Tuesday, March 12, 2024

What is a rhizome?

A brown, horizontal rhizome bearing a pair of whitish nubs (incipient shoots) and clusters of long, white roots.
Mayapple (Podophyllum peltatum) spreads by rhizomes. The two whitish nubs at the node in the middle are the beginning of shoots. Clusters of roots also grow from the nodes.

A rhizome (RY-zome), also called a creeping rootstock, isn’t a root at all. It’s a stem that runs roughly  horizontal under or just above the soil, producing roots and shoots along its length. Slender, aboveground rhizomes, like those of strawberries, are also called stolons (STOW-lons). In either case, they're stems, and they serve many purposes.

Rhizomatous (rhizome-bearing) plants are colony-formers. Mayapple rhizomes, pictured above, grow moderately fast to produce a steadily expanding colony. The compact rhizomes of large-flowered trillium (Trillium grandiflorum) grow much slower, producing closely spaced clumps of plants.  On the opposite end of the spectrum, weedy quackgrass (Elymus repens) and Japanese knotweed (Fallopia japonica) have vigorous rhizomes that quickly give rise to large, rapidly expanding colonies. That’s why they’re hard to manage. Even if they’re pulled or dug up, they can regrow quickly from even small bits of rhizomes left behind.

A set of three photos showing mayapple with its umbrella-like leaves, a clump of large-flowered trillium with several white, three-petaled flowers, and wild strawberry leaves and runners clambering over rocks.
Left: A mayapple colony. Each plant is 12-16 inches (30-40 cm) tall. Center: Large-flowered trillium grows in clumps from slowly growing rhizomes. The flowers are about 2 inches (5 cm) wide. Right: The slender red rhizomes of wild strawberry (Fragaria vesca) are also called stolons. Each is about as wide as a pencil tip. 

Rhizomes have several benefits.

Rhizomes are a form of vegetative reproduction. Compared to flowers and seeds, they’re a faster and energetically less expensive way to grow a population. Rhizomes won’t spread a plant far and wide – seeds are often better at that – but if a plant is growing in a favorable place, rhizomes can increase its numbers quickly, and without the risk of losing fragile seedlings.

Except for stolons, rhizomes also serve as storage organs. As winter approaches, sugars and nutrients are moved underground, forming a protected reserve that can be tapped to begin next year’s growth. Some rhizomes end in tubers, swollen organs specialized for storage. Potatoes are a familiar example, but other plants also have tubers. The small tubers of native enchanter’s nightshade (Circaea lutetiana) detach from their rhizomes in fall and function much like seeds, and the tubers of yellow nutsedge (Cyperus esculentus), also called earth almonds, are the edible but maddening means by which this plant persists.

Two photos showing yellow nutsedge plants with grass-like leaves and branched, yellow flower clusters, and a root/rhizome system with several light to dark brown, pea-sized tubers.
Left: Yellow nutsedge plants. Photo by Howard F. Schwartz, Colorado State University, Bugwood.org. 
Right: The thin, white rhizomes of yellow sedge bear small tubers. Photo by Steve Dewey, Utah State University, Bugwood.org.

Rhizomes have potential drawbacks, too.

Plants that produce seeds or spores combine DNA from different individuals to make genetically unique offspring. The young plants aren’t exactly like their parents or even like each other. In contrast, rhizomes produce genetically identical offspring. All shoots from a common rhizome are the same as their parents and the same as each other. In other words, they are clones.

If that uniform gene combination is adaptive in a certain environment, it’s an advantage. It’s like using the same, tried-and-true recipe over and over again, with great success. If conditions change, though, uniformity can be a drawback. If the plants don’t have the genetic makeup to adapt, say, to warmer or drier weather or shadier or lighter conditions, the population may not survive. Their genetic recipe may not serve them well anymore. Especially in a rapidly or drastically changing environment, plants that reproduce primarily by rhizomes may decline, while plants that reproduce by seeds or spores may survive if a few individuals have the genetic ability to adapt.

How to recognize a rhizomatous plant

In the field, there are several ways to know that a plant has rhizomes. One is to look for spreading growth. The presence of colonies can indicate that rhizomes lie below, although some plants without rhizomes also grow in spreading patches. They may have sprawling stems, for example, or seeds that land close to the parent plant.

Another option is to look underground. If possible and permissible, pull or dig up a stem and look at the root system. Rhizomes, if present, will grow horizontally or almost so. They will also have nodes, places where small, scale-like leaves are or were attached. That’s how to tell rhizomes from roots, which also grow from rhizomes. Some rhizomatous plants also produce aboveground leaves -- see the last section for an example. Wear gloves when you handle rhizomes; some can irritate skin or even cause poisoning if ingested. 

