Idaho’s Native Milkweeds

Concern for monarch butterflies has resulted in increasing interest in milkweeds. Understandably so, as they are the host plants and food source for the larval stage of these migrating butterflies. But milkweeds are an impressive group of plants in their own right, and their ecological role extends far beyond a single charismatic insect. Work to save the monarch butterfly, which requires the expansion of milkweed populations, will in turn provide habitat for countless other organisms. A patch of milkweed teems with life, and the pursuit of a single caterpillar helps us discover and explore that.

Asclepias – also known as the milkweeds – is a genus consisting of around 140 species, 72 of which are native to the United States and Canada. Alaska and Hawaii are the only states in the United States that don’t have a native species of milkweed. The ranges of some species native to the United States extend down into Mexico where there are numerous other milkweed species. Central America and South America are also home to many distinct milkweed species.

The habitats milkweeds occupy are about as diverse as the genus itself – from wetlands to prairies, from deserts to forests, and practically anywhere in between. Some species occupy disturbed and/or neglected sites like roadsides, agricultural fields, and vacant lots. For this reason they are frequently viewed as a weed; however, such populations are easily managed, and with such an important ecological role to play, they don’t deserve to be vilified in this way.

Milkweed species are not distributed across the United States evenly. Texas and Arizona are home to the highest diversity with 37 and 29 species respectively. Idaho, my home state, is on the low end with six native species, most of which are relatively rare. The most common species found in Idaho is Asclepias speciosa commonly known as showy milkweed.

showy milkweed (Asclepias speciosa)

Showy milkweed is distributed from central U.S. westward and can be found in all western states. It occurs throughout Idaho and is easily the best place to look for monarch caterpillars. Side note: the monarch butterfly is Idaho’s state insect, thanks in part to the abundance of showy milkweed. This species is frequently found growing in large colonies due to its ability to reproduce vegetatively via adventitious shoots produced on lateral roots or underground stems. Only a handful of milkweed species reproduce this way. Showy milkweed reaches up to five feet tall and has large ovate, gray-green leaves. Like all milkweed species except one (Asclepias tuberosa), its stems and leaves contain milky, latex sap. In early summer, the stems are topped with large umbrella-shaped inflorescences composed of pale pink to pink-purple flowers.

The flowers of milkweed deserve a close examination. Right away you will notice unique features not seen on most other flowers. The petals of milkweed flowers bend backwards, allowing easy access to the flower’s sex parts if it wasn’t for a series of hoods and horns protecting them. Collectively, these hoods and horns are called the corona, which houses glands that produce abundant nectar and has a series of slits where the anthers are exposed. The pollen grains of milkweed are contained in waxy sacs called pollinia. Two pollinia are connected together by a corpusculum giving this structure a wishbone appearance. An insect visiting the flower for nectar slips its leg into the slit, and the pollen sacs become attached with the help of the corpusculum. When the insect leaves, the pollen sacs follow where they can be inadvertently deposited on the stigmas of another flower.

Milkweed flowers are not self-fertile, so they require assistance by insects to sexually reproduce. They are not picky about who does it either, and their profuse nectar draws in all kinds of insects including bees, butterflies, moths, beetles, wasps, and ants. Certain insects – like bumble bees and other large bees – are more efficient pollinators than others. Once pollinated, seeds are formed inside a pod-like fruit called a follicle. The follicles of showy milkweed can be around 5 inches long and house dozens to hundreds of seeds. When the follicle matures, it splits open to release the seeds, which are small, brown, papery disks with a tuft of soft, white, silky hair attached. The seeds of showy milkweed go airborne in late summer.

follicles forming on showy milkweed (Asclepias speciosa)

Whorled or narrowleaf milkweed (Asclepias fascicularis) is widespread in western Idaho and neighboring states. It is adapted to dry locations, but can be found in a variety of habitats. Like showy milkweed, it spreads rhizomatously as well as by seed. Its a whispy plant that can get as tall as four feet. It has long, narrow leaves and produces tight clusters of greenish-white to pink-purple flowers. Its seed pods are long and slender and its seeds are about 1/4 inch long.

