Idaho’s Native Milkweeds (Updated)

As David Epstein said in an interview on Longform Podcast, “Any time you write about science, somethings is going to be wrong; the problem is you don’t know what it is yet, so you better be ready to update your beliefs as you learn more.” Thanks to the newly published Guide to the Native Milkweeds of Idaho by Cecilia Lynn Kinter, lead botanist for Idaho Department of Fish and Game, I’ve been made aware of some things I got wrong in the first version of this post. I appreciate being corrected though, because I want to get things right. What follows is an updated version of the original post. The most substantial change is that there are actually five milkweed species native to Idaho rather than six. Be sure to check out Kinter’s free guide to learn more about this remarkable group of plants.

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Concern for monarch butterflies has resulted in increased 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 halting milkweed losses and restoring milkweed populations, will in turn provide habitat for countless other organisms. A patch of milkweed teems with life, and our pursuits to protect a single caterpillar invite us to 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 U.S. 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. Asclepias species found in southern Africa are considered by many to actually belong in the genus Gomphocarpus.

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 five native species. The most abundant 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. In fact, the monarch butterfly is Idaho’s state insect, thanks in part to the abundance of showy milkweed, which 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, which would otherwise allow 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. Pollination is successful when the pollen sacs are 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) occurs across western and southern Idaho. Its distribution continues into 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. It’s a whispy plant that reaches one to three feet tall and occasionally taller. 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 narrowleaf milkweed (Asclepias fascicularis)

seeds escaping from the follicle of narrowleaf milkweed (Asclepias fascicularis)

Swamp or rose milkweed (Asclepias incarnata) is more common east of Idaho, but occurs occasionally in southwestern Idaho. As its common name 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 flower heads at the tops of its stems. Its seed pods are narrow and around 3 inches long.

swamp milkweed (Asclepias incarnata)

Asclepias cryptoceras ssp. davisii, or Davis’s milkweed, is a low-growing, drought-adapted, diminutive species that occurs in southwestern Idaho. 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. The range of Asclepias cryptoceras – commonly known as pallid milkweed or jewel milkweed – extends beyond Idaho’s borders into Oregon and Nevada, creeping north into Washington and south into California. Another subspecies – cryptoceras – can be found in Nevada, Utah, and their bordering states.

Davis’s milkweed (Asclepias cryptoceras ssp. davisii)

The final species is rare in Idaho, as Idaho sits at the top of its native range. Asclepias asperula ssp. asperula, or spider milkweed, has a single documented location in Franklin County (southeastern Idaho). Keep your eyes peeled though, because this plant may occur elsewhere, either in Franklin County or neighboring counties. 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. Its common name comes from the crab spiders frequently found hunting in its flower heads.

A sixth species, horsetail milkweed (Asclepias subverticillata), has been falsely reported in Idaho. Collections previously labeled as A. subverticillata have been determined to actually be the similar looking A. fascicularis.

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Book Review: What Weeds Are Thinking

Plant intelligence is a burgeoning field of research. Despite being predominantly sessile organisms, plants are able to sense their surroundings and make decisions based on environmental cues. In a certain sense, they can see, hear, smell, and remember even though they don’t have eyes, ears, noses, or brains. Not surprisingly, these fascinating findings have spawned books, podcasts, documentariesarticles, etc. The idea that plants could be intelligent beings like us is something that captures our attention and imagination.

For example, if plants are so smart, does this mean that they actually have thoughts? And if they have thoughts, what could they possibly be thinking? In her new book, What Weeds Are ThinkingErica Crockett takes a stab at what a particularly despised group of plants might think if, indeed, they could have thoughts. Weeds are, in Crockett’s words, “the deviants of the plant world.” This book was her chance to imagine what might be going on inside the minds of these deviants, despite the fact that they don’t have minds.Crockett conjures up the thoughts of 21 different weeds. Each thought is accompanied by an illustration by Sarah Ragan Olson. The drawings are charming, but the thoughts that juxtapose them aren’t always so sweet. Perhaps you imagine weeds to be potty mouthed? Well, so does Crockett. That being said, this book is not for kids, nor is it for anyone sensitive to adult words and themes. Each of the weeds in this book varies in its degree of irreverence – not all of them are so crass and some of them are actually pretty mild-mannered – but that’s just what you’d expect from such a diverse group of plants.

Comfrey is offended by cow manure being used as fertilizer and would rather be fertilized by “the decaying corpses of [its] relations.” Ground ivy is embarrassed and offended by inadvertently seeing the ankles of human passersby.  Plantain is apparently into being stepped on, and prostrate spurge is trying to “rebrand” to make itself more appealing and set itself apart from purslane. Cheatgrass, no surprise here, comes across as a big jerk. Originally from Eurasia, it is now a freedom-loving American, “choking out the rights of the natives.”

Most in line with what I would expect a weed to be thinking – especially one found growing in an urban area – is prickly lettuce. Upset after watching a fellow member of its species ruthlessly dug up, it laments: “Neither of us decided to seed down in the deep crack of this suburban driveway. We were blown here…It’s our home…Yet we are hunted.”

Milkweed is quite aware of its role as the sole food source of the monarch caterpillar. It sees how much humans appreciate monarchs, and admonishes us for killing off its kind: “Keep it up, and I’ll take every last one of those delicate darlings down with me.”

Botanical inaccuracies aside – and there are several – this was a fun book. The main appeal for me is that it is plant-themed and, more specifically, weeds-themed. If you follow this blog, you’ll know that pretty much anything involving weeds is going to get my attention. Beyond that, any project that puts plants in the spotlight and gives them a voice (even in a fictitious sense) is worth checking out. This book is no exception.

