Helianthus – Sunflowers: More than a Big Smiley Face

Who doesn’t smile when they see a field of sunflowers? Or feel happier when they put a few sunflowers in a vase? Most of us picture the familiar large-headed, folksy giant when we hear the word “sunflower.” This article will explore the different types of Helianthus sunflowers, including native perennials and annuals, and the many cultivars of the native annual sunflower. The Mexican annual sunflower in the genus Tithonia will also be discussed. 

HISTORY

The annual sunflower (Helianthus annuus) is one of the few crop species that originated in North America. It was probably a “camp follower” of several of the western native American tribes who domesticated the crop as a source of food. Native Americans selected a tall, single-headed variety for domestication before European explorers reached North America in the 16th Century. The first Europeans observed sunflowers cultivated in many places from southern Canada to Mexico.

The sunflower was probably first introduced to Europe through Spain and spread through Europe as an ornamental and medicinal curiosity until it reached Russia where the crop’s potential for oil production was established. According to data from 2016, Europe, Russia, and the Ukraine produce almost 70% of the world’s sunflowers.

PLANT DESCRIPTION

Native primarily to North and South America, Helianthus is a genus comprising 72 species (14 annual and 39 perennial) of flowering plants, plus 19 subspecies in the family Asteraceae.

The taxonomy gets very complicated; suffice it to say that the genus Helianthus is divided into four sections, including an annual section that includes both the cultivated native sunflower (common sunflower)  and the wild sunflower (sometimes referred to as  “cultivated wild relatives” or CWRs, an important source of genetic diversity used for breeding in commercial crop production). Most of the popular sunflower cultivars are hybrids of H. annuus.

Wild Helianthus annuus is a widely branched, annual plant with many flower heads. The branching sunflower grows 3-9’ tall, has small flowerheads and small seeds, and produces dozens of blooms over a peak bloom period of 3-4 weeks. The yellow blooms are 2-5” wide with dark centers. They look very much like black-eyed Susan (Rudbeckia), only the blooms are larger, and the plants are taller.

The domestic annual sunflower — the one grown for food — possesses only a single large inflorescence (flower head) atop an unbranched stem. The single-stem domesticated sunflower grows 5-10’; has coarse, hairy, ovate to triangular leaves up to 12” long; and a large seed head with yellow rays and a dark center disc. Flower heads are 3-6” across.

Both the single-stem and branching-stem native sunflowers are identified in the literature as Helianthus annuus L. and are referred to as “common sunflower.” H. annuus L. is known to be an aggressive spreader and is also considered a weed in many agricultural areas.  

Sunflower heads are made up of outer ray flowers and inner disc florets. The large petals around the edge of a sunflower head are individual ray flowers which cannot reproduce on their own and do not develop into seed. The disc florets have both male and female sex organs and can self-pollinate. The disc flowers give way to sunflower seeds.

USES

Sunflower seeds are typically used to make edible oil, bird seed, livestock feed, and food for snacking. Two basic types of sunflowers are typically grown: oil seed sunflowers and confection sunflowers. Oil seed sunflowers are primarily grown for producing sunflower oil and bird seed. Sunflower oil has been researched as a potential diesel fuel substitute, since sunflower oil has an energy content equivalent to 93 percent of number 2 U.S. diesel fuel.

Confection sunflowers produce large seeds on large heads and have black stripes along the seed coat. This type is primarily grown for human consumption. Growing sunflowers for decorative purposes, such as cut flowers in floral arrangements, is a fairly recent trend that has become more popular since the introduction of pollenless sunflowers.

HELIOTROPISM

Sunflowers have motor cells in the flower head, which move the head so that it faces the sun (Heliotropism). A sunflower “faces” or follows the sun east to west through the sky and returns at night to face the east, ready for the morning sun. Researchers say the young plant’s sun-tracking can be explained by circadian rhythms – the behavioral changes which follow a 24-hour cycle that is tied to an internal clock. The plant’s turning is a result of different sides of the stem elongating at different times of day. Studies have shown that this movement leads to increased vegetative biomass and enhanced pollinator visits. Heliotropism only occurs during early stages before the flowers form seeds. As the flower buds mature and blossom, the stem stiffens, and the flower becomes fixed facing the eastward direction.

