Bloodroot's Bet

This herbaceous perennial is among the first to bloom on the deciduous forest floor. It's also among the hardiest.

Bloodroot, Sanguinaria canadensis, in early April 2021.

Every year, bloodroot throws the dice. It blooms surprisingly early, risking the return of winter weather to grow and flower when light intensity is highest on the deciduous forest floor. Its way of surviving in shade is to mostly avoid it, and thanks to several adaptations to cope with the uncertain conditions of early spring, it has succeeded.

Sometime in late March or early April, single basal leaves spear through the leaf litter, wrapped around and protecting flower stalks bearing one, delicate bud. On dry, sunny days, the flowers open like bowls, each a brilliant display of eight or more petals around a mass of orange-yellow stamens and a central pistil.

Little else is blooming at the time, so bloodroot doesn’t have much competition for the few pollinators that are active in early spring. Although bloodroot has no nectar, insects nevertheless circle over the flowers in search of a sugary sip.  A few will land inside, and over the course of a few days, they will find that Bloodroot’s offerings change.

When a flower first opens, only the stigma is mature. The anthers need another day or two before they’re ready to release any significant amount of pollen. Although visiting insects won’t find much pollen to gather from a young flower, they can deliver what they collected from an older one. In rapid succession, the stigma receives the pollen, the eggs are fertilized, and seeds begin to develop. Such cross-pollination has the advantage of mixing genes from different plants, creating new combinations that might improve offspring survival.

Self-pollination isn’t likely to occur at this stage, in part because the anthers aren’t mature but also because the stamens initially bend away from the pistil. By day three in a flower’s life, however, the anthers are fully developed and the stamens change their position. Instead of bending away from the pistil, they bend toward it, increasing the odds that the flower will fertilize itself.

Left: In recently-opened flowers, the stamens and immature anthers bend away from the pistil.
Right: In an older flower (three days or so), the stamens bend toward and even arch over the pistil.

In a sense, this is bloodroot’s life insurance policy. If cross-pollination doesn’t happen – say in a stretch of cold, rainy or snowy weather that limits pollinator activity – self-pollination is a fallback. The resulting seeds will produce offspring much like the parent, but at least there will be seeds, tiny propagules that can be disseminated to expand the population.

Whatever ate these Bloodroot leaves might not have
enjoyed them for long. Bloodroot sap contains several
alkaloids, bitter molecules that can be harmful.

Of course, insects aren’t the only animals looking for food in early spring. Hungry herbivores are also on the prowl, and bloodroot provides some tempting, tender shoots. One chomp and bloodroot could lose its annual opportunity to grow and reproduce. The “blood” of bloodroot tends to discourage that activity, however. It contains several alkaloids, molecules that are distasteful if not harmful to animals that eat them. Alkaloids are most concentrated in the roots, but they are found in all parts of the plant, including the leaves and petioles. A mouthful of the bitter greens could be enough to put an herbivore off bloodroot for a long time.

It’s a gamble to grow early, but bloodroot has adapted to the risk. The plant's fragile appearance belies a durability born of countless generations on the forest floor. Through genetic trial and error, the dice came to be loaded in its favor.     

References

Hayden, W.J. (2005). Bloodroot pollination: Bet-hedging in uncertain times. Bulletin of the Virginia Native Plant Society 24(1): 5+

Matsuura, H., & Fett-Neto, A. (2015). Plant Alkaloids: Main Features, Toxicity, and Mechanisms of Action.

Schemske, D.W., Willlson, M.F., Melampy, M.N. et al. (1978). Flowering ecology of some spring woodland herbs. Ecology 59 (2): 351-366.

Hopeful News About Emerald Ash Borer

 “Attack fungi” could help manage this destructive insect.

University of Minnesota researchers recently discovered insect-attacking fungi in the larval galleries of emerald ash borer. Their finding offers hope that the fungi could help manage this destructive pest.

Emerald ash borer (EAB) is an introduced beetle that kills all species of ash trees. It has been present in the U.S. since at least 2002 and in Minnesota since at least 2009. Even with a federal quarantine (now removed), the insect spread rapidly. According to the USDA, its range now covers most of the eastern U.S. and the Midwest, with isolated infestations as far west as Colorado.

