The Right Place
Sunflowers grow best in locations with full sun. They are remarkably tough and will grow in any kind of soil as long as it is not waterlogged. They do fine in soils that are acid to somewhat alkaline (pH 6.0 to 7.5). Once sunflowers get started, they can tolerate drought, as befits plants whose ancestors developed in dry prairie lands.
Growing Sunflowers from Seed Outdoors
Although you can start sunflowers indoors it is easiest to sow seeds directly into the soil after all danger of spring frost is past. If your growing season is short, you can even safely plant them up to 2 weeks before the last expected spring frost. Sunflowers can take a chill or two. It is best, though, to wait until the soil temperature reaches 55° to 60° F.
Prepare the planting area by clearing away any stones and debris. Dig down into the soil about 8 to 10 inches with a trowel or shovel or spade and turn it over to loosen and aerate it. If the soil is thin mix in some organic matter such as peat moss, compost or chopped leaves to improve its drainage and ability to hold moisture. Break up any large soil clumps, then smooth and level the area. This is a good time to sprinkle some all-purpose slow-acting granular fertilizer on the soil and mix it in.
Plant sunflowers in groupings or in rows.
To plant in rows:
Trace a shallow indentation in the soil with a stick or pencil to guide planting.
Then sow the seeds by dribbling them through your thumb and forefinger into the indented rows. Plant the seeds 1 to 2 inches deep, spaced 6 to 8 inches apart. They will sprout in 5 to 10 days.
When sprouts are a few inches tall and showing their first true leaves, thin them by pulling up the weaker ones, to allow space --about 2 feet--for the remaining ones to grow. Depending on the variety, sunflowers will grow to maturity and develop seeds in their centers in 80 to 120 days.
To maximize seed production plant seeds as above and in rows 2 to 3 feet apart. Use traditional, tall, seed-producing varieties such as ‘Mammoth’ or ‘Peredovic.’ To grow smaller flowers for bouquets, space plants much closer together—about 6 inches apart in Maine, as close as 2 inches in dry places like Arizona. Skip fertilizing. The plants will be much smaller, with fewer branches, but the stems are longer and flower heads are a good size for cut flowers. Sow a new row of sunflowers every 2 to 3 weeks to get a continuous supply of cut flowers.