All varieties of annual sunflowers (Helianthus annuum spp.) do their growing during one year. Depending upon variety, this can mean a plant that grows to just 2 feet or one that grows to 12 feet. Most annual sunflowers are hardy in U.S. Department of Agriculture plant hardiness zones 4 to 8, are not frost-tolerant and must be replanted each year.
Most people are familiar with sunflowers such as "Mammoth Russian," characterized by broad flower heads on tall stiff stalks. They do all their growing within a single season that runs from mid-spring to late summer when their large heads produce hundreds of seeds that are an important food crop for humans as well as birds. "Mammoth Russian" and "Giganteus" can grow to 12 feet and sometimes need support particularly in windy areas. Other large varieties that grow to 10 feet or more include "Kong," "Russian Giant" and "Giant Grey Stripe," all grown for seeds produced on heads that measure nearly 2 feet across. Dwarf varieties that grow to no more than 34 inches in a year include "Big Smile," "Elf" and "Kid Stuff." Flower size on smaller sunflower plants ranges from 5 to 10 inches in diameter.
It takes about 11 days for an annual sunflower seed to germinate and appear through the soil, and roughly 90 days to reach maturity. Sunflowers attain their maximum height in about 50 days, followed by head development and seed production. Each sunflower seed produces a single plant that grows on a main stalk with no side branches; and sunflowers are phototropic, which means that they turn in the direction of the sun as the day progresses. Sunflowers are considered drought tolerant because their extensive root systems are capable of traveling deeply into the soil in search of water.
All varieties do best in loose, rich, well-drained soil and tolerate a broad range of soil pH, or acidity, values ranging from 5.7 to 8. While weed control is important in the early stages of growth, once established, sunflowers grow vigorously enough to outpace most weeds. In keeping with their name, sunflowers need at least six hours of direct sunlight each day and may weaken and fall over if deprived of enough light. Sunflowers need soil temperature of at least 50 degrees Fahrenheit. Plant large varieties as soon as possible to allow them time to mature, and smaller varieties anytime through July as they have a shorter growing period. Plant seeds about 6 inches apart and the plants thinned out to increase the distance to roughly 2 feet apart.
Grown for their dense shrubbery and, in some cases, for their tasty roots, perennial sunflowers grow to heights of 4 to 12 feet, depending upon the variety. The Jerusalem artichoke (Helianthus tuberosus) grows from 3 to 12 feet in a year. The flowers are smaller than those of many annual varieties, and the plants are known for their sweetly flavored white tubers. Plants die back each fall and are regenerated in the spring, with new shoots developing from the plants' extensive roots systems that can become invasive if not controlled. This spreading habit also characterizes other perennial sunflower varieties, including swamp sunflower (Helianthus augustifolius), which grows from 4 to 8 feet, Helianthus "Lemon Queen" and Helianthus giganteus, which grows to 12 feet. Perennial sunflowers are grown from seed or from root sections called rhizomes, and are hardy in USDA zones 4 to 9.
- Seattle Central Community College: About Logistic Growth of a Sunflower Plant
- Purdue University: Sunflower
- Kansas State Department of Agronomy: Sunflower Production
- Alabama Cooperative Extension: Sunflowers Add Big Splash of Color Along Fencerows
- Washington State University: Sunflowers
- Perennialreference.com: Helianthus
- Purdue University: Jerusalem Artichoke
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