The larvae of EAB cause the bulk of the damage. After eggs hatch, the larvae bore into the inner bark, the area that includes the water- and sap-conducting cells of the xylem and phloem. Their galleries of serpentine tunnels interfere with the flow of needed resources and kill trees in 2-4 years. Hundreds of millions of trees have succumbed to the insect.

Hope for managing EAB rests in part on biocontrols, organisms that prey on eggs, larvae or adults and so reduce their numbers. Three parasitoid wasps have been released in Minnesota as potential biocontrols. More information about that program is available here.

New biocontrols could emerge from recent research by the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC). In their study published in Fungal Biology, U of M researchers sampled affected trees from Rochester to Duluth and isolated the fungi associated with EAB larval galleries. They identified many types of fungi, including some that are entomopathogenic – fungi that attack insects.

One fungus they identified, Beauveria bassiana, has already been studied for EAB control. The other entomopathogenic fungi they found also need research to see if they, too, could be used to manage the insect.

This good news comes as EAB continues to spread in Minnesota. Since the November 2020 post about EAB, the insect has been confirmed in two more counties, Cottonwood and Blue Earth in southwest Minnesota. The state’s Department of Agriculture maintains a quarantine boundary that now includes 27 affected counties. 

Quaking Aspen Breaks the Ice

 

If you’re eager to see something in bloom, look for quaking aspen (Populus tremuloides). In southern Minnesota, the tree’s flower buds broke in early March and the flowers are almost mature. Don’t expect to see anything showy, however. Early bloomers like quaking aspen tend to be wind pollinated. There aren’t many insects around yet, so they don’t produce large flowers with colorful petals. Instead, quaking aspen produces dozens of tiny flowers on small spikes called catkins.

This is a lengthwise section through a catkin of male flowers. 

Each flower is just a few millimeters wide, composed of a light 

yellow cup, a brown bract with finger-like lobes and hairs, and

stamens. The red "bumps" are developing anthers.

This photo was taken on March 22, 2021.

The only flower parts you might see, if you look closely, are stamens or pistils. Individual trees usually bear only one kind of flower. In other words, trees are either male (pollen-producing) or female (seed-producing). Both male and female trees flower before leaves emerge, timing the wind-driven spread of pollen when there is the least interference.

You might also notice that all or most of the trees in a stand of quaking aspen bloom at the same time. That’s likely because they’re clones. Quaking aspen reproduces vigorously by root suckers, with young plants emerging amid or around a stand of older trees. All individuals in a clone are connected by a common root system and are genetically identical. In a sense, they are one being. 

The stand pictured here, found in western Hennepin County, covers about 2,000 square feet. Most of the trees in the stand bloom at the same time, so they probably belong to the same clone. In other words, that’s one organism covering 2,000 square feet. That’s big, but it’s far from the biggest clone of quaking aspen. A stand in Fishlake National Forest in central Utah, called Pando (Latin for “I spread”), comprises more than 40,000 individuals, all male, together covering 106 acres and weighing an estimated 13 million pounds. Pando was once the largest known organism on Earth, and one of the oldest. Although its exact age is uncertain, the stand is thought to have originated at the end of the last glacial period, about 11,000 years ago.

Unfortunately, Pando may be declining. Scientists have noticed that the stand is producing fewer young trees. Grazing and browsing of root suckers is one possible reason, but diseases, insects and lack of disturbance, which favors vegetative reproduction, may also be causes. Efforts are underway to understand why the stand isn’t regenerating and to slow or prevent its further decline.

Far from Utah, in this small stand of quaking aspen, female trees will soon be pollinated and release their cottony seeds. Like the pollen, the seeds are carried by wind, and in a month or so there will be a blizzard of them. If they happen to land where soil is moist – even for just a few hours – they will germinate and an infant stand may be born. It’s a chancy way of reproducing, but even Pando started this way. Big clones from little aspen seeds grow.

