Historians believe people first began growing sunflowers (Helianthus annuus) more than 3,000 years ago in the western United States. Today sunflowers thrive throughout U.S. Department of Agriculture plant hardiness zones 2 through 11. Your sunflowers will be in full bloom within 80 days of sowing the seeds and are ready to harvest within 120 days of planting. Knowing not just when to plant your sunflower seeds, but how to plant them best, is essential for the tallest, biggest and healthiest sunflower plants.
Check Your Calendar
Wait to plant your sunflowers until the last frost date in your region has passed. Chilly, frosty conditions can kill young sunflower seedlings or stunt flower plant growth. In general, sunflower seeds will germinate best when the soil's temperature has reached a minimum of 55 to 60 degrees Fahrenheit.
Count the Sunny Hours
Before planting your sunflower seeds, watch your chosen planting site and monitor how much sun it gets. Sunflower plants thrive in full sun, meaning they need to receive at least six to eight hours of direct sunlight every day. More is better. For optimal results, you may wish to consider a planting site located on an east- or west-facing side of your property to maximize your sunflower garden's sun exposure.
Prepare the Soil
Once temperatures and sunlight hours indicate it's time to plant the sunflower seeds, pull out your garden spade and till your flowerbed's soil. Sunflowers have an extensive root network. Before planting, break up the soil to a depth of at least 24 inches. Then, mix in 3 to 4 inches of compost. Not only does this improve soil drainage -- essential for sunflower health -- but it also establishes all the nutrients your sunflower plant will need. Adding fertilization is optional. Should you want to, use a slow-release 12-4-6 or similar fertilizer at a rate of 1 pound for every 50 square feet of flowerbed.
You've laid the groundwork for healthy sunflower plants. Now it's time to sow the seeds. Use high-quality seeds from a garden store or nursery that's been certified as free of disease. Roasted sunflower seeds from a grocery store won't work, as the heat kills the seed. Bury the sunflower seed approximately 1 inch deep. If you're planting more than one seed, space them apart by 6 inches. This spacing works for most sunflower varieties, which tend to reach a mature height of 2 to 5 feet. If you're planting giant sunflowers that grow taller than 5 feet, space them apart by at least 12 inches.
Provide Post-Planting Care
Water your flowerbed once a day or as necessary to keep the soil moist. Once seedlings emerge and the plants are established, you can reduce watering to once a week. Infrequent yet deep watering -- applying enough water to moisten the soil to a depth of a couple feet -- encourages deeper root growth, which in turn creates healthier sunflower plants. When watering, apply the water at the sunflower plant's base instead of sprinkling water overhead, which can damage sunflowers. To help conserve soil moisture, add a 3-inch-thick mulch layer to your sunflower planting site.