flowers of Mexican whorled milkweed (Asclepias fascicularis)

seeds escaping from the follicle of Mexican whorled milkweed (Asclepias fascicularis)

Swamp or rose milkweed (Asclepias incarnata) is more common east of Idaho, but occurs occasionally in southwestern Idaho. As its common names suggests, it prefers moist soils and is found in wetlands, wet meadows, and along streambanks. It can spread rhizomatously, but generally doesn’t spread very far. It reaches up to four feet tall, has deep green, lance-shaped leaves, and produces attractive, fragrant, pink to mauve, dome-shaped inflorescenses at the tops of its stems. Its seed pods are narrow and around 3 inches long.

swamp milkweed (Asclepias incarnata)

Asclepias cryptoceras, or pallid milkweed, is a low-growing, drought-adapted, diminutive species that occurs in southwestern Idaho. It can be found in the Owyhee mountain range as well as in the Boise Foothills. It has round or oval-shaped leaves and produces flowers on a short stalk. The flowers have white or cream-colored petals and pink-purple hoods.

pallid milkweed (Asclepias cryptoceras)

The two remaining species are fairly rare in Idaho. Antelope horns (Asclepias asperula) is found in Franklin County located in southeastern Idaho. It grows up to two feet tall with an upright or sprawling habit and produces clusters of white to green-yellow flowers with maroon highlights. Horsetail milkweed (Asclepias subverticillata) occurs in at least two counties in central to southeastern Idaho and is similar in appearance to A. fascicularis. Its white flowers help to distinguish between the two. Additional common names for this plant include western whorled milkweed and poison milkweed.


Tiny Plants: Draba verna

Draba verna is a small but memorable plant. Common names for it include early whitlowgrass, vernal whitlowgrass, and spring whitlow-mustard. Sometimes it is simply referred to as spring draba. As these common names suggest, Draba verna flowers early in the spring. It is an annual plant that begins its life by germinating the previous fall. While its flowers are minuscule, multiple plants can be found packed into a single section of open ground, making their presence more obvious. This and the fact that it flowers so early, are what make it so memorable. After a cold, grey winter, our eyes are anxious for flowers, and even tiny ones can be enough.

Draba verna

Draba verna is in the mustard family (Brassicaceae), which is easy to determine by observing its flowers and fruits. The flowers are about 1/8 inch across, with four, deeply-lobed petals. The fruits are oblong, “football-shaped,” flattened capsules that are divided into two chambers and hold up to forty seeds or more. Flowers and fruits are borne at the tips of branched stems that are leafless, hairless, and very thin. Stems arise from a small rosette of narrow leaves that are green to purplish-red and slightly hairy. The plant itself is generally only an inch or two wide and a few inches tall, easily missed other than its aforementioned tendency to be found en masse.

flowers of Draba verna via

Draba verna occurs throughout much of eastern and western North America, but is said to be introduced from Eurasia. A few sources claim that it is native to North America, but as far as I can tell, that is unverified. Either way, it is naturalized across much of its present range, and even though many of us consider it a weed, it doesn’t seem to be causing too much concern. It’s too tiny and short-lived to really be a problem. It makes its home in disturbed and neglected sites – along roadsides; in fields, pastures, and garden beds; and in abandoned lots. The one place it may be trouble is in nurseries and greenhouses, where it might be able to compete with young plants in pots.

open capsule and seeds of Draba verna via

The flowers of Draba verna are self-fertile, but they are also visited by bees that have ventured out in early spring. The foliage might by browsed by rabbits and other small mammals, but otherwise this plant is of little use to other creatures. Being in the mustard family, it is likely edible, but again it is so small that harvesting it would hardly be worth it. Instead, maybe its best to leave it in place and enjoy it for what it is: a tiny, brave reminder that spring is on its way and an encouragement to get down low once in a while to admire the little things.

An attempt at sketching Draba verna fruits on a raceme.