References

The Diminishing Pando Clone: History and Forest Management
https://history.utah.gov/the-diminishing-pando-clone-history-and-forest-management/
 
Pando – (I Spread)
https://www.fs.usda.gov/detail/fishlake/home/?cid=STELPRDB5393641
 
Smith. W.R. 2008. Trees and Shrubs of Minnesota. Minnesota Department of Natural Resources. University of Minnesota Press, Minneapolis.

The Two Lives of Cedar-Apple Rust

 

The dimpled, red galls on this eastern red cedar (Juniperus virginiana) are signs of cedar-apple rust, a fungus that divides its time between two completely different hosts. One part of its life cycle is completed on junipers, where these golf ball-like masses can be spotted in winter. The other part is completed on plants in the rose family, such as apple trees. On each host, the appearance of the fungus is so different that it can be hard to connect the two as belonging to the same organism.

In spring, cedar-apple galls on junipers sprout gelatinous, orange “horns.” These gummy tentacles produce and release spores that can infect the leaves of apples, crabapples and sometimes hawthorns. As the fungus grows on apple trees, the leaves develop yellow or orange spots on their upper and lower surfaces. In summer, spores released from the spots on the lower surfaces of the leaves are spread by wind back to junipers, where they form overwintering galls. And so the cycle is continues.  

Left: Cedar apple gall with orange, spore-producing "horns" in spring. Right: Spots on an apple leaf caused by the cedar-apple rust fungus. Spores produced on apple leaves then infect junipers.

Photos by James Chatfield, Ohio State University, Bugwood.org, through forestryimages.org.

Cedar-apple rust usually doesn’t have severe effects, although infection can cause susceptible apples and crabapples to lose their leaves early. Fruits may also develop unattractive spots. Many varieties of apples and crabapples are resistant to cedar-apple rust. For a list, see the University of Minnesota Extension Service link below.

Spots on apple leaves can also be caused by other fungi. Apple scab is one example.

References

Cedar-apple rust and related rust diseases. R. Koetter and M. Grabowski, University of Minnesota Extension Service.  Accessed online 2/27/21.

Plant of the week: Cedar apple rust (Gymnosporangium juniperi-virginianae Schwein.). D. Taylor, U.S. Forest Service. USDA. Accessed online 2/27/21.

Agrios, G. N. 1988. Cedar-Apple Rust. Pages 462-466 in Plant Pathology, third edition. Academic Press, Inc. New York.

Crusty Clues to Plant ID

In this patch of winter woods, black knot has a grip on the understory. Dark, lumpy galls crust over many stems and branches, flagging them against the snow. From a distance it looks like scat, but this isn’t animal stuff. It isn’t plant stuff either, not entirely. It’s a fungus, and it’s dropping hints about the plants growing here.

Many fungi that grow on plants, including black knot, have specific hosts. Some grow so consistently on one plant or another that when they’re found, they can help identify a plant to its genus, if not its species. This can be especially helpful in winter, when plant ID is challenging without leaves.

Many fungi serve as reliable guides to plant identification, but here are three that are especially common or commonly sought and easy to recognize.

Prunus and Black Knot

Black knot, Apiosporina morbosa, infects trees and shrubs in the genus Prunus, a group that includes native and introduced cherries and plums. In Minnesota, the disease is especially common on chokecherry, Prunus virginiana, a small tree found in open woods and woodland edges throughout the state. It also affects American plum (P. americana), Canada plum (P. nigra), pin cherry (P. pensylvanica), black cherry (P. serotina) and sand cherry (P. pumila). All these are native shrubs and trees, but several introduced species and cultivars of Prunus are also susceptible to the disease.

Black knot is spread in spring, when fungal spores produced in these knots are carried by wind or rain to new hosts. A year later, after the fungus has stimulated its host to grow a mass of large cells, the infection appears as a swelling with a light brown or olive-green surface. In the second summer after infection, the gall turns black. Some galls may have white or pink patches on the surface from other fungi parasitizing the knot.

Tiny pores on the surface of a gall mark the exit holes for spores. They’re easiest to see with magnification. Pores are one way to distinguish black knot from chaga, a similar dark, crusty fungus that grows primarily on birch trees. Unlike black knot, chaga is sterile -- it doesn't produce spores. More on that next. 