A single, whitish rhizome and clusters of thin, white roots of Canada goldenrod. The rhizome has dark marks at regular intervals that indicate the position of nodes.
Canada goldenrod (Solidago canadensis) has pencil-thick rhizomes and much thinner and more numerous roots.

Two photos, the first showing bloodroot plants with closed, white flowers and lobed green leaves wrapped around the flower stalks; the second showing a thick, orange-red rhizome.
Left: Bloodroot (Sanguinaria canadensis) plants grow in slowly expanding colonies from their rhizomes. These plants, photographed in early spring, will eventually unfold their leaves and open their flowers. Right: A mature bloodroot rhizome is about as thick as a thumb. If cut it will "bleed" an orange-red latex. So will the aboveground parts. The latex is poisonous in large doses. Photo by Joseph O'Brien, USDA Forest Service, Bugwood.org.

A thorough plant guide will tell you if a plant has rhizomes. A plant’s name can be another clue. If the  common name includes “creeping” or “crawling,” it’s a good bet it has rhizomes. Creeping Charlie and creeping bellflower are good examples. Sometimes plants creep by other means, such as low-growing or arching stems that root at nodes where they touch the soil. This kind of creeping habit, though, can be easily spotted above ground.

Scientific names, too, can be revealing. Look at the specific epithet, the second word in a plant’s scientific name, which identifies the species. If you see repens or reptans, from Latin words meaning creeping or crawling, the plant likely has rhizomes. As mentioned above, the Eurasian import Elymus repens, or quackgrass, spreads aggressively by rhizomes. Native Polemonium reptans, or spreading Jacob’s ladder, also has rhizomes, but they grow slowly. The plant also spreads with its sprawling stems and self-seeding habit.

Looking for an easy rhizome to study? Try clover.

Introduced Dutch or white clover, Trifolium repens, is a convenient plant to see rhizomes. Its stem grows just above or below the soil, so it’s easy to pull up. This is the only stem the plant has. The vertical “shoots” are actually petioles, or leaf stalks, and scapes, structures that support clusters of flowers. Notice that the rhizome has nodes, but the petiole and scape do not. 

Two photos, the first showing a mass of clover with clusters of white flowers and the second showing a narrow, red clover rhizome.
A white clover colony spreads by rhizomes. They can grow quickly, forming patches. 

True roots will also come up, and they lack nodes, too. Some of them may have tiny nodules attached. These aren’t tubers, but rather small bodies containing nitrogen-fixing bacteria. The bacteria convert nitrogen gas in the air to a form the plant can use. For more information about that, see The Boon of Biological Nitrogen Fixation.

A white clover rhizome with roots bearing many small nodules.
A white clover rhizome and roots with nodules.

Sunday, January 28, 2024

Look Closely at Wildflower and Pollinator Seed Mixes

A mock-up of a seed package showing a field of wild flowers and the words "Wildflower Mix."

If you’re thinking about sowing a wildflower or pollinator seed mix this season, you have lots of company.

Many people have been inspired to support pollinators by planting wildflowers to provide nectar and pollen. This increasing demand has spurred many companies to offer one or more seed mixes online or in retail stores. Caution is best, though, before choosing one. Look closely at the mix contents. “Wildflower” means different things to different people, and there could be surprises.

Consider the contents of this mix advertised for Minnesota:

Sweet William, Prairie Coneflower, Mexican Hat, Red Corn Poppy, Lance Leaf Coreopsis, Shirley Poppy, Wild Cosmos, Blanket Flower, Black Eyed Susan, Wild Perennial Lupine, Purple Coneflower, Russell Lupine, Plains Coreopsis, Siberian Wallflower, Scarlet Flax, Annual Red Phlox, Cornflower, Gloriosa Daisy, California Poppy, Perennial Blue Flax, Candytuft. 

Although these plants will likely survive in Minnesota, some are undesirable because they can grow aggressively, alter soil chemistry, or replace food sources that native insects or other animals are long adapted to using.

One example is Russell Lupine, also called Bigleaf Lupine. Known by the scientific name Lupinus polyphyllus, the plant is a garden favorite native to several western states. It has been introduced to the Upper Midwest, the Northeast, eastern Canada, and even Europe primarily for its ornamental value, but also for its deep, nitrogen-fixing roots that can stabilize and enrich soils.

In Minnesota, Russell Lupine has become especially abundant in the Arrowhead region, especially along roadsides through the north shore of Lake Superior. In spring and early summer, masses of the plants bloom in spectacular displays of white, pink, blue and purple. Their striking colonies are a big draw to that area each season. That’s good for the regional economy, but not so good for its environment. Beautiful though they are, the plants have some drawbacks.