See Also: Tiny Plants: Duckweeds

Awkward Botanical Sketches #1

At the beginning of the year I unveiled my plan to share some of my sketches with you as I learn how to draw. This is to make up for not writing quite as many posts so that I can spend time working on some other projects. It also serves as a great motivator to actually draw, which isn’t something I do very often. Turns out that if you want to get better at something, you actually have to do it.

To help me in my quest, I collected a few books. Some are instructional and others simply feature inspirational artwork. I’ve included links to a few of these books with my drawings below. If you have any books you would like to recommend, particularly a book that has helped you learn to draw, please let me know in the comment section below.

And now on to my dumb drawings…

My first drawing in Drawing Nature by Jill Bliss

Drawing of a hibiscus flower with help from Illustration School: Let’s Draw Plants and Small Creatures by Sachiko Umoto

A sketch inspired by Carcassonne: Over Hill and Dale

Sketch of an old tree inspired by a drawing in Clare Walker Leslie’s book, Drawn to Nature

Sketch of agave in bloom inspired by an image on the back of some guy’s shirt at Treefort Music Fest

Sketch of a tiny tuft of grass I was trying to identify. It’s still a bit of a mystery.

A Few Fun Facts About Pollen

Sexual reproduction in vascular plants requires producing and transporting pollen grains – the male gametophytes or sperm cells of a plant. These reproductive cells must make their way to the egg cells in or order to form seeds – plants in embryo. The movement of pollen is something we can all observe. It’s happening all around us on a regular basis. Any time a seed-bearing plant (also known as a spermatophyte) develops mature cones or flowers, pollen is on the move. Pollen is a ubiquitous and enduring substance and a fascinating subject of study. In case you don’t believe me, here are a few fun facts.

Bee covered in pollen – photo credit: wikimedia commons

Pollen is as diverse as the species that produce it. Pollen grains are measured in micrometers and are so tiny that the only reason we can see them with the naked eye is because they are often found en masse. Yet they are incredibly diverse in size, shape, and texture, and each plant species produces its own unique looking pollen. With the help of a good microscope, plants can even be identified simply by looking at their pollen. See images of the pollen grains of dozens of plant species here and here.

Pollen helps us answer questions about the past. Because pollen grains are so characteristic and because their outer coating (known as exine) is so durable and long-lasting, studying pollen found in sediments and sedimentary rocks helps us discover all sorts of things about deep time. The study of pollen and other particulates is called palynology. Numerous disciplines look to palynology to help them answer questions and solve mysteries. Its even used in forensics to help solve crimes. Criminals should be aware that brushing up against a plant in bloom may provide damning evidence.

Pollen oddities. While all pollen is different, some plants produce particularly unique pollen. The pollen grains of plants in the orchid and milkweed families, for example, are formed into united masses called pollinia. Each pollinium is picked up by pollinators and transferred to the stigmas of flowers as a single unit. A number of other species produce other types of compound pollen grains. The pollen grains of pines and other conifers are winged, and the pollen grains of seagrass species, like Zostera spp., are filamentous and said to hold the record for longest pollen grains.

The pollinia of milkweed (Asclepias spp.) look like the helicopter-esque fruits of maple trees. photo credit: wikimedia commons

Pollen tube oddities. In flowering plants, when pollen grains reach the stigma of a compatible flower, a vegetative cell within the grain forms a tube in order to transport the regenerative cells into the ovule. This tube varies in length depending on the length of the flower’s style. Because corn flowers produce such long styles (also know as corn silk), corn pollen grains hold the record for longest pollen tube, which can measure 12 inches or more. Species found in the mallow, gourd, and bellflower families produce multiple pollen tubes per pollen grain. Hence, their pollen is said to be polysiphonous.

Pollen is transported in myriad ways. Plants have diverse ways of getting their pollen grains where they need to be. Anemophilous plants rely on wind and gravity. They produce large quantities of light-weight pollen grains that are easily dislodged. Most of this pollen won’t make it, but enough of it will to make this strategy worth it. Hydrophilous plants use water and, like wind pollinated plants, may produce lots of pollen due to the unpredictably of this method. Some hydrophilous plants transport their pollen on the surface of the water, while others are completely submerged during pollination.