Birch Trees and Chaga

Chaga, Inonotus obliquus, is a parasitic fungus found in northern forests around the world, including the United States and Canada. It has been used in folk medicine for centuries and is still harvested and consumed for its purported health benefits. Formal studies of its medicinal use continue.

In this region, chaga grows mostly on birch trees, especially paper birch, Betula papyrifera. It also grows on yellow birch, Betula alleghaniensis, and much less frequently on alders, beech, oaks, maples and aspen.

On living birch trees, chaga looks like a hard, irregular, black mass erupting from the trunk. Likened to burned charcoal, the surface is a melanin-rich mass of dead fungus over an orange or brown interior. The masses, popularly called conks, appear on trunks or large branches, but not on small branches. An infected tree may bear 1-3 conks.

Chaga conks are the visible part of the fungus. The hidden part lives mostly in the heartwood, where it causes white rot. The tree may live with the infection for up to 80 years, producing slow-growing, sterile conks that take many years to mature.

Inonotus, the fungus that produces chaga, reproduces only after its host dies. The fruiting body (actually a spore-producing body called a basidiocarp) is a mat of slender, vertical tubes formed under the bark, typically above a sterile conk. As the fruiting body develops, it exerts so much outward pressure that it will suddenly rupture the bark. It’s a fascinating find, but a rare one. Fruiting bodies are produced only once in the life cycle of the fungus, and they live for just a few days.

Chaga look-alikes include black knot and several shelf mushrooms in the genus Phellinus. Unlike chaga, these fungi release spores through tiny pores. Black knot pores cover the gall, wheareas shelf mushrooms have a pore layer on their lower surfaces. 

A note about harvesting chaga:

Because of its purported medicinal value, chaga is in high demand, so harvest from some public lands is regulated. According to Ed Quinn, Natural Resource Program Supervisor for the Division of Parks and Trails at the Minnesota DNR, harvesting chaga is illegal in state parks, state recreation areas, state monuments and state waysides. Although state park rules permit collection of­­­­ mushrooms for personal use, chaga is technically not a mushroom because it isn’t a fruiting body -- it doesn’t produce spores. In addition, use of spikes, ladders, knives and hatchets to reach and collect the fungus can damage trees and create wounds that may leave them vulnerable to pathogens and insect pests. Finally, because the fungus slowly rots the heartwood of infected trees, climbing them can be unsafe.  

The rules in state forests differ from those in state parks. According to Dave Schuller, State Land Programs Supervisor for the Division of Forestry at the Minnesota DNR, collecting chaga for personal use is allowed without a permit, but host trees must not be damaged. Commercial harvest is allowed with a special products permit from a local DNR forestry office. The permit requires harvesters to take only the visible part of the fungus, without cutting into the stem or damaging live trees. More information about chaga harvest is available from the University of Minnesota Extension Service publication linked in the references below. Again, safety is paramount when harvesting chaga. Because trees bearing the fungus may have significant interior decay, climbing them is risky and can result in serious injury. 

Harvesting chaga on other public lands, such as regional or local parklands, may also be regulated. Check with the appropriate authority before heading out.

There are also concerns about the potential effects of collection on chaga biology and ecosystem health (Thomas et al., 2020). One issue is that overharvesting chaga has unknown effects on the ability of the fungus to reproduce. Taking too much, too often could delay or deny Inonotus its once-in-a-lifetime opportunity to make fruiting bodies, with consequent effects on its spread.  

Overharvesting also has unknown effects on the function of ecosystems where chaga is found. Other living things – insects, for example – may depend on chaga in ways not yet understood. Without that understanding, aggressive collection of chaga could have ripple effects on ecosystem health. More study is recommended.

 

White Oaks and Smooth Patch

Smooth patch looks like it sounds: It’s a smooth patch on otherwise rough bark. Usually the area is low on the trunk and sunken and lighter than surrounding bark. Smooth patch disease is caused by any of several fungi, especially Aleurodiscus oakesii. This fungus prefers trees in the white oak group, In this region the most common hosts are white oak, Quercus alba, and bur oak, Quercus macrocarpa. Less often, smooth patch is also found on birch, ash, willow and basswood.