ALLELOPATHY

Allelopathy is a biological phenomenon where one plant inhibits the growth of another through the release of allelochemicals.  This protective system reduces competition from plants nearby. Research that I found is focused on the effect of sunflowers on agricultural crops. Sunflowers  give off toxins from their roots, decomposed leaves, and stem leachates. Field studies have shown that the soil around sunflowers are rich in chemicals, with greatest concentrations under the aerial canopy of the plant, and decreasing with distance away from the plant. In the following season, crops in these fields did poorly not because of lack of nutrients, but because of sunflower residues and remaining chemicals. The efficacy of sunflower toxin is such that the sunflower extracts are being considered as potential organic herbicides. Potatoes are very sensitive to sunflower toxins, but not all plants are affected. Although I found some lists of plants that are considered immune to the toxic effects, I didn’t find supporting studies. The photo of  H. Anuus ‘Mammoth Russian’ shows that Russian sage (Salvia yangii, previously known as Perovskia atriplicifolia) is apparently not affected by sunflower toxins. Its height did prevent the sunflowers from getting enough sun in the early growth stage, and so I would not plant sunflowers in this spot next season. I’ll be interested to see if the Russian sage, a perennial, shows negative effects next year.

 

GROWING SUNFLOWERS

Sunflowers are easy to grow, and seeds can be sown directly outdoors into the ground or in containers, once the danger of frost has passed and the soil has warmed to above 50°F.  If you’ve had a problem with squirrels or birds eating the seeds, plants can be started indoors in pots for transplanting outside.  When plants emerge, thin every other seed, allowing 12-15” between plants. Rows should have a minimum of 2-3’ of space between them. Sunflowers do best when grown in well-draining, slightly acidic soil with a pH from 6.0-6.8 in locations that receive 6-8 hours of direct sunlight. Sunflowers, especially the multi-flowered, branching type or single, large-flowered cultivars, can be heavy soil feeders. Work organic matter into the bed before planting, and then apply a slow-release fertilizer as a side dressing a couple of times during the season. Irrigate soon after the application of fertilizer to allow it to move into the root zone of the plant.  Water them regularly 20 days before and after flowering to encourage root growth, which helps anchor taller sunflowers bearing top-heavy blooms. Once established, sunflowers can withstand some drought. Sunflowers generally grow well with 1 inch of water per week, whether from rain or supplemental irrigation. It is important to control weeds to prevent competition with the sunflowers for nutrition and moisture. As they mature, large sunflowers may need support to deal with strong winds or top heaviness from seed production.

EXTEND THE BLOOM

You can have sunflowers blooming all season long through succession planting. There are three ways of doing this:

• Plant all at one time multiple varieties that have different days to maturity.
• Plant the same variety at one- to two-week intervals.
• Plant multiple varieties with varying days to maturity at three- to four-week intervals.

Another interesting recommendation comes from Master Gardeners Olson, Liskey, and Habeck at the Oregon State University Extension Service who designed and cared for a sunflower demonstration garden. They planted 18 varieties of sunflowers arranged by height from 15”-12’. They found that the shortest sunflowers bloomed earliest. Based on their demonstration results, they recommend planting the tall varieties first, mid-sized varieties three to four weeks later, and the short sunflowers about two weeks after that.