Employing animals to move pollen is a familiar strategy. Entomophily (insect pollination) is the most common, but there is also ornithophily (bird pollination) and chiropterophily (bat pollination), among others. Plants that rely on animals for pollination generally produce pollen grains that are sticky and nutritious. They attract animals using showy flowers, fragrance, and nectar. The bodies of pollinating insects have modifications that allow them to collect and transport pollen. Certain bees, like honey bees and bumblebees, have pollen baskets on their hind legs, while other bees have modified hairs called scopae on certain parts of their bodies.

Pollen is edible. Some animals – both pollinating and non-pollinating – use pollen as a food source. Animals that eat pollen are palynivores. Bees, of course, eat pollen, but lots of other insects do, too. Even some spiders, which are generally thought of as carnivores, have been observed eating pollen that gets trapped in their webs.

Pollen is thought to be highly nutritious for humans as well, and so, along with being taken as a supplement, it is used in all sorts of food products. To collect pollen, beekeepers install pollen traps on their beehives that strip incoming worker bees of their booty. Pollen from various wind pollinated plants, like cattails and pine trees, are also collected for human consumption. For example, a Korean dessert called dasik is made using pine pollen.

pine pollen – photo credit: wikimedia commons

Pollen makes many people sick. Hay fever is a pretty common condition and is caused by an allergy to wind-borne pollen. This condition is also known as pollinosis or allergic rhinitis. Not all flowering plants are to blame though, so here is a list of some of the main culprits. Because so many people suffer from hay fever, pollen counts are often included in weather reports. Learn more about what those counts mean here.

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Charles Darwin and the Phylogeny of State Flowers and State Trees

This is a guest post by Rachel Rodman. Photos by Daniel Murphy.


Every U.S. state has its own set of symbols: an official flower, an official tree, and an official bird. Collectively, these organisms form the stuff of trivia and are traditionally presented in the form of a list.

But, lists…well. As charming as lists can sometimes be, lists are rarely very satisfying.

So I decided to try something different.

I am not, of course, the first person to be unhappy with the eclectic, disordered nature of many biological assemblages. In the 18th century, Linnaeus developed a classification system in order to make sense of that untidiness. Kingdom, Phylum, Class, and so on.

In the 19th century, Darwin set biodiversity into an even more satisfying intellectual framework, outlining a model that linked organisms via descent from a series of common ancestors. And, as early as 1837, he experimented with a tree-like structure, in order to diagram these relationships.

Following Darwin’s lead, I’ve worked to reframe the state flowers and state trees in terms of their evolutionary history (*see the methods section below). And today, in honor of Darwin’s 209th birthday, I am delighted to present the results to you.

Let’s start with the state flowers.

In this tree, Maine’s “white pine cone and tassel” forms the outgroup. Among all the state “flowers,” it is the only gymnosperm—and therefore, in fact, not actually a flower.

Notice, also, that the number of branches in this tree is 39—not 50. Most of this stems from the untidy fact that there is no requirement for each state to select a unique flower. Nebraska and Kentucky, for example, share the goldenrod; North Carolina and Virginia share the dogwood.

With the branch labeled “Rose,” I’ve compressed the tree further. The state flowers of Georgia, Iowa, North Dakota, New York, and Oklahoma are all roses of various sorts; with my data set (*see methods below), however, I was unable to disentangle them. So I kept all five grouped.

This is a rich tree with many intriguing juxtapositions. Several clades, in particular, link geographical regions that are not normally regarded as having a connection. Texas’ bluebonnet, for example, forms a clade with Vermont’s red clover. So, similarly, do New Hampshire’s purple lilac and Wyoming’s Indian paintbrush.

Texas bluebonnet (Lupinus texensis) – the state flower of Texas

The second tree—the tree of state trees—is similarly rewarding. This tree is evenly divided between angiosperms (19 species) and gymnosperms (17 species).