The fungi that cause smooth patch feed only on dead outer bark and don’t directly harm the tree. Small, cup-like fruiting bodies are often seen within the smooth patch. They are light brown or gray with curled edges. In winter they may be shriveled and look like lichens. The genus Aleurodiscus translates to “flour disc,” named for the whitish, dusty appearance of the disks in spring and summer.

 

References

Black knot

American Phytopathological Society. Black knot. Website accessed February 11, 2021.

University of Illinois Extension, Department of Crop Sciences, University of Illinois at Urbana-Champaign. Black knot of plums and cherries. RPD No. 809 September 2000.

University of Minnesota Extension Service. Black knot. Website accessed February 11, 2021.

Chaga

Millman, L. 2012. Chaga’s Significant Other. Fungi 5:3, 11-12.

Min-Woong Lee, Hyeon Hur, Kwang-Choon Chang, Tae-Soo Lee, Kang-Hyeon Ka, L. Jankovsky. Introduction to Distribution and Ecology of Sterile Conks of Inonotus obliquus. Mycobiology. 2008 Dec; 36(4): 199–202. Published online 2008 Dec 31. doi: 10.4489/MYCO.2008.36.4.199

Natural Resources Canada. Sterile conk trunk rot of birch. Date modified:2015-08-04. Website accessed February 11, 2021.

Spinosa, R. and Bunyard, B. No, That’s NOT Chaga! Fungi 5:3, 45-47

Thomas P.W., Elkhateeb W.A. & Daba G.M. 2020. Chaga (Inonotus obliquus): a medical marvel becomes a conservation dilemma? Sydowia 72: 123–130.

University of Minnesota Extension Service. 2013. Chaga (Clinker Polypore). Pages 99-101 in the Minnesota Harvester Handbook. Available at https://conservancy.umn.edu/handle/11299/173824.

Smooth patch of oak

Smith, W. R. 2008. Quercus macrocarpa Michx., Bur oak. Pages 382-383 in Trees and Shrubs of Minnesota. Minnesota Department of Natural Resources. University of Minnesota Press, Minneapolis.

University of Minnesota Extension Service. Non harmful tree conditions. Website accessed February 11, 2021.

Vann, S. R. Undated. Smooth patch of oak trees. FSA7578, University of Arkansas, Division of Agriculture.

Volk. T. April 2006. Aleurodiscus oakesii, the oak parchment, cause of "smooth patch disease."https://botit.botany.wisc.edu/toms_fungi/apr2006.html, accessed February 16, 2021.

How to Identify Oriental Bittersweet in Winter

The twining stems, yellow capsules and scarlet arils of Oriental Bittersweet are distinctive in winter.

Winter is an ideal time to look for Oriental bittersweet (Celastrus orbiculatus), an invasive vine that is spreading in Minnesota. Left unchecked, this aggressive vine can smother and strangle its hosts and shade out native vegetation. For those reasons, Oriental bittersweet is considered a noxious weed in several states, including Minnesota. The state’s Department of Agriculture includes it on the Prohibited-Eradicate list, which means all above- and below-ground parts should be destroyed.

What to Look For

Like native American bittersweet (C. scandens), Oriental Bittersweet, also called Asian Bittersweet, is a twining vine. Oriental Bittersweet vines are often tightly wrapped around their hosts, sometimes girdling them, whereas American Bittersweet vines are usually looser. In both species, buds are mound-shaped, 3-5 mm long (about an eight of an inch) and pointed at right angles to the stem. As the stems age they develop ridges and furrows in irregular diamond-shaped or striped patterns. Vines are either male (pollen-producing) or female (fruit-producing).

Clockwise from top left: A bud, tightly wrapping vine, and mature bark
of Oriental Bittersweet.

Male and immature vines are hard to identify to species, but mature female vines can be identified by the color and placement of their fruits. Oriental bittersweet capsules are yellow, opening in fall to reveal scarlet arils. Fruit clusters grow from the axils, the angles between the buds and the stem. 