Although Russell Lupine, L. polyphyllus, isn’t considered a noxious weed in Minnesota, the plant has several traits associated with invasive plants. It reproduces prolifically and grows in dense patches that exclude other plants, including native plants that have long supported pollinators. Although the plant declines in summer heat, it can grow in a range of other conditions, from moist to dry soils and full to part sun. That adaptability means it can grow in many habitats, from lakeshores and wetland edges to upland forest edges and roadsides. Its deep roots do help stabilize and enrich erodible, nutrient-poor soils – a useful trait for reclamation in the West – but deep roots also make the plant hard to remove from places where it isn’t wanted.

A mass of Lupinus polyphyllus along a roadside in northeast Minnesota. Photo copyright Peter Dzuik, 2004, and used with permission granted on the Minnesota Wildflowers website. 

Unfortunately, L. polyphyllus is sometimes mistaken for (and mislabeled as) our native “wild” lupine, Lupinus perennis. Now called Sundial Lupine to avoid confusion with “wild” Russell varieties, this perennial grows in prairies and savannas primarily in southeast and east central Minnesota and the eastern U.S.  

Like Russell Lupine, Sundial Lupine is beautiful, but more importantly it's critical for the survival of the federally endangered Karner Blue butterfly, Plebejus samuelis. Karner Blue larvae feed only on Sundial Lupine. Females lay their eggs on or near the plant – or what they judge to be the plant – and the caterpillars eat the leaves. Karner Blues are endangered in large part because Sundial Lupine has lost habitat, and as this larval host has become less plentiful, so has the Karner Blue.

Efforts are underway to bolster populations of Sundial Lupine, but where it overlaps with plantings of Lupinus polyphyllus, competition and hybridization present challenges. L. polyphyllus grows more aggressively and can displace L. perennis. The two species also hybridize, and unfortunately, neither L. polyphyllus nor its hybrids support Karner Blue larvae. If female butterflies lay eggs on either one, the larvae are unlikely to survive. 

Wildflower seed mixes, then, can include species that have both benefits and major drawbacks. To avoid problems like those caused by L. polyphyllus, it’s best to pause before choosing a mix. Study the contents and try to avoid species that can do more harm than good. Here are some steps to take:

  • The surest and easiest solution is to buy seeds and seed mixes from a local native plant nursery. The Minnesota DNR maintains a list of native plant suppliers. These growers are familiar with what should and shouldn’t be included in a mix, but if you prefer to do the work yourself, keep reading.
  • Look for scientific names of the plants in the mix. Different species may have the same common name, and plants with different common names may be the same species. Scientific names avoid this confusion by revealing a plant’s identity. If none are given, either don’t buy the mix or try to use the common names of the species to look them up.
  • Using scientific names, search for information about each species. Where is it from? Does it have a reputation for invasiveness? Good sources include the USDA Plants Database, EDDMapS, the Invasive Plant Atlas, and your state’s department of natural resources. Alternatively, search online using the scientific name of the plant followed by “invasive.”  
  • Be aware that plants native to the U.S. or North America are not necessarily native to your area. This may seem unimportant, but plants that come from another part of the country or continent can behave differently where they’re introduced.
  • Also be aware that maps showing native ranges can be wrong. The USDA Plants Database map for L. polyphyllus, for example, shows that the plant is native to much of the U.S. and Canada. But a plant guide from the USDA states that it’s native only to several western states and two western Canadian provinces..
A map of North America showing states and provinces where Russell Lupine is now found. Yellow stars placed on Alberta and Britsh Columbia in Canada and Washington, Oregon, California, Nevada, Montana, and Utah in the U.S. show where it originated.
Range of Lupinus polyphyllus, according to the USDA. States and provinces
shaded green are where the plant is said to be native. Yellow stars are added to
show where the plant originated.

  • As you read about the plant, look for invasive traits, such as rapid growth and prolific reproduction by seeds or vegetative parts. Phrases like “forms large colonies” and “naturalizes easily” are red flags, especially if the plant isn't native here. Comments from other growers can also be informative. One customer who grew Russell Lupine posted, “Lupines are growing everywhere, even the few seeds . . . tossed at the edge of the woods.” Sometimes pictures of a species provide a clue. If photos show an extensive carpet of plants (a monoculture), that plant is probably aggressive and could displace other species.

References and More Information

Wildflower and pollinator plantings

 Russell Lupine, Lupinus polyphyllus

Karner Blue butterflies

Tuesday, December 5, 2023

Battling Buckthorn: Research Continues

Common buckthorn, Rhamnus cathartica, fills the understory of this hardwood forest. Buckthorn retains its green leaves longer than most other woody plants.

Anyone who’s tried to manage invasive common buckthorn (Rhamnus cathartica) knows it’s a battle. Pull it out and seedlings take its place. Cut it and it grows back, especially if the cut stump isn’t treated with herbicide. Buckthorn baggies or other covers for cut stumps are an option, but not a practical one for large, dense infestations. The search for insects that target buckthorn hasn’t succeeded yet, either.

