The common sunflower (Helianthus annuus), with its showy flowers and large stature, is a favorite summer plant that almost everyone recognizes. There are also about 70 other species of sunflower. Mostly native to North and Central America, sunflowers can be annuals or perennials and most display the cheery yellow flowers that make these plants such a perfect expression of summer. For gardeners seeking variety, sunflower cultivars are also available with red, orange, and cream flowers.
A sunflower plant's inflorescence is a complex structure, typical of plants in its family (Asteraceae) and designed to maximize attraction of pollinating insects. Each flowering structure is made up of hundreds to thousands of individual flowers, depending upon the variety, in complex patterns. The yellow petals are infertile ray florets, and the many small, interior disc florets are fertile and available for pollination.
Sunflowers are known and named for their ability to orient themselves in relation to the direction of sunlight when they are immature. While they are growing, these plants adjust the direction that the flowerhead is facing as the sun travels through the sky during the day. At night, they return to an east-facing orientation to await sunrise. They manage this movement by matching their stem growth to an internal circadian clock. The east-facing side of the stem grows more during the day and the west-facing side at night. This diurnal rhythm is called heliotropism and is different than the more common phototropism which has less dynamic reorientation during the day. While the growth is coordinated and conducted by internal chemistry, the movement also requires the external cue provided by the presence of the sun; plants grown in a lab without a moving light source do not exhibit the same dynamism. As plants mature and become more rigid in their stem structure, the rotating of the flower ceases and the flower fixes in an east-facing direction.
Recent research has determined that by facing the warm, morning sun, more bees are attracted to flowers at a time of day that they are warmth-seeking. The strong, direct sunlight also reveals markings visible to bees on the flowers that direct them to pollen. By facing east into the warming sun, plants produce larger seeds and release pollen earlier in the day, both competitive advantages. Additional research demonstrated that by facing east, plants grow larger and are able to pollinate more nearby plants than if they were otherwise oriented. The light responsive growth regulated by a natural clock is believed to represent an evolutionary adaptation.
Common sunflowers (Helianthus annuus) are native to the Western United States and Mexico. The dense concentration of pollen in a single flower makes this sunflower a pollinator's paradise. They need a sunny location and soil that drains well to prosper. Their large flowerheads will orient themselves facing east as they mature so choose a site that takes advantage of that feature; they are most delightful to view if you are facing west towards them.
These plants are annuals and do not survive the winter but are easy to start as seeds for a fresh display each summer. Each seed takes only about 2 1/2 to 3 months from sowing to flowering maturity. You can extend the floral display by starting seeds at different times so that the flowers are staggered through the summer. Amend the planting area in advance with organic compost to improve richness and drainage. Begin seeds outdoors after the last frost date for your area*, according to the instructions for depth and spacing on your seed packet. Large varieties need greater spacing to ensure robust growth, larger flower size and reduce the chance of fungal disease related to poor air circulation. You can find tips for successful seed sowing in our guide Seed Sowing and Planting Outdoors.
In our area, starting seeds outdoors in early May will result in flowers in late July and August. You can have flowers a few weeks sooner by starting your seeds indoors earlier in the season. Don't start too soon. Seeds will have germinated and be ready to plant outdoors in as little as three weeks. If you wait too long to move them, the plants may be weak and have less chance of maturing into vigorous plants. Plant these out a few weeks after the last frost date, when soil has begun to warm. Take time to transition the young plants to the new environment by bringing them outdoors for longer periods each day over approximately a week. You can find information for success in our Guide to Staring Seeds Indoors.
Flowerheads left on the stalk provide seed for migrating and overwintering birds. This is also the type of sunflower that produces seeds consumed by humans. If you would prefer to harvest the seeds you can cover the flowerheads with netting to reduce use by birds.
NYBG's Van Gogh's Flowers Exhibition features more than 25 common sunflower cultivars. For cut flowers, select non-pollen producing cultivars to avoid mess. Called male-sterile or CMS varieties, these flowers will still attract pollinators by offering nectar and will still form seeds if there are male-fertile sunflowers nearby.