PESTS AND INSECTS

Deer can destroy young, developing plants quickly. If deer have access to your sunflowers, repellents or fencing may be necessary. (See this article, “Deer, Deer, Deer!” from The Garden Shed.) Stink bugs, leaf-footed bugs, Japanese beetles, and aphids can be a problem. Be mindful that sunflowers also attract an enormous number of beneficial insects and pollinators. Use extreme caution if considering an insecticide. According to the Native Plant Finder by Zip Code, 66 native butterflies and moths use sunflowers as caterpillar hosts. See the top 15 caterpillars identified at this LINK. Sunflowers may also exhibit occasional fungal infections such as rusts and mildews. Powdery mildew and downy mildew are occasionally found on the leaves of sunflowers, typically affecting the oldest leaves first. If the plants are old enough and have vigor, they can normally grow despite these diseases.

SPECIES Native to the Charlottesville/Albemarle County Area

The Native Plant Finder by Zip Code shows 9 perennial sunflowers native to our area. See the table below for a summary of requirements by sunlight and soil. A brief description for each of these species is included after the summary table so that you can read more about the sunflowers best suited to your landscape.

• Ashy sunflower (Helianthus mollis)
• Giant sunflower (Helianthus giganteus)
• Maximilian sunflower (Helianthus maximiliani)
• Paleleaf woodland sunflower (Helianthus strumosus)
• Purpledisc sunflower (Helianthus atrorubens)
• Sawtooth sunflower (Helianthus grosseserratus)
• Smooth sunflower (Helianthus laevigatus)
• Thinleaf sunflower, ten-petaled sunflower (Helianthus decapetalus)
• Woodland sunflower (Helianthus divaricatus)

FULL SUN
Ashy sunflower	Well-drained, average, sandy, and poor rocky soil; avoid clay
Giant sunflower	Wet-to-moist sandy soil
Maximilian sunflower	Moist, clay-like soil; wide range of soils
Sawtooth sunflower	Moist, fertile loamy soil, high organic matter
Smooth sunflower	Dry, shale barrens, roadside, powerline corridors

PARTIAL SUN
Thinleaf or ten-petal sunflower	Moist to mesic conditions, rich loamy soil
Woodland sunflower	Dry to medium, average, well-drained, rocky or sandy soil; wide range of soils
FULL SUN TO SHADE
Paleleaf sunflower	Dry, well-drained, acidic soil
Purpledisc sunflower	Rocky, clay, or sandy soil

 

PERENNIAL SPECIES Native to the Charlottesville/Albemarle County Area

H. mollis – Ashy Sunflower, Hairy Sunflower, Downy Sunflower

This sunflower typically grows 2-4’ tall. Conspicuously pubescent (covered with short, soft hair), gray-green leaves and stems, project a grayish ash-like color, thus giving rise to the common name of ashy sunflower. Flowers develop singly from the upper stems, with each flower having a 4” diameter. Flowers bloom from July-September. Plant in full sun. Ashy sunflower generally tolerates a wide variety of well-drained soils, including average, sandy, and poor rocky soils. Avoid clay soils. Plants spread by rhizomes into broad clumps that are often difficult to properly maintain in a formal border. It is not described as deer resistant.

 

 

 

 

 

H. giganteus – Giant Sunflower

This wildflower grows 3-9′ tall, with a branched, purplish stem that bears terminal, yellow flowers that bloom July-September. Despite this plant’s name, its flower heads are comparatively small, 2-3″ across; the common and species names refer to the plant’s overall height. The root system has fleshy fibrous roots and shallow rhizomes. Small colonies of plants often develop from the rhizomes. It prefers full sun, wet-to-moist conditions, and sandy soil. Because giant sunflower occurs in wetlands, its large stalks are sometimes used by beavers in the construction of their dams and lodges. The stems and leaves can cause skin irritation in humans. The plant can be fatal to animals if ingested. 

