Helianthus annuus / sunflower

Adjectives: annual, disturbed, dramatic, edible, exposed, highly visible, interesting bits, invasive, native, ruderal, showy, weed

Also known as: annual sunflower, common sunflower, sunflower
See also: Helianthella uniflora / little sunflower

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The wild sunflower is a widely branching, stout annual growing up to 8 ft. tall. It is commonly found along roads, fences, fields and in waste areas.

Given that, it’s a bit surprising that it could be the state flower of Kansas. But there’s a difference. The sunflower known as a crop and fed to birds and squirrels and in trail mix and foods as oil is not the same as the wild one, albeit they are the same species. Hybridization has produced many cultivars that have higher yields and it is these that farmers use. The wild progenitor is now a weed, sometimes invasive, and in some states considered noxious; perhaps as many of us considered our own parents at some time in our lives.

The leaves of sunflower are alternate and large, up to 15 x 15 inches, and coarsely hairy. They droop downwards from long petioles. In shape, they are typically ovoid, deltoid or lanceolate. The margins are finely toothed (dentate), and the veins create channels for water to flow off.

Sunflowers inflorescences are like very large yellow daisies ranging from 3 to 6 inches across, but each flower, or floret, is teeny, only 1/8 in across. The 100 to 150+ disk florets are yellow to brown. With a hand lens, you can see the distinct tips of 5 petals in each floret. It is these florets that are fertile and produce the seeds. The ones toward the outside of the disk open first. The much larger heads on commercial varieties have more disk florets, reflecting the over-all size of the inflorescence.

By contrast, the ray florets number only 20-40 and are definitely yellow. It is these that make up the “petals”.  On the “back side” of the heads are conspicuous floral bracts, dull green and stiffly hairy.

Compared to the little sunflower, the disk of the true sunflower is much larger and the ray florets much more numerous.

After pollination, the disk florets are replaced by a large seed that is ovoid and somewhat flattened. The fruits are edible in many forms and recipes. Steamed buds supposedly have an artichoke flavor. Petals of open flowers have a bitter-sweet flavor.

Sunflower is native to North America and was originally a species of the prairie and other grasslands. Now, it can be found especially in disturbed sites and openings, abandoned fields, pastures, roadsides, along railroads, and waste areas.

Interesting bits – there are lots of them. This is a very cool plant.

First, the roots of sunflowers may exude an allelopathic chemical that can inhibit the growth of other plants. The conditions for this happening are probably limited to cultivated fields where it seems to have sometimes been used for weed control. On roadsides, for example, the competition is probably just too much to observe this.

Next, in a 2008 article in the Proceedings of the National Academy of Sciences,  Lenz et al. 2008 reported a study showing that sunflower was domesticated in Mexico ca.2600 BCE. It was widely cultivated in Mesoamerica by 1000 BCE. It was well known to the Aztecs, and it is still in use by traditional cultures today. Sunflower was associated with indigenous solar religion and warfare in Mexico. This may have led to its suppression after the Spanish Conquest.

Third, the disk flowers are arranged spirally. Generally, each floret is oriented toward the next by approximately the golden angle, 137.5°, producing a pattern of interconnecting spirals, where the number of left spirals and the number of right spirals are successive Fibonacci numbers. Typically, there are 34 spirals in one direction and 55 in the other; however, in a very large sunflower head, there could be 89 in one direction and 144 in the other. This pattern produces the most efficient packing of seeds mathematically possible within the flower head, and it’s pretty. (Wikipedia)

Fourth, the sunflower genome is huge, perhaps as big as 3.5 billion base pairs (3500 MBp). That is about the same size as the human genome and perhaps 1000x as large as the best studied plants. At the same time, it is infinitely more useful than the best studied plants.

Fifth – sunflower can be used in phytoremediation to extract toxic ingredients from soil, such as lead, arsenic and uranium. It can also be planted to filter radioactive and other toxic ingredients and harmful bacteria from water. For example, sunflowers were used to remove caesium-137 and strontium-90 from a pond near the Chernobyl disaster. A similar planting was done after the Fukushima nuclear disaster in Japan.

Sixth – sunflowers are often appreciated because they track the sun through the day. This phenomenon occurs from early development, through the young flower stage, and up to full maturity of the flower heads (anthesis). At dawn the young flower heads face east and rotate through south, to west through the day. Since plants don’t have muscles, this movement reflects differential growth rates around the stem. It stops when the flowers reach maturity because stem growth is also done then. At that point, they just stay facing east. During the younger stages, the rotation is not directly caused by light, but by an innate circadian rhythm. That means, essentially, that it happens even on cloudy days or in experimentally manipulated constant light. If you try to mess up the plants, say in pots, by turning them 180˚, within a few days they will have figured it out and resynchronize.