Plant Story--the Prairie Sunflower, Helianthus petiolaris

They remind me of pinwheels

bright yellow flowers all along the highway. From western Nebraska and Kansas west across Colorado, they are prairie sunflowers, Helianthus petiolaris.


Actually, the USDA lists the prairie sunflower as occurring in all but five of the lower 48 states, and across central Canada. link 

prairie sunflower, Helianthus petiolaris

Sunflowers are native American plants, in the genus Helianthus, in the daisy, or sunflower, family, Asteraceae. North America has 52 Helianthus species and three other species can be found in Central America.

Calling something a "sunflower" was--and is--an obvious sort of name, so England used that name for a number of plants before America was discovered, and in the languages of Europe and Asia, other plants are "sunflowers." Helianthus plants, however, have been planted all over the world, since their seeds and oil make them a valuable crop and the flowers are attractive. So, increasingly, this is THE sunflower. The scientific name, created by Linneaus, in Sweden in the mid 1700s where they had already been grown for decades, says "sun flower", heli- from helios for sun, anthus for flower.

common sunflower, Helianthus annuus

The common sunflower, Helianthus annuus, is the species that is the source of sunflower seeds and sunflower oil, planted all over the world. It is the only uniquely North American plant that has become a major crop. The cultivated sunflowers, including the Russian sunflower, were all bred from  common sunflowers. The common sunflower was and is widespead in North America, but, being big, 6' tall is normal, requires more water than the prairie sunflower. In drier areas and especially on sandy soils, the abundant sunflower is the prairie sunflower, Helianthus petiolaris. Common sunflowers line the roads in the Corn Belt, prairie sunflowers replace them in the West, waving along roads and splashing yellow over fields and pastures.

prairie sunflowers

Both the common sunflower and prairie sunflower are annuals. The plant germinates, grows, flowers and produces all the seeds it will ever have within one growing season. That is a hazardous life-style:  what if there is a killing frost in June or is a long drought in July? If all the seeds had started to grow but no rain fell for six weeks (it happens in the West!) for miles and miles all the little sunflowers would die, leading to regional extinction.

The obvious solution is that not all the seeds germinate, even if it seems a VERY good year. Dormancy--not germinating--is a complex process. Why sunflower seeds remain dormant and how some germinate sooner than others is not well-understood. Right after harvest, barely 2% of wild seeds will germinate no matter how they are treated. After a winter in the soil, heavily watered, more than half of the seeds germinate. But not all. Those that remain dormant form a critical reserve so that if some weather event (or disease or plague of grasshoppers or...) kills every plant, there will be seeds to germinate next year. It will be several years, likely decades, before the very last seed from this year germinates.

Plant breeders select for reduction of dormancy and farmers try to provide excellent conditions for germination, reducing the impact of dormancy on crops.

Not only is the sunflower a native crop, it is a native weed. Most of the crop weeds of North American agriculture came from Europe, having adapted in the Old World to the rigors of growing quickly from seed to flowering before the farmer harvests the crop. The annual cropping systems of agriculture fit the annual sunflowers' lifestyle very well and they moved right in.

The result is that some parts of agricultural research are breeding better sunflowers and others are looking for better ways to eliminate sunflowers.

Both common and prairie sunflowers count on their showy flowers, abundant pollen and the drop of nectar in each tiny floret, to attract numerous insect pollinators. Growing well on disturbed sites is the first step to being effective weeds. Another crucial trait is to produce seeds under almost any condition and many, many seeds if possible. Some crop weeds multiply easily because they can produce seeds in complete isolation, either because they self-pollinate, putting pollen on their own stigmas (common poppy, common knotgrass) or bypass pollination altogether (dandelion). Flowering in an isolated room, sunflowers produce 1-2% of the seeds that are produced outdoors under natural conditions. The pollen from each little flower within the head will pollinate neighboring flowers, but the plants require insects to carry pollen between flowers. Most places have lots of bees, butterflies, moths and flies available to visit sunflowers and a flower head can produce several hundred seeds, one from each small floret.