Although these difficulties are frustrating, researchers at the University of Minnesota haven’t given up. Just the opposite: They are leading several investigations of potential solutions. Their studies of biodiversity, fungi, goats and other possible buckthorn remedies are shedding light on what works, what doesn’t and what to try next. Here are summaries of some of their efforts.

Using biodiversity to control buckthorn

The University’s “Cover It Up!” studies investigate the use of plants to suppress regrowth of common buckthorn after it is removed. Two phases are completed and a third is in progress, all asking questions about which plants are best at outcompeting buckthorn, how best to plant them, and how managing fire and deer browsing affects the success of using plants to control buckthorn.

The researchers were surprised by one of their findings: Most buckthorn seeds remain viable in the soil for only 1-2 years, not the commonly thought 6 years. That means seedlings will be abundant for the first year or two after buckthorn is removed but should taper after that, assuming new seeds aren’t introduced.

Phase 3 of the project is underway. When it’s done, researchers hope to offer buckthorn solutions that are both affordable and practical. For more information, see the Cover It Up! project page.

Can fungi help?

In some parts of Minnesota and Wisconsin, buckthorn is dying, possibly from canker rot, root rot and other fungal diseases. Researchers are trying to identify the fungi infecting the trees and how they may be used to manage buckthorn stands. Employing fungi would reduce the use of herbicides, an important benefit when working near water or other places where protection of other resources is paramount. The project began in January 2023 and will continue for another two years. See the project website for more information and updates.

Orange growths of crown rust on common buckthorn.
Another fungus being investigated for control of common buckthorn is Puccinia coronata, the rust fungus that causes crown rust of oats. The fungus uses buckthorn as an intermediate host and is visible during summer as orange, fuzzy-looking spots on leaves and stems. Some negative effects of the fungus have been found on buckthorn, so researchers have started identifying those strains and studying their effects on buckthorn growth and mortality. The study began in January 2023 and continues for two more years. More information is on the project website.

What about goats?

A study to evaluate the effectiveness of goat grazing on buckthorn ended in 2021. Researchers found that although goats can control buckthorn, the benefit is temporary. Buckthorn can rebound after grazing unless other control measures follow.

Grazing goats are also at risk of eating snails or slugs that are intermediate hosts for the brainworm Parastronguloides tenuis, which can cause fatal neurological disease. Co-grazing waterfowl with goats has been suggested to reduce that risk, so the study also examined the effect of waterfowl on the abundance of snails or slugs. In a 2022 paper published in EcoHealth, the researchers found that where goats grazed alone, the abundance of snails and slugs increased, but where ducks and geese were included with goats, the increase didn’t occur. They also found that waterfowl didn’t affect the overall diversity of the snails and slugs, which is important to protect populations of native gastropods.

The researchers point out that while waterfowl can lower the numbers of snails and slugs and therefore reduce the risk to goats of acquiring brainworms, more study is needed to learn if this also reduces the incidence of the disease.

More challenges of goat grazing are discussed in this paper and several others linked at the project website.

Thursday, November 9, 2023

Moonseed Vine: A Poisonous Grapevine Look-Alike

Moonseed vines with twining stems and fading leaves.
Moonseed vine, Menispermum canadense, in early fall. Although most of its leaves have now fallen, it can still be identified -- and avoided, if foraging.

Moonseed vine (Menispermum canadense), also called Canada moonseed, is a twining vine of deciduous forests and riverbanks, thriving in moist or mesic soils and partial shade. Although the vines have now shed their leaves, moonseed vine can still be identified by other characteristics, and for foragers of wild foods, the distinctions could be lifesaving.

The danger comes from mistaking moonseed vine for riverbank grape (Vitis riparia). Both vines produce clusters of deep blue or purple, spherical fruits that can remain on the vines through winter. Unlike edible grapes, however, moonseed fruits are poisonous.

That’s because moonseed vine contains alkaloids, compounds many plants produce for defense against herbivores. Although alkaloids have been and are used medicinally, they can also be poisonous. Several sources report that eating moonseed vine, especially the fruits and seeds, can be fatal because of its alkaloids. The Minnesota Poison Control System includes moonseed on its list of toxic plants and advises calling Poison Control if any part of the plant is eaten.

Fortunately, poisoning can be avoided. There are reliable ways to tell the difference between moonseed and riverbank grape, even in fall and winter. Here’s what to look for.

Mash fruits and crescent seeds of moonseed vine.

Moonseed fruits contain single, flat seeds that look like crescent moons. In contrast, riverbank grape seeds are egg-shaped, and there are usually several inside each fruit. This photo from the University of Wisconsin shows them side by side. 

