It's best to sow sunflower seeds directly into the soil after the danger of spring frost is past. Ideally, the soil temperature has reached 55 to 60 degrees F.
Give plants plenty of room, especially for low-growing varieties that will branch out. Make rows about 30 inches apart. (For very small varieties, plant closer together.)
Plant the large seeds no more than 1 inch deep about 6 inches apart after it has thoroughly warmed, from mid-April to late May. You can plant multiple seeds and thin them to the strongest contenders when the plants are six inches tall.