As a bonus, I asked Erica Crockett what other plants (apart from weeds) are thinking. This was her response: “Probably really precious or intellectual things. I imagine tomato plants are fairly self-important and zonal geraniums are divas. Bougainvilleas are likely social climbers and oak trees are dull, but honest.”

More Weeds-themed Book Reviews on Awkward Botany:

When Milkweed Kills

When you think of milkweed, you probably think of the life it supports. The monarch butterfly, for one. As the sole food source for its leaf-eating larvae, monarchs would be a thing of the past if milkweeds disappeared. Numerous other insects feed on its foliage as well, and there are a plethora of organisms that feed on its nectar, including bees, butterflies, beetles, wasps, and other insects, as well as hummingbirds. And speaking of birds, some birds use the silky hairs attached to the seeds to line their nests, while other birds strip stringy fibers from the stems for nest building. And while it is not a major food source for mammals, deer and other animals have been known to sample it. Indeed, milkweed is a veritable life force.

red milkweed beetle (Tetraopes sp.) feeds on milkweed

But it’s also a poisonous plant. The latex sap of milkweed contains cardiac glycosides, among a variety of other toxic chemicals. The plant produces these chemicals to defend itself from herbivory, and so the insects that feed on it have adapted a variety of strategies to avoid being poisoned. Some bite a hole in a leaf vein and wait for the milky sap to drain before proceeding to eat the leaf. Others are able to consume the toxic foliage without being poisoned by it. Some even store the toxic chemicals in their bodies, making themselves poisonous to other organisms that dare consume them.

Aphids on Mexican whorled milkweed (Asclepias fascicularis). One species commonly found on milkweed is the oleander aphid (Aphis nerii), an introduced species that feeds on milkweeds and other plants in the dogbane family.

While milkweed is generally found to be unpalatable to most livestock, those that venture to eat it risk being poisoned and even killed. A guide to milkweed written by the Xerces Society states, “sheep and goats are the most likely to be poisoned because they are browsers and often prefer to feed on weeds over other forages.” Weeds of the West calls Utah milkweed (Asclepias labriformis) “the most poisonous of all western milkweeds,” claiming that “as little as one ounce of green leaf material … can kill an adult sheep.” It also lists swamp milkweed (Asclepias incarnata) as “suspected of causing livestock deaths.” To make matters worse, dead and dried milkweed plants retain their toxicity, which is a problem when they end up in animal feed.

Despite their toxicity, humans have been consuming milkweed for centuries. Young shoots and leaves can be eaten after boiling them several times, refreshing the water each time, and a medicinal tea can be made from the roots. While fatal poisonings of humans haven’t been reported, Nancy Turner and Patrick von Aderkas warn in their book The North American Guide to Common Poisonous Plants and Mushrooms that “uncooked shoots and the mature plants should never be consumed”

But milkweed’s toxic sap is not its only method for killing.

In fact, it may not even be its most deadly. And this is where things get interesting. Last month I arrived at work one morning to find a portion of a dried-up milkweed inflorescence on my desk that had been left there by a friend and co-worker. Stuck to the inflorescence were three, dead, dried-up honey bees, their legs trapped in the slotted hoods of the flowers. Apparently this is a common occurrence; one that is mentioned in nearly every resource about milkweeds that I have read now, and yet I had never heard of it nor seen it until this gift was left for me. I then went out to a patch of milkweed to see this for myself. Sure enough, I found a few dead bees trapped in the flowers of showy milkweed.

dead honey bee stuck in the flowers of showy milkweed (Asclepias speciosa)

Milkweed flowers do not always give up their pollen sacs easily. The slits where the pollinia are found can, on occasion, trap the legs of visiting insects. John Eastman describes this in The Book of Field and Roadside, “insects sometimes become permanently wedged as the fissures trap their feet or the pollinia entangle them, and they die hanging from the flowers.” While milkweed species are native to North America, honey bees are not; they have not evolved alongside the flowers of milkweed, yet they are drawn in, like so many other insects, to the nutritious and abundant nectar.

Native or not, honey bees are not the only insects getting trapped in the flowers. Eastman reports seeing various species of butterflies ensnared as well, and a paper by S.W. Frost lists cluster flies, soldier beetles, and a couple species of moths as unsuspecting victims of these unruly flowers. Frost goes on to observe that, “in spite of the hazards,” bees, wasps, and various other insects “visited the flowers of milkweeds freely.”

In a paper published in 1887, Charles Robertson describes the insect visitors of several different milkweed species. He found an occassional dead insect on the flowers of swamp milkweed, adding that “this occurs only when all or most of the feet are entangled simultaneously, so as to render the insect absolutely helpless.” Observing common milkweed (Asclepias syriaca), Robertson finds that “even when small and short-legged insects succeed in extracting pollinia and inserting them into the stigmatic chambers, they have great difficulty in breaking the retinacula, and often lose their lives in consequence.”

Honey bees were easily the most common victims observed in Robertson’s study, leading him to quip, “it seems that the flowers are better adapted to kill [honey bees] than to produce fruit through their aid.” And a honey bee’s trouble doesn’t always end when she escapes the grasp of the flowers. Pollinia and its connecting tissues can get so tangled around her legs and other body parts that she can no longer forage, subjecting herself to starvation and predation.

To add insult to injury, dead and dying insects stuck to flowers result in another interesting phenomenon. Robertson writes, “many fall prey to predacious insects. I have seen them while still alive, attacked by ants, spiders and [predatory stink bugs].” Eastman adds daddy longlegs to the list of “scavengers” or “cleanup specialists” that come to feed on “flower trapped insects.” As it turns out, visiting the flowers of milkweed can be a dangerous, even deadly, game.

See Also: Idaho’s Native Milkweeds

Idaho’s Native Milkweeds

An updated version of this post was posted on August 14, 2019. See it here.

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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.