Twining vs. grasping vines

Moonseed is a twining vine. It clambers over other vegetation or climbs upward by wrapping its stems around small trees or shrubs. Riverbank grape is a grasping vine. It uses tendrils, modified leaves, to clutch onto supports and hold the vines upright. 

A panel of two photos showing moonseed vines with twining stems and grapevine stems with coiled, grasping tendrils.
Twining stems of moonseed vine, left, and grasping stems of riverbank grape, right.

Stems and bark

The young stems of moonseed vine are brown or greenish brown and hairy, although as the stems age, they lose the hairs. Stems grow as large as 2 cm in diameter ( a little less than an inch) and have ridged or fluted bark. 

Younger stems of grapevine are brown or reddish brown and hairless. Mature stems grow up to 20 cm in diameter (about 8 inches) and have brown, shaggy bark.

Young moonseed stems, left, are brown or brownish green and hairy. Young stems of riverbank grape, center, are reddish brown and smooth. Mature stems of riverbank grape, right, are brown and shaggy. 

Leaf scars and buds

Leaf scars are marks on woody stems left by petioles when leaves are shed in fall. Inside them are small, often raised dots called bundle scars, created when strands of vascular cells (water- and food-conducting cells) are severed when leaves fall off the stem. Buds – next year’s protected growth – sit above the leaf scars.

The leaf scars of moonseed vine are typically 2-3 mm long and wide, oval to circular in outline, and concave. They are often notched or split at the top. Inside the leaf scar, the bundle scars are arranged in a faint, broken circle or oval. The bud is barely visible above the leaf scar; it appears to be embedded in the stem, protruding a millimeter or less above the surrounding tissue.

Leaf scar and bud of moonseed vine viewed from the front (left) and side (right). 

The leaf scars of riverbank grape are typically 2-4 mm long and wide and roughly triangular, semicircular, or U-shaped. The perimeter of the leaf scar may be ridged, but the leaf scar is not noticeably concave. To the side of the leaf scars are linear marks called stipule scars.(Stipules are thread-like or leafy structures at the bases of petioles in some plants.) Bundle scars are hard to see, but there are several. The bud is brown or reddish brown and 3-5 mm long. 

Leaf scar, bud and stipule scar of riverbank grape viewed from the front (left) and side (right).

Leaves, if still present

Both moonseed vine and riverbank grape have alternate, lobed leaves, but they have different margins (edges) and points of petiole attachment. These features can also be found on fallen leaves if they’re still intact.

The margins of moonseed leaves are smooth, although the lobes may have pointed tips. In addition, the petioles are attached inside the leaf blade, even if just barely, making a peltate or shield-shaped leaf blade. The leaves of riverbank grape are sharply and coarsely toothed, and the petiole is attached at the edge of the blade, at its base. 

Riverbank grape, left, has lobed, coarsely toothed leaves. Moonseed vine (right) also has
lobed leaves, but the margins are smooth, not toothed.

The petiole of riverbank grape, left, is attached at the edge of the blade. The petiole of
moonseed vine, right, is attached just inside the blade. 

Rhizomes and roots

Although aboveground characteristics are enough to positively identify moonseed and grapevine, belowground structures can also be helpful if it's possible -- and permissible -- to uncover or uproot them.

Moonseed vines have yellow rhizomes, underground stems that grow horizontally and produce roots and shoots along their length. Because of their color and traditional use to treat various ailments, Dakota Indians call it yellow medicine, the namesake of Yellow Medicine River and Yellow Medicine County in southwest Minnesota. The homeland of the Dakota is Pezihutazizi Kapi, “the place where we dig for yellow medicine.”

No source for this article describes grapevine as having rhizomes, but its woody, brown stems can become buried and grow roots at their nodes. These rhizome-like stems look nothing like the slender, yellow rhizomes of moonseed vine, however. 

The rhizomes of moonseed vine, left, are yellow. The roots and rhizome-like stems of riverbank grape, right, are brown.

Moonseed vine is found in much of Minnesota but is more abundant in the southern part of the state. Its broader range generally covers the eastern half of the US. The vine flowers in late spring and early summer. For more information, see the webpages from Minnesota Wildflowers and The Friends of the Wildflower Garden, Inc. Both are linked below.

Range maps of moonseed vine, from the USDA Plants Database.

Riverbank grape is also found in most of Minnesota except for several counties in the far north and northeast. More broadly, its range extends from the Mid-Atlantic states into New England and west through the Dakotas.


Wednesday, October 11, 2023

Flinging Spores and Fern ID

Sori on the back of a lady fern frond. Dozens of brown sporangia emerge from beneath the edge of a nearly translucent indusium.

