The sunflower (Helianthus annuus) grows well in U.S. Department of Agriculture plant hardiness zones 1 through 11. These annual plants are tall, with a height of 8 to 15 feet. The plant produces a large head of seeds that can be saved for planting the following season or left as an ornamental feature that birds eat. Sunflower seeds are simple to plant in either the winter or spring season, depending on your preference.
Sunflower seeds planted either indoors or outdoors require a minimum soil temperature of 46 to 50 degrees Fahrenheit in order to stimulate germination. Seeds can germinate in a soil temperature lower than 46 degrees Fahrenheit, but the germination rate significantly drops.
Test the soil temperature when you are ready to plant seeds in spring by inserting a thermometer a minimum of 1 inch into the soil to verify the temperature before planting. The best planting date varies by geographical location and can range from February to May.
Start sunflower seeds indoors in late winter to produce hardy seedlings for planting directly into the garden. Sow the seeds into a growing tray about six weeks before the approximate last spring frost for your area. The seedlings can be transplanted directly into the garden when the risk of frost is gone.