Sunflower head showing florets 

If you haven't looked closely at a sunflower, there are two types of flower in the flower head. (Floret just means "little flower" and is used for the two types of flower in a sunflower family flower head. A good general term for the flower head is inflorescence). The ray florets each put out one big petal, making the show. The disc florets are the ones in the center and don't contribute a petal, but are more numerous. In the photo above disc florets form the central ring, the yellow dots are anthers with yellow pollen. See a diagram here: link. One of the strengths of the sunflower family is that the disc florets continue to open for many days, the open ones moving toward the center in a spiral, while the ray florets maintain the bright petals. That's why sunflowers, daisies, chrysanthemums and other members of the Asteraceae last so long in bouquets: it may seem an old inflorescence but new disc florets are still opening.

Prairie sunflowers generally have 10-30 ray florets and 50-100 disc florets; common sunflowers 17-30  ray florets and more than 150 disc florets. Each makes a single seed, so one plant, well-visited by pollinators, with a dozen flower heads should leave behind more than 1,200 seeds. That's a plus for the cultivated crop, and plant breeding has pushed those numbers much higher. That same ability to produce many seeds is annoying to farmers trying to grow corn or soybeans.

(A couple of sources said the ray florets of Helianthus annuus are sterile. I don't know if that applies to Helianthus petiolaris or to all the complex hybrids and varieties of cultivated sunflowers.)

wild common sunflowers, Helianthus annuus
the red is a parked pickup truck

More is known about the common sunflower than the prairie sunflower because it is a major crop worldwide. But in the western U.S., the prairie sunflower paints the landscape and roadsides yellow. It is smaller than the common sunflower, rarely taller than 3', with smaller inflorescences, smaller seeds and smaller leaves than the common sunflower. Where I live wild common sunflowers are not just bigger, but have triangular rather than long thin leaves, have obviously jagged edges to the leaves not smoothish leaf edges and are a different (browner) shade of green. The best character to recognize prairie sunflowers, though, is a white area, created by white hairs, in the middle of the inflorescence. ( See photos). Key in Flora of North America link

Where they grow together, the common sunflower and prairie sunflower can cross. The same pollinators are attracted to both and fertile hybrids form. Those have a mix of characters from both parents, so if you look at a plant and find it has characteristics that make you say "common sunflower, no prairie sunflower, no common sunflower..." you may be absolutely right, you are looking at a hybrid. Both species are now found all over North America, so sunflowers of mixed ancestry might occur anywhere.

prairie sunflowers

Native Americans domesticated the common sunflower some 5,000 years ago. But Moorman's Native American Ethnobotany (link) gives may historical uses for the prairie sunflower as well. Of course you can eat the seeds--they're just small. The Ramah Navajo sprinkled an infusion of prairie sunflower inflorescences onto clothes for luck in hunting. (Since wild sunflowers have a distinct odor, which I don't like, the scent would have helped conceal the hunter). The Hopi dried the yellow petals and powdered them to make a yellow ceremonial face powder for women.

And when the Hopi found prairie sunflowers with many flowers, they took it as a sign that there would be ample rain and a good harvest. This summer the plains around me are alight with prairie sunflowers in bloom. A good sign indeed.

Roadsides in a good prairie sunflower year--
and they are the yellow of the plains beyond, too

Comments and corrections welcome.

References
Collison, V. L. and R. L. Wilson. 1985. Comparison of honey bees vs hand and self pollination for obtaining seed from wild type sunflowers in cages. The Southwestern Entomologist. 10 (4) 268273.
Main structure of a sunflower pbiosunflower link
Pollination Sunflower pollination link
Marchetti, R. 2012. Evaluation of four treatments to break seed dormancy in sunflower inbreds. U. Tennessee, Martin link
Moorman, D. E. 1998. Native American Ethnobotany. Timber Press. Online: link
Schilling, E. 299. Helianthus Linnaeus Flora of North America link
Vujaković, M., V. Radić, V., Miklič, D. Jovičić, S.Balešević-Tubić, J. Mrđa, and D. Škorić. 2012 Seed dormancy of hybrids and parent lines of sunflower  (Helianthus annuus L.)

Helia 35, (56): 111-118. link

Kathy Keeler, A Wandering Botanist

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