The back of this lady fern frond (Athyrium filix-femina) is covered with sori, clusters of spore-forming bodies called sporangia. Each sorus holds dozens of them, all covered by a protective flap of tissue called an indusium. When a sporangium matures and dries, a line of cells over the top of the sporangium contracts, causing it to fling open and catapult its spores. You can watch it happen here in a sorus of an unidentified fern.

A spore print made by a lady fern frond. The spores are so small they look like dust. 

Unlike seeds, spores don’t contain embryonic plants. They’re little more than tiny packages of DNA that give rise to the next generation of ferns. They do this by first growing a small, heart-shaped prothallus, a body that produces egg and sperm cells. The flagellated sperm cells swim through a film of water to fertilize the egg cells, which then grow into the ferns we recognize. Because water is needed for this kind of reproduction, many ferns rely on damp or humid habitats.

A closeup of the back of a lady fern frond with a labeled sorus, sporangia and indusium.
Lady fern is identified in part by the shape of its sori. They are usually curved or
horseshoe-shaped. They appear in late summer and early fall.

A cluster of brown fertile fronds of ostrich fern. Their shapes resemble ostrich feathers.
Fertile fronds of ostrich fern.
Not all ferns have sori that look like those of lady fern. Some are located along the edges of the frond, some have indusia of different shapes or sizes, and some have no indusia at all. Other ferns, such as ostrich fern (Matteuccia struthiopteris) and sensitive fern (Onoclea sensibilis), produce spores on fronds specialized for that purpose – in other words, the whole frond is devoted to forming spores. For all these variations, timing is important. Sori and reproductive fronds (aka fertile fronds) may appear only at certain times of the year, and the appearance of sporangia and sori can vary depending on how old they are. 

Both reproductive and vegetative characteristics are helpful to identify a fern. Here are some resources to learn more about fern anatomy, biology and identification.

  • Ferns. U.S. Forest Service. This site may take a few tries to load.
  • Dichotomous Key to Ferns of Wisconsin, by Tim Gerber, UW-La Crosse.
  • Key to Fern Traits, by Areca Treon. This is a key to ferns in Cedar Creek Ecosystem Science Reserve near Bethel, MN.
  • Ferns of Minnesota, by Rolla Tryon. Illustrated by Wilma Monserud. University of Minnesota Press, 1980. ISBN 0-8166-0932-2.
  • Ferns and Lycophytes of Minnesota: The Complete Guide to Species Identification, by Welby R. Smith (author) and Richard Haug (photographer). University of Minnesota Press, 2023. ISBN 1517914663.

 For more information about lady fern, try these sites:

Sunday, September 17, 2023

Snakeroot's Secret

White snakeroot, Ageratina altissima, flowering in a woodland edge in August.

In the fall of 1818, a 35-year-old pioneer woman fell ill and took to her bed in a crude dwelling near Pigeon Creek in Indiana. She had been caring for her sick relatives and a neighbor before she came down with the same symptoms: lethargy, abdominal pain, fever, nausea, and worse.

She had no medical care, so her health declined quickly. In a matter of days, she slipped into a coma, but before she lost consciousness, she called her two children to her side. When she died, her nine-year-old son, Abraham, is said to have been devastated. He would later write that his mother, Nancy Hanks Lincoln, made him all that he was.

Called sick stomach and later milk sickness, the mysterious illness was a menace on the 1800s wooded frontier. It sickened and killed thousands and terrified thousands more, because its cause was unknown. Faced with the agonizing and unexplained deaths of their family and friends, many pioneers abandoned their settlements for what they hoped would be healthier locations. In some cases, entire towns were deserted, as told by a writer to the Farmers’ Register in 1834:

A Village Depopulated by the Milk Sickness

The following extract is of a letter from a traveler dated at St. Louis:

A few miles below Alton, on the Mississippi, I passed a deserted village, the whole population of which had been destroyed by the “milk sickness.” The hamlet consisted of a couple of mills and a number of frame houses, not one of which was now tenanted; but the dried weeds of last year choaked [sic] the threshold of the latter, and the raceways of the mills were lumbered up with floating timber, while the green slime of two summers hung heavy on the motionless wheels. Not an object but ourselves moved through the town; and the very crows themselves seemed to make a recruit around the fatal place when they came in view of the thickly sown burial ground on the skirts of the deserted village. (1)

Although the settlers often found the illness again in their new homes, their knowledge was building. They recognized that cattle stricken with “the trembles,” a shaking weakness that progressed to more severe illness, could cause a similar condition in people who drank the cows’ milk or ate their beef, butter, or cheese. The illness tended to appear later in the season, from mid-summer through fall, and it was worse in dry years. Newcomers to areas stricken with the illness were advised to avoid eating beef or dairy products from July to the first frost.