 

H. Maximilinia – Maximilian Sunflower, Max Sunflower

This sunflower has several tall, leafy, branched stems that grow to 3-10’ tall. Leaves vary in length from 10” near the bottom of the stem to 2” near the top. Flowers are 5” across with yellow rays and darker yellow-to-brown centers. Bloom time is August-September. Although it prefers full sun in moist clay-like soil, it can tolerate a wide range of soils. This sunflower is eaten by many livestock but is reportedly NOT attractive to deer. A heavy crop of seeds makes it valuable for wildlife. This is a very big, vigorous grower that can form large colonies. It spreads via vegetative sprouting from a thick rhizome, and from seed. It should not be planted in the middle of flower beds as it eventually overtakes its neighbors.

 

H. strumosus – Paleleaf Sunflower, Paleleaf Woodland Sunflower, Woodland Sunflower

The stout, erect, mostly smooth stem of paleleaf woodland sunflower is branched toward the top and grows to 7’ tall. Narrow, oval leaves are up to 8” long and whitish underneath, giving the common name of paleleaf. Flower heads, 2-4” across with yellow rays and a yellow center, grow in loose clusters at branch tips from July-September. The plant grows in full sun to part-shade to shade and likes dry, well-drained acidic soils. It is found in dry, open upland woods and wood edges. It spreads by rhizomes and colonizes quickly. This plant is reportedly deer and rabbit resistant.

 

 

 

H. atrorubens – Purpledisc Sunflower, Appalachian Sunflower

This 3-4’ tall plant is found in rocky, clay, or sandy soils of woodlands and along road banks in full sun to partial shade. Daisy-like flowers are 2” wide with yellow rays surrounding a dark purple-brown disc. Several flowers bloom on individual 3″ pedicels (stems that attach a single flower to the inflorescence). It excels in open woodland gardens, cottage gardens, and mixed perennial beds. It has shallow rhizomes and is not described as an aggressive spreader. It blooms from July until the first frost. This is a larval host plant that supports Silvery Checkerspot butterfly (Chlosyne nycteis). It is deer and drought resistant.

 

H. grosserratus – Sawtooth Sunflower

Notwithstanding its name, the sawtooth sunflower has leaves that are toothless or only slightly serrated. This plant is 3-12′ tall, with considerable variation in the size of plants across different locations. Plants that grow in dense colonies are only 3-5′ tall, but “lone wolf” plants can achieve considerable height. Sawtooth sunflower has smooth reddish stems, with very little branching, except for some flowering stems that occur along the upper half of the plant. Each flowerhead is 2½-4″ across. The plant thrives in both disturbed and high-quality sites. It prefers full sun, moist soil, and fertile loamy soil with high organic content. However, this robust plant will tolerate other kinds of soil. Its roots are fibrous and rhizomatous, sometimes forming large colonies that exclude other plants. Sawtooth sunflower is reported to be both deer and rabbit resistant.

 

 

H. Laevigatus – Smooth Sunflower

This sunflower grows to 7’ tall and is abundant in Virginia. It produces umbels (short flower stalks which spread from a common point) of brilliant, greenish-yellow stellate (star-shaped) flowers from August-November. Smooth sunflower requires high light availability. In Virginia, West Virginia, and Maryland, its habitats include glades and shale barrens, woodland borders, dry roadsides, and powerline corridors. It is threatened primarily by fire suppression and by invasion of exotic plant species, such as spotted knapweed and Japanese honeysuckle. Other threats include development, mining for limestone, herbicide use, mowing during the growing and flowering seasons, and road construction. It spreads by means of underground rhizomes. It is not described as deer resistant.

 

H. decapetalus – Thin-leaf Sunflower, Ten-Petal Sunflower, Narrow-leaved Sunflower

This wildflower is about 3-5′ tall; it is erect and branches occasionally. The leaf blades are up to 7″ long and 3″ across; leaf margins are sharply serrated; and the tips of its leaf blades are unusually long and slender. Each flowerhead is about 2–3½” across, with bright yellow rays and tubular yellow disc florets. The bloom period is July-October. It prefers partial sun or dappled sunlight, moist to mesic conditions, and a rich loamy soil. This species may spread aggressively in some situations. It is attractive to deer. It is a larval host plant to many moths and butterflies, including the silvery checkerspot butterfly (Chlosyne nycteis), and painted lady butterfly (Vanessa cardui).

