Sunflowers have a very short growing season, going from seed to towering flower in about 2 to 3 months. Under the right conditions, they can grow 12 inches in a day, because of the efficiency with which they harness the sun's energy. Sunflowers are phototropic. In their bud stage, they faithfully follow the sun, which increases light exposure and photosynthesis. "After pollination, the disk faces down and east permanently to protect the seeds from solar radiation," says Dimitrov.
Despite their bright collars of petals, sunflowers, which come in single- and multistem varieties, aren't really single flowers but 1,000 to 2,000 individual flowers. The ray petals around the circumference have no stamens or pistils. Their function is to attract pollinators to the landing pad or "disk," since sunflowers rely heavily on bees and other pollinators to reproduce, says Dimitrov. Pollenless hybrid varieties were developed for the cut-flower industry (to avoid that "messy" yellow dust), so they're starlet pretty and a boon to the allergic gardener.
Sunflowers can grow just about anywhere. "You'll see them by the side of the road in New Mexico growing right through the pavement," says Dimitrov, who grew up in Bulgaria, where he recalls acres of golden sunflowers dotting the landscape every summer.
But they do have a few picky requirements. They prefer 6 to 8 hours of direct sunlight—even more if the purpose is record-breaking growth. Without it, plants grow spindly and fall over. Sunflowers are also heavy feeders, so it's best to plant in compost-enriched, well-drained soil after all danger of frost has passed, when the soil temperature is at least 50°F. Direct-sow seeds (to a depth of 1/4 to 1 inch) every couple of weeks to ensure a bumper crop through fall. Sunflowers are only middling drought-tolerant, so they may need some judicious watering throughout the season.