That good advice likely prevented many cases of illness, but the ultimate cause of milk sickness remained unknown, or at least debated, for decades. In hindsight, it didn’t have to be. Unfortunately for many who would later become ill, an early and accurate warning was largely missed, in part because it came from a woman. Actually, from two women.

In Illinois around 1830, Anna Pierce Hobbs Bixby, a nurse and midwife called Doctor Anna, was grieved about the cause of milk sickness. It had killed her mother and sister-in-law and it had disabled her father, who developed a chronic, disabling form of the illness called “the slows.” She suspected the cause was something cattle were eating, so she followed them into their wooded pasture to record what they ate.

While she was there, she is said to have met an elderly Shawnee woman hiding from forced relocation to a reservation in Kansas. After the elderly woman learned what Doctor Anna was looking for, she identified white snakeroot as the plant that was making animals and people sick. The women parted, and the fate of the Shawnee elder is lost in history.

Now known by the scientific name Ageratina altissima (formerly Eupatorium rugosum, E. ageratoides, and E. urticaefolium), snakeroot’s phenology matched the seasonality of the sickness. It flowers in mid-summer into early fall, coinciding with the time milk sickness tended to occur. Its habitat was another good match. Snakeroot grew in woodlands, including the forested pastures where cattle then commonly grazed, and it persisted in drought. When they had no choice, cattle ate snakeroot.

White snakeroot range in North America (left) and the upper Midwest (right). USDA NRCS 2023.

With this new-found knowledge, Doctor Anna began experimenting. She fed the plant to animals, including calves, and found that they developed the trembles. Convinced that she had found the cause of milk sickness, she spread the word. She grew a garden of white snakeroot to teach others what it looked like, and she urged farmers to pull it out of their pastures. They did, and her advice is thought to have saved many lives, at least in southeastern Illinois.

But that’s as far as it went. Whether her work was dismissed or not widely published or both, it didn’t get much traction. Instead, physicians and settlers alike continued to speculate about the cause of milk sickness. They blamed all kinds of things: arsenic or other metals, bacteria, bad water, poison oak, poison ivy, and other agents. Some blamed miasmas, imaginary, poisonous exhalations from the earth that misted the vegetation and sickened the cattle.

As the debate continued through the 1800s, milk sickness nearly vanished. That was another mystery, although a welcome one. Two hundred years on, we know why it disappeared: Cattle came to be pastured not in the woods but in cultivated pastures where snakeroot was excluded, and commercial operations combined and diluted milk from many sources. If the contaminant was present in the milk, it was at lower concentrations, too low to produce the severe illness caused by chronic consumption of tainted meat and dairy products.

Even as milk sickness waned, research continued into its cause. The poisonous-plant hypothesis eventually held after other possibilities were eliminated, and snakeroot was finally confirmed as the cause of the illness in the early 1900s, almost 100 years after the Shawnee woman and Doctor Anna warned of its dangers.

In 1928 or 1929, James F. Couch, a chemist with the USDA, identified the toxin in snakeroot that had caused so much suffering. He described it as “a viscous . . . oil with a pleasant aromatic odor” and named it tremetol after the tremors it caused (2). The compound is present in all parts of the plant and is also found in rayless goldenrod, aka jimmyweed (Isocoma pluriflora), a plant native to the Southwest.

Milk sickness, or chronic tremetol poisoning, is rare now, but the University of Minnesota includes snakeroot among the plants known to be poisonous to livestock. While there is some concern that a return to small-scale, “natural milk” could result in cases of (now treatable) milk sickness, today white snakeroot is more often appreciated as a late-season source of nectar or pollen for bees, wasps, and flies and as a likely host plant for moth larvae (3). It’s available from many native plant nurseries – with some history attached.

To learn how to identify white snakeroot, see this page from the Friends of Eloise Butler Wildflower Garden.

Cited References

(1) A Village Depopulated by the milk sickness. Farmers' Register. Oct1834, Vol. 2 Issue 5, p308-309. 2p. [Obtained through the Hennepin County Library’s database of  American Antiquarian Society (AAS) Historical Periodicals Collection: Series 2.]

(2) Trembles (or milk sickness). James F. Couch. Circular No. 306, United States Department of Agriculture. 1933. https://archive.org/details/tremblesormilks306couc/page/n1/mode/2up

(3) White snakeroot. Illinois Wildflowers, website accessed 9-17-23. 

Additional References

Milk Sickness. Curtis Wood, NCPedia, 2006.

The “Slows”: The Torment of Milksickness on the Midwest Frontier. Walter J. Daly, Indiana Magazine of History 102 (1): 29-40, March 2006.

The Death of Nancy Hanks Lincoln. Philip D. Jordan, Indiana Magazine of History 40 (2): 103-110, June 1944.