 

 

 

 

 

H. divaricatus – Woodland Sunflower, Rough Sunflower

This plant is easily grown in average, dry-to-medium, well-drained soil in part shade. It is found in rocky or sandy woodlands and in dry open sites and is tolerant of a wide range of soil conditions. Flowers are 2″ across with bright yellow rays, and slightly-darker-yellow center discs. The bloom period is July-September. Smooth stems grow 2-6’ tall with short-stalked 6” leaves. This sunflower is well-suited to a native/pollinator garden, naturalized area, or woodland garden, and is a good cut flower. Woodland sunflower is rhizomatous and forms colonies over time. Divide every 3-4 years to control spread and maintain vigor. It is reportedly deer and rabbit resistant.

 

 

 

 

PERENNIAL SUNFLOWER CULTIVARS

There are also cultivars of perennial sunflowers. The Chicago Botanic Garden Plant Finder is a good reference for information on both annual and perennial cultivars. Cultivars of the perennials native to our area include: H. maximiliani ‘Lemon Yellow’,  H. maximiliani ‘Dakota Sunshine’, and H. divaricatus ‘Chartreuse Butterflies’.

 

ANNUAL SUNFLOWER CULTIVARS

One of the most recognizable garden flowers is the classic annual sunflower Helianthus annuus. It has been hybridized into many varieties, including those with red, orange, and bicolored petals, dwarf and mammoth varieties, and those with double flower heads. The University of Minnesota Extension describes annual cultivars according to 5 categories: branching, pollenless, dwarf, semi-dwarf, and giant. It also provides a sample of popular cultivars for each. The following chart provides a summary. ‘Kong’ and ‘Tiger Eye’ were added as examples.

Branching	Pollenless	Dwarf	Semi-dwarf	Giant
Sonja	ProCut Series	Teddy Bear*	Cappuccino	Cyclops
The Joker	Double Quick	Sunspot*	Chianti	Mammoth Russian
Shock O Lat	Sunrich	Ms. Mars	Italian White	American Giant
Teddy Bear			Moulin Rouge
				Kong
				Tiger Eye

 

ANNUAL CULTIVAR CATEGORY SUMMARIES

Branching cultivars get quite large and produce an abundance of blooms over a long period. They require a good deal of room, so space them 18-24” apart.

Pollenless cultivars are used primarily as cut flowers or garden plants.  Pollen-free types do not contain any of the bright yellow pollen that can stain clothing or tablecloths. Heights range from 2-8’ with a variety of flower colors and forms.

Dwarf cultivars are 3’ tall or less. They generally work well in front of beds or borders, in limited spaces or in containers. *I planted both ‘Teddy Bear’ and ‘Sunspot’ cultivars this spring from seed. The first problem was that birds ate the planted seed. The second problem was that deer devoured the plants both before bloom and after bloom. See phot

Semi-dwarf cultivars are 3-8’ tall and usually do not require staking. These cultivars generally work well in the beds or borders of most home landscapes.

Giant cultivars grow to a height of 8’ or more. These cultivars sometimes require staking due to the size of the plants, flowers, and seed heads. Space plants about 2’ apart for good air-circulation.

 

TIPS FOR HARVESTING SUNFLOWER SEEDS

Sunflowers are mature when the back of the heads turn yellow and eventually brown and dry. Most of the yellow petals will have dried and fallen, with the seeds still plump. Seed heads can ripen on the plant, but they will need protection from birds. Try covering the heads with a paper sack or cheesecloth once the petals start turning brown if you want to protect the seeds.  See this article for information on how to prepare seeds for consumption by both birds and people.