Iowa’s state tree is simply the “oak”—no particular species was singled out. To indicate Iowa’s selection, I set “IA” next to the node representing the common ancestor of the three particular oak species: white oak, red oak, and live oak, which were selected as symbols by other states.

Arkansas’ and North Carolina’s state tree, similarly, is the “pine,”—no particular species specified. I’ve indicated their choice in just the same way, setting “AR” and “NC” next to the node representing the common ancestor of the eight particular pine species chosen to represent other states.

In this tree of trees, as with the tree of flowers, several clades link geographical regions that are not usually linked—at least not politically. Consider, for example, the pairing of New Hampshire’s white birch with Texas’ tree, the pecan.

Another phylogenetic pairing also intrigued me: Pennsylvania’s eastern hemlock and Washington’s western hemlock. It evokes, I think, a pleasing coast-to-coast symmetry: two states, linked via an east-west cross-country bridge, over a distance of 2,500 miles

The corky bark of bur oak (Quercus macrocarpa). Oak is the state tree of Iowa.

In this post, I’ve presented the U.S. state flowers and U.S. state trees in evolutionary framework. The point in doing that was not to denigrate any of the small, human stories that lie behind these symbols—all of the various economic, historical, and legislative vagaries, which led each state to select these particular plants to represent them. (Even more importantly, I have no wish to downplay the interesting nature of any of the environmental factors that led particular plants to flourish and predominate in some states and not others.)

The point, instead, was to suggest that these stories can coexist and be simultaneously appreciated alongside a much larger one: the many million year story of plant evolution.

With Darwin’s big idea—descent with modification—the eclectic gains depth and meaning. And trivia become a story—a grand story, which can be traced back, divergence point by divergence point: rosids from asterids (~120 mya); eudicots from monocots (~160 mya); angiosperms from gymnosperms (~300 mya), and so on and so on.

So today, on Darwin’s 209th, here, I hope, is one of the takeaways:

An evolutionary framework really does make everything—absolutely everything: U.S. state symbols included—more fun, more colorful, more momentous, and more intellectually satisfying.

Thanks, Darwin.


To build these two trees, I relied on a data set from, a website maintained by a team at Temple University. At the “Load a List of Species” option at the bottom of the page, I uploaded two lists of species in .txt format; each time, TimeTree generated a phylogenetic tree, which served as a preliminary outline.

Later, once I’d refined my outlines, I used the “Get Divergence Time For a Pair of Taxa” feature at the top of the page in order to search for divergence time estimates. As I reconstructed my trees in LibreOffice, I used these estimates to make my branch lengths proportional.


Rachel Rodman has a Ph.D. in Arabidopsis genetics and presently aspires to recontextualize all of history, literature, and popular culture in the form of a phylogenetic tree. Won’t you help her?

Botany in Popular Culture: Laura Veirs

I love music for its ability to conjure up emotions, create a mood, and inspire action. The music of Laura Veirs has always inspired me to get out into nature and be more observant of the wild things around me. Her music is rich with emotions, and I feel those, too. However, when I think of her music, I can’t escape images of the natural world and the creatures that inhabit it.

Found within her nature-centric lyrics are, of course, numerous botanical references. After all, plants and their actions make excellent subject matter for all types of art. And with that in mind, Veirs asks rhetorically in the song Rapture, “Doesn’t the tree write great poetry?”

When it comes to botanical references, the song that jumps first to mind is Lonely Angel Dust, starting off right away with these lyrics: “The rose is not afraid to blossom / though it knows its petals must fall / and with its petals fall seeds into soil / Why toil to contain it all? / Why toil at all?” Plants produce seeds in abundance, as mentioned in Shadow Blues: “Thousand seeds from a flower blowing through the night.” And, as in Where Are You Driving?, they’re seeking a suitable spot to plant themselves: “Through clouds of dandelions / seeds sailing out on the wind / hoping you’ll be the one to plant yourself on in.”