In contrast, American bittersweet capsules are orange with darker arils. The capsules may fade to tan during winter, but typically they retain at least a few specks of orange. Fruit clusters grow at the ends of stems and branches rather than from the axils. 

Fruits of Oriental Bittersweet (left) and American Bittersweet (right). 

Hybrids Can Have Intermediate Characteristics

Hybrids of Oriental and American Bittersweet are uncommon, but of the few that have been found, at least one displayed intermediate characteristics. A hybrid female identified during one field study  produced clusters of flowers and fruits both at the ends of the stems and at the axils. Its capsules were light orange, intermediate between the yellow of Oriental Bittersweet and the darker orange of American Bittersweet. More information about hybrid research follows the references below.

Where to Look

Oriental Bittersweet grows in a variety of conditions. Typical habitats include fields, forest edges, and open woods, but even forest interiors can support this shade-tolerant vine. Removing Oriental Bittersweet from residential properties is as important as removing it from natural areas, because birds can spread the seeds wherever they fly.

How to Manage Oriental Bittersweet

Managing Oriental Bittersweet typically involves applying herbicides to the basal (lower) bark, leaves or cut stems. Seedlings can be hand-pulled, but they're hard to identify at that stage. Most of the references below include management advice. 

References

Oriental bittersweet. EDDMapS. Website accessed November 5, 2022.

Oriental bittersweet. Minnesota Department of Agriculture. Website accessed November 5, 2022.

Oriental bittersweet. Minnesota Department of Natural Resources. Website accessed November 5, 2022.

Differentiating Oriental and American Bittersweets. Minnesota Department of Agriculture. Website accessed November 5, 2022.

Asian Bittersweet. Woody Invasives of the Great Lakes Collaborative. Website accessed January 29, 2021.

 

Research on Hybrids

Hybridization between Oriental and American Bittersweet is suspected of contributing to the decline of the native species. One study (Pooler et al. 2002) found that crossing male Oriental Bittersweet with female American Bittersweet produced seeds that germinated sooner (with shorter dormancy) and seedlings that grew faster than American Bittersweet. Although these results hint at possible dominance of the hybrid vines, the study did not follow the hybrids into maturity.

A later study (Zaya et al. 2015) looked for naturally occurring hybrids across the eastern half of the U.S. to determine their prevalence and their potential impact on American Bittersweet. Using DNA analysis, the researchers found that 4-8% of 475 individuals sampled were hybrids. Only one female hybrid was confirmed among the sampled vines, and it displayed intermediate field characteristics. Flower clusters were produced both at the ends of stems and from the axils, and capsule color was light orange, intermediate between the yellow of Oriental Bittersweet and the darker orange of American Bittersweet. The hybrid capsules were smaller than either parent and produced few or no viable seeds. 

Two male hybrid vines were also confirmed among the vines sampled. When researchers examined their pollen grains, they found that more than 90% of them were smaller than the average for either parent, so they were judged to be inviable. Like the female hybrid, the male hybrids in this study could not reproduce as successfully as their parents.

The 2015 research suggests that the harm to American Bittersweet caused by hybridization isn’t from the hybrids themselves. Unlike some crosses between native and introduced species, hybrids of American and Oriental Bittersweet appear to be uncommon, and they do not successfully reproduce to form aggressive populations. Instead, the harm comes from wasting the reproductive potential of female American Bittersweet vines.  All the hybrids identified in the 2015 study resulted from Oriental Bittersweet pollen fertilizing American Bittersweet eggs. Because female flowers so pollinated could not then accept pollen from a male American bittersweet, they were not able to produce seeds that could maintain the native population.

References:

Pooler, MR, Dix, RL, and J Feely. 2002. Interspecific hybridizations between the native bittersweet, Celastrus scandens, and the introduced invasive species, C. orbiculatus. Southeastern Naturalist 1(1): 69-77.

Zaya, D.N., Leicht-Young, S.A., Pavlovic, N.B. et al. 2015. Genetic characterization of hybridization between native and invasive bittersweet vines (Celastrus spp.). Biol Invasions 17, 2975–2988. https://doi.org/10.1007/s10530-015-0926-z

How to Identify Common Buckthorn in Winter

A female Buckthorn displays a persistent leaf, fruits and terminal thorn that help with winter identification.

