-sized cultivars to 1′ apart. Water well after planting.
Apply a 3″-4″ layer of mulch to conserve moisture and keep down weeds. Sunflowers are drought-resistant, but they’ll grow better if you water regurlarly from the time the flowers begin to develop until they’re mature.
Sunflowers are remarkably trouble-free. Rotate crop if leaf mottle, a soil fungus that produces dead areas along leaf veins, becomes a problem. An early autumn may interfere with pollination and cause the plant to form empty seeds; plant earlier the next year. To protect seeds from birds, cover flowers with mesh bags, cheesecloth, old pantyhose, or perforated plastic bags.
Harvest as soon as seeds start to turn brown or the backs of the seed heads turn yellow. The heads usually droop at this time. Cut them along with 2′ of stem and hand upside down in a dry, well-ventilated place, such as a garage or attic, until fully dry; store in plastic bags for birds and animal food. To eat, soak overnight in water (or strong salt water, if a salty flavor is desired), drain, spread on a shallow baking sheet, and roast for 3 hours at 200°F or until crisp.
Guide from http://www.organicgardening.com/learn-and-grow/sunflowers-growing-guide