Flowers come up often in the songs of Laura Veirs. In White Cherry, “cherry trees take to bloom.” In Nightingale, “her heart a field in bloom.” In Make Something Good, “an organ pipe in a cathedral / that stays in tune through a thousand blooms.” In Sun is King, “innocent as a summer flower.” In Cast a Hook, “with watery cheeks down flowered lanes.” In Life Is Good Blues, “Messages you sent to Mars came from a crown of flowers.” Grass and weeds get a few mentions, too. In Summer Is the Champion, “let’s get dizzy in the grass.” In Life Is Good Blues, “tender green like the shoots of spring / unfurling on the lawn.”

Trees are the real stars, though. Veirs makes frequent references to trees and their various parts. This makes sense, as trees are real forces of nature. So much happens in, on, and around them, and images of the natural world can feel barren without them. First there is their enormousness, as in Black Butterfly, “evergreen boughs above me tower / were singing quiet stories about forgiveness, ” and Don’t Lose Yourself, “we slept in the shadow of a cedar tree.” Then there is their old age, as in Where Are You Driving?, “tangled up in the gnarled tree,” and When You Give Your Heart, “falling through the old oak tree.” There is also their utility, mentioned in Make Something Good, “I wanted to make something sweet / the blood inside a maple tree / the sunlight trapped inside the wood / make something good.” And then, of course, there is the fruit they bear, as in July Flame, “sweet summer peach / high up in the branch / just out of my reach,” and then in Wandering Kind, “a strange July / a storm came down / from the North and pulled out the salt / and it tore out the leaves from the pear tree / my canopy.”

Many of Veirs songs create scenes and tell stories of being in the wilderness among rivers, lakes, mountains, and caves. Chimney Sweeping Man, for example, is a “forest resident” who “walks[s] quiet through the forest like a tiny, quiet forest mouse.” In Snow Camping, Veirs tells a story about sleeping in a snow cave in the forest, where “a thousand snowflakes hovered,” “a distant songbird [was] singing,” and “the weighted trees” were her “only home.” But sometimes those forests burn, which is captured in Drink Deep: “Now the raging of the forest fires end / and all the mammals fled / I smell in the charred darkness / a little green / a little red.” Later in the song: “the fire closed his eyes / tipped his flame hat and slipped through the dire rye / we wandered romantic / we scattered dark branches / with singing green stars as our guide.”

Nature can also be empowering, and Veirs often refers to things in the natural world as metaphors or similes for the human experience. In Cast a Hook, Veirs adamantly asserts, “I’m not dead, not numb, not withering / like a fallen leaf who keeps her green.” This line comes up again in Saltbreakers: “You cannot burn me up / I’m a fallen leaf who keeps her green.” In Lake Swimming, Veirs addresses change and how some of life’s changes may wound us but we can still shine – “shucking free our deadened selves / like snakes and corn do / … / Old butterfly / I’ll dance with you / though our wings may crumble / we can float like ash / broken but the edges still shine.”


The botanical references Veirs makes in her songs are not the only things that excite me. Birds, insects, mammals, fish, and worms all find a place in Veirs’ lyrics. This is why, after more than a decade of listening to her songs, I find myself coming back to them again and again. There is a sort of kinship we feel for each other when we share in common a love of the natural world. I find that in the music of Laura Veirs.

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The Agents That Shape the Floral Traits of Sunflowers

Flowers come in a wide array of shapes, sizes, colors, and scents. Their diversity is downright astounding. Each individual species of flowering plant has its own lengthy story to tell detailing how it came to look and act the way it does. This is its evolutionary history. Unraveling this history is a nearly insurmountable task, but one that scientists continue to chip away at piece by piece.

In the case of floral traits – particularly for flowers that rely on pollinators to produce seeds – it is safe to say that millennia of interactions with floral visitors have helped shape not only the way the flower looks, but also the nature of its nectar and pollen. However, flowers are “expensive” to make and maintain, so even though they are necessary for reproduction, plants must find a balance between that and allocating resources for defense – against both herbivory and disease – and growth. This balance can differ depending on a plant’s life history – whether it is annual or perennial. An annual plant has one shot at reproduction, so it can afford to funnel much of its energy there. If a perennial is unsuccessful at reproduction one year, there is always next year, as long as it has allocated sufficient resources towards staying alive.

