By Botanical Interests
Photographs courtesy of Botanical Interests
The sunflower’s botanical name, Helianthus, comes from the Greek words, helios meaning "sun" and anthos meaning "flower." The sunflower is the state flower of Kansas. Sunflower buds (immature flowers) and leaves turn to follow the sun from east to west each day (a response called "heliotropism"), and once the flowers have fully opened, they stay facing east. By facing east, sunflower heads warm up quickly with the morning sun, attracting more pollinators than if they were west-facing and cooler. Also, remaining east-facing may protect them from possible sun-scald from the strong afternoon sun. Wild sunflowers, from which all other sunflowers come, grow in prairies, dry, open areas, and along roadsides throughout North America from central Canada to northern Mexico.
Sunflowers bloom 65–85 days from sowing depending on variety; add one to two weeks if they are transplanted.
When to sow outside: Recommended. One to two weeks after your average last frost date.
When to start inside: Two to four weeks before your average last frost date. Sunflowers are sensitive to root disturbance; sow in biodegradable pots that can be planted directly in the ground.
Use a lightweight seed-starting mix/medium (sterile, and lighter than potting mix), and sow two to three seeds per pot, ¼"–½" deep. Sunflowers have a long taproot, which is sensitive to transplanting, which is why direct sowing is our recommendation. However, sowing in 3"–4" biodegradable paper pots that can be planted directly into the ground will minimize root disturbance.
Thinning: Thin to one plant once leaves appear (clip extra plants at the soil level using scissors). The strongest plant may not be the tallest; look for thick, strong stems and deep color. By thinning early, you minimize the negative impact of crowding, like stretching for light. Transplanted sunflowers are usually not as vigorous as direct-sown sunflowers.
Transplanting: Be sure to harden-off seedlings before transplanting. Hardening off is the 7 to 10-day process of introducing pampered seedlings to the intense outdoor sun and temperature swings.
Transplant to an area of full sun (six or more hours a day), after your average last frost. Transplant on a cloudy day or in the evening to reduce stress. Remove the bottom of the biodegradable pot when transplanting into the planting hole. This allows roots to escape easily while the remainder of the pot breaks down.
Soil: Amend the soil ahead of time. We suggest submitting a soil test periodically, which gives you detailed information on your soil and how to improve it. Over-fertilization can invite pests, reduce flowers in favor of leaf growth, impact flavor, burn plants, or be a pollutant. Work the soil so it is clump free, allowing it to drain well.
How to sow: Sow single stem, large-headed sunflowers as three seeds every 1'–2' apart and ¼"–½" deep. Sow multi-stem sunflowers 2' apart. Professional cut-flower growers reduce this spacing by half resulting in longer stems and smaller flowers, which are more desirable for bouquets. By sowing single-head sunflowers monthly, you ensure continuous blooms, whereas multi-stem sunflowers bloom until the weather cools.
Fertilization: Fertilizer isn't needed, but a monthly side dressing of compost or fertilizer can result in larger flowers if the soil is lean. Use a nitrogen-rich or balanced fertilizer (encouraging green growth) for four to six weeks, followed by a phosphorous-rich fertilizer (encourages large blooms) as the time for flowering approaches.
Water: Sunflower stems are hollow like straws and will take a lot of water, although they will not tolerate being waterlogged. They are also drought tolerant once established, but as with lean soil, the size of the plants and flowers can be impacted by drought conditions.
Flowers: Cut flowers as they just begin to unfurl their petals. Harvest flowers in the morning if possible, and place cut stems in water as soon as possible. For more information see our article on Making Cut Flowers Last Longer.
Seeds: Sunflower seeds take about four weeks to ripen, once pollinated. To help protect sunflower seed heads from birds and squirrels as the seeds ripen, cover the heads with pantyhose, a small-mesh lingerie bag, cheesecloth, or similar type of fabric that provides ventilation. Wait to cover the flower until you no longer see bees flying around to ensure good pollination. Tie the protective covering securely at the base of the flower with twine or a twist tie. You can also cover the seed head with a paper bag but it may disintegrate during rainy weather. Don't use a plastic bag as it may cause seeds to rot due to lack of ventilation.
Seeds are ready to harvest when the back of the head has dried and turned brown. Petals will have wilted and fallen off, and the seeds will be plump and colored white with gray/black stripes. Seeds ripen around the outside of the flower first; the ones in the center will ripen last.