Religion and Removal among the Shawnee from Ohio into Kansas. Brady DeSanti, International Journal of Humanities and Social Science 3 (4): 46-56.

How an 1800s Midwife Solved a Poisonous Mystery. Will McCarthy, Smithsonian Magazine, July/August 2023. [Note: A photograph in the article incorrectly labels white snakeroot flowering in spring. It flowers in mid-summer to fall.]

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 09/17/2023). National Plant Data Team, Greensboro, NC USA.

Saturday, August 19, 2023

Once Upon a Milkweed

A black and gray sweat bee walking on top of a group of pink swamp milkweed flowers.
A sweat bee (genus Lassioglossum) on swamp milkweed (Asclepias incarnata) is in a precarious position. 

Milkweeds are familiar to many as essential for Monarch butterflies, but there’s much more to their story. A close look at their flowers shows an intricate structure with a tricky way to snag insects – literally.

The flowers of swamp milkweed (Asclepias incarnata), like many other milkweeds, are composed of five reflexed petals, five upright hoods, and five narrow horns around a gynostegium, a central column of fused stamens and pistils. The bases of the hoods hold nectar, and between them are narrow slits bordered by two “guide rails.” Each slit leads to a chamber that contains the reproductive parts of the flower, including the stigma, the part that receives pollen. For that reason, it’s called the stigmatic chamber.

A group of swamp milkweed flowers with the petals, horns, hoods, gynostegium and stigmatic slits labeled.

What’s missing from the flowers are anthers shedding dust-like pollen grains. Unlike typical flowers, milkweeds don’t offer individual grains for insects to carry away. Instead, their pollen is packed into waxy sacs called pollinia. Each chamber holds two pollinia connected by a pair of arms and a central gland or corpusculum, Latin for “little body.” The whole structure, called a pollinarium, looks like a small pair of winged maple seeds.

Pollinia are rare. Only orchids also make them. The advantage of packing pollen grains together is that they can be carried as one unit to deliver hundreds of grains to the stigma of another flower. That improves the odds that every ovule in an ovary will be fertilized and develop into a seed. 

[Sidebar: In seed plants, ovules contain egg nuclei and develop into seeds. One or more ovules reside in an ovary, which sits at the base of a pistil, the “female” reproductive part. Above the ovary is a neck-like style and the stigma, the surface that receives pollen. Ovary walls develop into fruits.]

The challenge for milkweeds is to somehow get the pollinia out of the chamber and onto another flower. To accomplish this, milkweeds rely on bees, wasps, flies and butterflies as carriers. The insects visit the flowers to get nectar, and in the process, take the pollinia. And that’s where it gets tricky.

As an insect walks across the waxy surface of a milkweed flower, a leg or other body part can accidentally slip into one of the slits between the hoods. The corpusculum then catches its leg, forcing the insect to pull hard to get it out. Sometimes the insect doesn’t succeed, and it either leaves behind a leg or dies trying to get it loose. But if the insect can manage, it extracts its leg with the pollinarium attached. Then it’s off to another flower and perhaps another slip into a chamber, where the pollinia are deposited and the pollen can reach the stigma.

A bristly tarsus of a digger bee to which a dangling yellow pollinium is attached.
Pollinarium with dangling yellow pollinia on the tarsus (lowest leg segment) of a digger bee. 
Photo by Allan Smith-Pardo, Bees of the United States, USDA APHIS PPQ, Bugwood.org.

That’s a lot of effort, for both the insect and the plant. The reward for the insect, if it isn’t snagged forever in a milkweed flower, is a source of nectar that is almost pure sucrose, the same as in your sugar bowl. The reward for the plant, as mentioned above, is an abundant and directed source of pollen. No other plants but milkweeds can receive pollinia, so little pollen is lost on plants that can’t use it. Even a different milkweed species is unlikely to accept pollinia from, say, a swamp milkweed, because the size and shape of the receiving chamber may not fit the arriving pollinia. Hybrids are therefore uncommon.

To listen to an ecologist talk about milkweed pollination and why it’s so unusual (and cool!), see this video by Dr. Thomas Rosburg of Drake University for Iowa PBS.

To see milkweed pollination in action, see this video from the Master Gardeners of Northern Virginia (scroll down at the site) or another video from Monarch Butterfly USA. Some of the terms differ, but the process – and the pitfalls – are the same. 

To learn more about swamp milkweed in particular, see this page from Minnesota Wildflowers.


Minnesota Wildflowers

Illinois Wildflowers

Milkweed pollination biology. By Eric P. Eldredge, USDA NRCS. November 2015. 

Milkweed pollination: A series of fortunate events. By Chris Helzer in The Prairie Ecologist, January 2021. 

Wyatt, R. and Broyles, S. B. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25: 423-441. https://www.jstor.org/stable/2097319

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