Where a plant exists in the world also influences how it looks. Abiotic factors like temperature, soil type, nutrient availability, sun exposure, and precipitation patterns help shape, through natural selection, many aspects of a plant’s anatomy and physiology, including the structure and composition of its flowers. Additional biotic agents like nectar robbersflorivores, and pathogens can also influence certain floral traits.

This is the background that researchers from the University of Central Florida and University of Georgia drew from when they set out to investigate the reasons for the diverse floral morphologies in the genus Helianthus. Commonly known as sunflowers, Helianthus is a familiar genus consisting of more than 50 species, most of which are found across North America. The genus includes both annuals and perennials, and all but one species rely on cross-pollination to produce viable seeds. Pollination is mainly carried out by generalist bees.

Maximilian sunflower (Helianthus maximiliani)

Helianthus species are found in diverse habitats, including deserts, wetlands, prairies, rock outcrops, and sand dunes. Their inflorescences – characteristic of plants in the family Asteraceae – consist of a collection of small disc florets surrounded by a series of ray florets, which as a unit are casually referred to as a single flower. In Helianthus, ray florets are completely sterile and serve only to attract pollinators. Producing large and numerous ray florets takes resources away from the production of fertile disc florets, and sunflower species vary in the amount of resources they allocate for each floret form.

In a paper published in the July 2017 issue of Plant Ecology and Evolution, researchers selected 27 Helianthus species and one Phoebanthus species (a closely related genus) to investigate “the evolution of floral trait variation” by examining “the role of environmental variation, plant life history, and flowering phenology.” Seeds from multiple populations of each species were obtained, with populations being carefully selected so that there would be representations of each species from across their geographic ranges. The seeds were then grown out in a controlled environment, and a series of morphological and physiological data were recorded for the flowers of each plant. Climate data and soil characteristics were obtained for each of the population sites, and flowering period for each species was collected from various sources.

The researchers found “all floral traits” of the sunflower species to be “highly evolutionarily labile.” Flower size was found to be larger in regions with greater soil fertility, consistent with the resource-cost hypothesis which “predicts that larger and more conspicuous flowers should be selected against in resource-poor environments.” However, larger flower size had also repeatedly evolved in drier environments, which goes against this prediction. Apart from producing smaller flowers in dry habitats, flowering plants have other strategies to conserve water such as opening their flowers at night or flowering for a short period of time. Sunflowers do neither of these things. As the researchers state, “this inconsistency warrants consideration.”

The researchers speculate that “the evolution of larger flowers in drier environments” may be a result of fewer pollinators in these habitats “strongly favoring larger display sizes in self-incompatible species.” The flowers are big because they have to attract a limited number of pollinating insects. Conversely, flowers may be smaller in wetter environments because there is greater risk of pests and diseases. This is supported by the enemy-escape hypothesis – smaller flowers are predicted in places where there is increased potential for florivory and pathogens. Researchers found that lower disc water content had also evolved in wetter environments, which supports the idea that the plants may be defending themselves against flower-eating pests.

Seed heads of Maximilian sunflower (Helianthus maximiliani)

Another interesting finding is that, unlike other genera, annual and perennial sunflower species allocate a similar amount of resources towards reproduction. On average, flower size was not found to be different between annual and perennial species. Perhaps annuals instead produce more flowers compared to perennials, or maybe they flower for longer periods. This is something the researchers did not investigate.

Finally, abiotic factors were not found to have any influence on the relative investment of ray to disc florets or the color of disc florets. Variations in these traits may be influenced instead by pollinators, the “biotic factor” that is considered “the classic driver of floral evolution.” This is something that will require further investigation. As the researchers conclude, “determining the exact drivers of floral trait evolution is a complex endeavor;” however, their study found “reasonable support for the role of aridity and soil fertility in the evolution of floral size and water content.” Yet another important piece to the puzzle as we learn to tell the evolutionary history of sunflowers.