When to Plant Sunflowers

The first sowing: soil temperature, not the calendar

The last frost date tells you when the risk of frost ends. The soil thermometer tells you when sunflower seed will actually grow. Extension services set the soil threshold at different points:

• Clemson Extension (South Carolina): sow after frost danger has passed and the soil is above 50°F.
• UGA Extension (Georgia): wait several weeks after the last frost, until the soil is at least 60°F. In Georgia that means March in the south of the state and early May in the north.
• Utah State University Extension: sow once the soil reaches 45°F, starting 7 to 14 days before the last frost in northern Utah.

The differences come from climate. In Utah’s dry springs, seedlings came through 28°F in on-farm trials without damage, so an early start costs little. In the humid Southeast, the season is long enough that waiting for 60°F soil costs nothing.

The dates for your zone above begin at the average last frost and run about three weeks past it. In a cold spring, use the later part of that window.

Keep sowing every two weeks

A single-stem sunflower makes one flower and is finished, so a row sown on one day blooms all at once and then finishes. For flowers all summer, sow a short row every two weeks. NDSU Extension sows ProCut varieties every two weeks; UC Master Gardeners in Sacramento sow every 2 to 3 weeks; Utah State growers sow every 1 to 3 weeks. Branching varieties flower over a longer period and need fewer sowings.

Another way to stretch the season from a single sowing date is to mix varieties: NDSU notes that the Sunrich series blooms about 10 days after ProCut.

The last sowing date

Count back from your first fall frost. Mississippi State Extension stops setting out new sunflowers 60 days before the first frost, which fits varieties that bloom in about 50 days. Utah State’s last sowing in northern Utah falls 7 to 14 weeks before the first hard frost. Use the longer figure for slower or giant varieties, and check the days to bloom on the seed packet: if a variety needs 90 days, it needs 90 frost-free days after sowing, plus time for flowers to open.

If you want seed to eat or feed birds, plant early. Seed heads need 80 to 120 days to mature (UC ANR), and they are checked for ripeness in early fall, so a summer sowing of a seed type may not finish before frost in the North.

Timing by region

Cold north. In most of North Dakota, NDSU sows after the last frost in mid-May. The window from then to the first fall frost is short, so fast cut-flower varieties give the most successions.

Southeast and Gulf Coast. Start once the soil reaches 60°F and keep going all summer. A Mississippi State agent on the zone 7b/8a line starts seedlings in late March and has harvested flowers until November 1.

Texas. Texas A&M AgriLife in Bexar County (San Antonio) notes that late May is still not too late to direct-sow sunflowers.

Arizona low desert. The University of Arizona lists planting from February through mid-July, with blooms from May to October.

Florida. UF/IFAS says sunflowers are seeded from late winter to early spring, bloom in mid-spring and decline quickly as late-spring heat builds. Its calendar lists March, April and June for all three regions of the state. Fall plantings give a shorter display of several weeks.

Transplants change the timing slightly

Sunflowers are usually sown in place, but where birds or mice dig up seed you can start them in cell trays. A Mississippi State agent sows in trays and sets seedlings out 8 to 10 days later, just after the last frost. Utah State starts seed 2 to 3 weeks before transplanting and sets plants out once the danger of freezing has passed. Do not start them much earlier than that, because sunflowers are sensitive to root disturbance and older, root-bound seedlings transplant poorly.