POLLENLESS SUNFLOWERS

Pollenless sunflowers originated as a mutation, a genetic error. They are male-sterile: the hundreds of individual florets that compose the inflorescence of pollenless sunflowers produce no pollen. Although the flowers have nectar, bees would still need to collect pollen from another source. Pollen is a source of amino acids for bees and is used to feed young larvae. Making additional trips to get pollen and nectar from different flowers is less efficient. Other pollinating insects may or may not frequent pollenless sunflowers. Most pollinating beetles, for example, are pollen eaters and will avoid pollenless sunflowers. Butterflies and hoverflies, however, feed mostly on nectar and are unfazed by the lack of pollen. If there are bisexual sunflowers within pollinating distance of pollenless sunflowers, the pollenless flowers can be pollinated to some extent and produce seeds. But otherwise, no food for the birds! Pollenless sunflowers make good cut flowers but are not very beneficial to wildlife.

 

TITHONIA ROTUNDIFOIA – MEXICAN SUNFLOWER

Mexican sunflower is an annual in the family Asteraceae, but it is in the genus Tithonia, rather than Helianthus. It is native to Mexico and Central America. The species Tithonia diversifolia is listed by the University of Florida as being very invasive. It spreads by self-seeding and by stolons. The species T. rotundifolia is not invasive. This species will be discussed in this article.

T. rotundifolia grows 4-6’ tall (sometimes up to 10!), with a large central stalk. The solitary 3” flowers have orange to orange-red rays and orange-yellow disks. The flowers, which make nice cut flowers, are attractive to a wide variety of bees, butterflies, and hummingbirds. Flowers are produced from mid-summer until frost. The foliage and stems are covered with a soft downy fuzz, and the underside of the leaves are hairy. T. rotundifolia grows best in full sun in poor to average, well-drained soil. Avoid planting in rich soil and avoid heavy fertilizing; these practices will promote excess foliage and weak stems. Cut off the spent flowers to promote more blooming. Shelter from strong winds if possible; staking is also very helpful. The flowers are followed by grey-to-black flattened triangular seeds that are easy to collect for future planting. Tithonia can be grown from seed, either planted directly in the garden after the last frost date or, for earlier blooms, started indoors 6-8 weeks before the average last date of frost. Sow shallowly as light is required for germination. Although an annual, this plant will generously self-seed. Because of its tall stature and coarse texture, this sunflower is best used at the back of borders and beds to form a backdrop for shorter plants. It has few pests or disease problems and is not favored by deer. Watch for slugs and snails. Tithonia needs warm sunny weather to grow well so it may not do much early in the season. Be patient and you will be rewarded with blooms and pollinators!

 

SUMMARY

Although most people are familiar with the tall, large-headed annual sunflower, Helianthus annuus, there are many more sunflower varieties available. There are both native perennial species and numerous cultivars of H. annuus. They come in different sizes, forms, and colors to enliven the late-season garden, although some perennials start blooming as early as July and continue flowering until late fall! Cultivars range from a dwarf at 15” for the front of the garden, to mammoth varieties that tower in the back. You should be able to identify different native perennial species that match your landscape conditions. Both annuals and perennials can easily be grown from seed, although seeds for the less familiar perennial varieties may be more difficult to find. For the annual hybrids, remember that seeds of hybrids will not produce plants that are “true to seed” the next season. If you want consistency in plants from year to year, it is best to start with new seed. Perennial varieties can also be propagated by division. The annual varieties are very susceptible to deer damage, but some perennial varieties are described as deer resistant. Check for deer resistance by species. Perennial sunflowers are rhizomatous and most need room to spread. Some species are described as good additions to perennial borders; others are described as aggressive colonizers and are recommended for natural areas. The Mexican sunflower, Tithonia rotundifolia, is an annual sunflower that attracts many pollinators and offers a bright splash of orange-red in the summer-fall garden.