For an explanation of the terms used here, see a previous post, “How to Identify Deciduous Trees and Shrubs in Winter.”  A two-page guide to Buckthorn ID is free to download. 

Common Buckthorn, Rhamnus cathartica, is an invasive shrub or small tree that was introduced to the U.S. in the late 1700s as an ornamental and a hedge plant. Thanks to its prolific fruit production, it soon spread to a range of habitats, from forests and savannas to prairies and wetland edges. According to Bell Museum herbarium records, it has been present in Minnesota since at least 1937, when it was found at a barge terminal on the Mississippi River in Minneapolis.

This aggressive species degrades habitat in many ways. Typical of many invasive plants, established stands of Buckthorn shade out native species and reduce diversity. Their nitrogen-rich leaves are quickly consumed by soil invertebrates, including invasive earthworms that deplete the duff layer of organic matter. The resulting bare, mineral soils erode more easily and are more quickly colonized by additional Buckthorn or other invasive plants, such as Garlic Mustard (Alliaria petiolata).

Everything green in this understory is Common Buckthorn. The photo was taken in November.

 

Because Buckthorn retains its leaves late into fall, it is easy to recognize by the green layer it forms under a leafless canopy. Even after it sheds its leaves, however, there are easy ways to recognize this species. In fact, winter can be an ideal time to map and manage Buckthorn. This post shows how to identify the plant by its twigs, buds, bark and fruits.

“Buckthorn” Describes the Ends of the Twigs

Buckthorn's common name comes from the pair of curved buds at the ends of the twigs. They resemble a buck's hoof, and because a thorn is often found between them, the plant is called "buck-thorn." All buds are covered by multiple, dark brown scales. Lateral buds are mostly subopposite. 

Bark is Warty to Flaky with an Orange Inner Layer

Buckthorn saplings have silver or brown bark with light, horizontal, warty lenticels. As the trunk grows, its bark turns dark brown and flaky. Inner bark is bright orange. The bark of cherries and plums can look like Buckthorn, but none have bright orange inner bark or brown, subopposite buds.

Females Have Blue-Black Fruits – Lots of Them

Common buckthorn has separate male (pollen-producing) and female (fruit-producing) plants. Females produce hundreds to thousands of globose, dark blue to black, berry-like fruits that are clustered at the nodes. The fruits may stay on the plant through winter. Ingesting them causes nausea and vomiting, so this plant is also called purging buckthorn. 

Buckthorn Look-alikes

Glossy Buckthorn (Frangula alnus), another invasive, prefers wetter habitats than common buckthorn. Its twigs are not tipped with thorns. Buds are alternate and in winter are covered by rust-colored hairs rather than scales. Bark of younger trunks is speckled. Inner bark is yellow.

Left: A twig of Glossy Buckthorn showing alternate buds and a terminal bud covered with rust-colored hairs.
Right: Lower stems of Glossy Buckthorn showing speckled bark. The largest stem is about 5 cm (2 inches) in diameter.

Alder-leaved  Buckthorn (Rhamnus alnifolia), a native shrub typically found in moist or wet soils in forested parts of Minnesota, rarely reaches more than 3 feet tall, and its stems are typically no more than 1 inch in diameter (Smith 2008). Its buds are clearly alternate and lack thorns.

References

Buckthorn. Minnesota DNR. Website accessed January 12, 2021.

Buckthorn Management. Minnesota DNR. Website accessed January 12, 2021.

Buckthorn: What You Should Know, What You Can Do. Minnesota DNR. Guide EWR_395_17. (This is formatted as a printable guide. )

Common Buckthorn. University of Minnesota Extension Service. Website accessed January 12, 2021.

Common Buckthorn. Woody Invasives of the Great Lakes Collaborative. Website accessed January 14, 2021.

Common or European Buckthorn. Minnesota Department of Agriculture. Website accessed January 12, 2021.

Winter Control of Buckthorn (video). University of Minnesota Extension Service. December 2013.

Smith, W.R. 2008. Trees and Shrubs of Minnesota. University of Minnesota Press, Minneapolis.