Plants grow rapidly from seed to flower, just one flower per seed with the plants below unless indicated, and you can affect the size of the flower by offering the plant more or less space to grow. The longest lasting flowers come from the naturally colored golden varieties, harvested just as the flowerhead is opening. Here are varieties that you may have seen at the Van Gogh Exhibition to try growing at home.
Perhaps less well known than their annual relatives, most sunflower species are perennial plants. More than 30 of these are U.S. native plants. Perennial sunflowers generally need a sunny location and moist, well-drained soil (some exceptions are noted below), but can be resilient in challenging environments. They may need staking as some species grow tall. You can grow plants from seed, starting them indoors in spring. Flowers appear in late summer into fall. Leave the flowerheads on the plants through the winter to support birds in your environment.
NYBG's Van Gogh's Flowers Exhibition features numerous perennial sunflowers and cultivars, but there are also five varieties that are part of NYBG's living collection in the Native Plant and Perennial Gardens. These plants are area natives with the adaptability and benefit to wildlife, birds and pollinators that implies.
Previously known as Coreopsis angustifolius, this sunflower is a New York native plant that grows in moist to wet, acidic soil. It does best in full sun to partial shade, growing over 5 feet tall as an upright plant with small, 2-inch flower structures, gold with dark center. Attractive to mammals, birds in all seasons and pollinating insects, this is an excellent plant for supporting a rich variety of caterpillars. USDA zone 5 to 9.
A tall, easy care, New York native plant that grows in partial shade and reaches up to six feet tall. Bright yellow, daisy-like flowers with gold centers in July and August, earlier than most perennial sunflowers. Spreads on rhizomes to form a colony and needs dividing every four years. A valuable food source for native bees. USDA zone 3 to 8.
An Eastern United States native plant, small woodland sunflower is a host for painted lady and spring azure butterflies. Late-summer blooming flowers are just an inch in size on tall plants up to 6 feet. Flower structures have bright yellow rays with gold centers on deep red stems. Good for massing, these sunflowers form a spreading clump and grow in sun or partial shade in average soils. Hardy in USDA zones 4 to 8.
A New York native plant that grows well in partial shade to full sun and average soil, as long as it drains well. Named for its fuzzy, gray leaves, this plant grows 2 to 4 feet tall on spreading rhizomes and is ideal in a informal growing area. Plant in a protected spot or stake if in a windier area. A favorite of bees, birds and caterpillars so plant in the expectation that these sunflowers will have some chewed upon leaves as they support biodiversity. Hardy in USDA zone 4 to 9.
A Central and Eastern United States native plant that blooms in late summer. The fewleaf sunflower is more stalk and flower than leaves and one of the shorter sunflowers, reaching about 3 feet. Hardy in USDA zones 4 through 8, this sunflower thrives in average soil and full sun. Flowers are golden with cone-shaped golden centers and attract birds, painted lady butterflies and native bees. Spreads on rhizomes to form a colony and needs dividing every four years.
Notes: * The average day for the last spring freeze in Central Park, New York City is April 1 to 15th. In our tri-state metro area, the average date can be as late as April 30th. The date indicates the time when, on average, the chance of staying above freezing for the rest of spring is higher than the chance of falling below freezing again. Click here for an interactive map from NOAA to find the average date of last frost in your location in the United States. Bear in mind that these dates are averages and it is safer to err on the side of caution when planting. The actual experienced last frost date (or first frost-free date) can vary by several weeks in any given year. Always consult your local long-range weather forecast before making planting decisions
** DMR = Downy mildew resistant
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Understanding Perennials
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Perennials: the gardener's reference
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Diseases of Herbaceous Perennials
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The Perennial Gardener's Design Primer
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Annuals for Every Garden
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The Gardener's Practical Guide to Annuals, Bulbs and Perennials
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