SOURCES

Native Plant Finder by Zip Code, https://www.nwf.org/NativePlantFinder/Plants/1561

“Sunflowers,” University of Minnesota Extension, https://extension.umn.edu/flowers/sunflowers#giant-cultivars-2103964

“Utilization of Sunflower Crop Wild Relatives for Cultivated Sunflower Improvement,” Crop Science, https://acsess.onlinelibrary.wiley.com/doi/10.2135/cropsci2016.10.0856

“Helianthus,” Chicago Botanic Garden Plant Finder, https://www.chicagobotanic.org/plantcollections/plantfinder?plant_search=helianthus

“Growing Sunflowers in the Home Garden,” University of Georgia Extension, https://extension.uga.edu/publications/detail.html?number=C1121&title=Growing%20Sunflowers%20in%20the%20Home%20Garden

“What is Heliotropism,” West Virginia University Extension Service, https://extension.wvu.edu/lawn-gardening-pests/news/2021/06/01/what-is-heliotropism

“Allelopathic Plants. 7. Sunflowers (Helanthus annuus L.),” Allelopathy Journal (2003), https://www.researchgate.net/profile/A-Azania/publication/287761119_Allelopathic_Plants_7_Sunflower_Helianthus_annuus_L/links/56e8a0ef08ae9bcb3e1cd8d3/Allelopathic-Plants-7-Sunflower-Helianthus-annuus-L.pdf

“Eliminating the Competition – Allelopathy in Plants,” Illinois Extension, https://extension.illinois.edu/blogs/naturalist-news/2021-02-12-eliminating-competitionallelopathy-plants

“Why All the Buzz About Pollenless Sunflowers?” Laidback Gardener, Welcome to Larry Hodgson’s World, https://laidbackgardener.blog/2018/02/26/why-all-the-buzz-about-pollenless-sunflowers/

“Giant Sunflower, Helianthus giganteus,” Illinois Wildflowers, https://www.illinoiswildflowers.info/wetland/plants/gnt_sunflower.html

“Helianthus atrorubens,” NC State Extension, https://plants.ces.ncsu.edu/plants/helianthus-atrorubens/

“Helianthus divaricatus,” Missouri Botanical Garden Plant Finder, http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=k390

“Helianthus divaricatus, Rough Sunflower, Woodland Sunflower,” NC State Extension, https://plants.ces.ncsu.edu/plants/helianthus-divaricatus/

“Sawtooth Sunflower, Helianthus grosseserratus,“ Illinois Wildflowers, https://www.illinoiswildflowers.info/prairie/plantx/swt_sunflowerx.htm

“Helianthus mollis,” Missouri Botanical Garden Plant Finder, http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=277330

“Helianthus laevigatus,” NatureServe Explorer, https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.140707/Helianthus_laevigatus

“Know Your Natives – Woodland Sunflower – Helianthus strumosus,” Arkansas Native Plant Society, https://anps.org/2016/09/21/know-your-natives-woodland-sunflower/

“Helianthus decapetalus,” NC State Extension, https://plants.ces.ncsu.edu/plants/helianthus-decapetalus/

“Ten-Petal Sunflower, Helianthus decapetalus,” Illinois Wildflowers, https://www.illinoiswildflowers.info/savanna/plants/tp_sunflower.htm

‘Sunflowers Burst onto the Scene with New Personalities,” Oregon State University Extension Service, https://extension.oregonstate.edu/news/sunflowers-burst-scene-new-personalities

“Sunflower Seeds Are for the Birds – and People,” Michigan State University Extension, https://www.canr.msu.edu/news/sunflower_seeds_are_for_the_birds_and_people

“Mexican Sunflower, Tithonia rotundifolia,” Wisconsin Horticulture Division of Extension, https://hort.extension.wisc.edu/articles/mexican-sunflower-tithonia-rotundifolia/

“Tithonia rotundifolia,” Missouri Botanical Garden Plant Finder, http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=b763

FEATURE PHOTO: H. annuus ‘Kong’ with bee. Photo: Melissa King