New Invasive Plants to Watch For

The Minnesota Department of Agriculture has added several new plants to its 2023 noxious weed list. This post focuses primarily on additions to the Prohibited-Eradicate category, but three other categories – Prohibited-Control, Restricted and Specially Regulated – are covered at the end of the post.

Prohibited-Eradicate: The Early-Detection List

The plants in this group either aren’t here yet or are present in low numbers. They're in the early-detection stage, when ideally, they can be found and removed before they become widespread. As the name of the group suggests, these plants should be eradicated by destroying all above- and below-ground parts. Transporting, propagating or selling them is illegal.

Three plants are new to the list this year: Johnsongrass, pale swallow-wort and red hailstone.

Johnsongrass
Sorghum halepense

USDA NRCS Plants Database (see references). Johnsongrass is
confirmed in counties shaded blue. Lakes and rivers are also
shaded blue.

This perennial grass was introduced to the U.S. in the 1800s as a forage crop. It is now found in many
habitats, including pastures, roadsides, ditches, old fields and wetlands. It reproduces by seeds and rhizomes and can spread aggressively to form dense mats that exclude other plants. It will not tolerate drought or extreme cold, so where winters are severe, Johnsongrass may be a facultative (optional) annual. Stressed plants can produce toxic levels of cyanide.

Johnsongrass grows 8-12 feet tall when flowering. Leaves are alternate, hairless and up to 2 feet long with white midribs. Ligules are 3-4 mm long, membranous and sometimes toothed. Johnsongrass flowers from mid-summer to fall in loose, purplish panicles.

Leaves of Amur silvergrass (Miscanthus sacchariflorus), another introduced plant, also have white midribs, but this plant is shorter at 6-8 feet. Its ligules are a hairy fringe, and its panicles are silvery and silky in fall.

Clockwise from left: Johnsongrass stem and dense stand by Chris Evans, University of Illinois, Bugwood.org; Johnsongrass rhizomes and panicle by Steve Dewey, Utah State University, Bugwood,org. 

Pale swallow-wort
Cynanchum rossicum (aka Vincetoxicum rossicum)

EDDMapS (see references). Pale swallow-wort is confirmed in
counties shaded green.The plant has also been confirmed in Ontario.

Pale swallow-wort, also called European swallow-wort, is a perennial, twining vine that was imported
to the U.S. in the late 1800s, probably as an ornamental. It thrives in disturbed sites but can grow in a variety of habitats, including fields, pastures and woodland edges and understories. Fast-growing and shade tolerant, it can quickly overrun and outcompete other plants. Because swallow-wort is in the milkweed family, female monarchs will sometimes lay their eggs on it if common milkweed is unavailable. There is concern, however, that swallow-wort is toxic to monarch larvae.

The vine has opposite, shiny, oval or heart-shaped leaves with pointed tips. Stems grow to 7 feet long, wrapping around other plants or structures for support or clambering over the ground. Sap is clear, not white. Clusters of pink to reddish-brown, star-shaped flowers bloom in June and July. Pods are slender, smooth, and 2-3 inches long.

Black swallow-wort (Cynanchum nigrum) is also on the Prohibited-Eradicate list. It looks like pale swallow-wort but has dark purple flowers.

Clockwise from left: Pale swallow-wort vines by Rob Routledge, Sault College, Bugwood.org; Pods and seeds by Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. Leaves and flowers by Rob Routledge, Sault College, Bugwood.org.

 

Red hailstone
Thladiantha dubia

EDDMapS (see references). Red hailstone is confirmed in 
counties shaded green. 

Also called golden creeper or tuber gourd, red hailstone is a perennial vine introduced to North America
in the late 1800s or early 1900s as an ornamental. This adaptable plant grows in abandoned fields, roadsides, gardens, crop fields, railroad corridors and natural areas. Red hailstone can quickly overgrow and smother other plants, including crops.

Vines grow up to 20 feet long with tendrils that grasp other plants or structures for support. Leaves are alternate and heart-shaped. Stems, leaves and petioles (leaf stalks) are hairy. Yellow, tubular flowers bloom from July to September on separate male (pollen-producing) and female (seed-producing) plants.

So far, all plants found in Minnesota are male, so the vines aren’t spreading by seed. Instead, they reproduce vegetatively by small tubers carried along waterways; many of the mapped infestations are along rivers or streams. Where both male and female plants grow, 2-inch oblong fruits may form on female vines. The fruits turn red when mature, thus the name red hailstone.

Before it flowers, red hailstone resembles other tendril-bearing vines in the same family. Wild cucumber (Echinocystis lobata) is a hairless vine with five-lobed leaves and white flowers that bloom in late summer. Bur cucumber (Sicyos angulatus) is hairy but also has five-lobed leaves. Its flowers are white or greenish-white. Neither cucumber vine has red fruits.

Clockwise from left: Red hailstone leaves and infestation by Katy Chayka, Minnesota Wildflowers; Red hailstone flowering plants by Peter Dzuik, Minnesota Wildflowers.

 

Clockwise from left: Red hailstone flowers by Peter Dzuik, Minnesota Wildflowers; male flower closeup by Katy Chayka, Minnesota Wildflowers; rhizomes and tuber by Katy Chayka, Minnesota Wildflowers.

Other Categories of Noxious Weeds

The MDA defines three other categories of noxious weeds. Two were expanded in 2023.

·         Prohibited-Control: Plants in this category are already established here, so eradication isn’t practical. Management aims at preventing them from reproducing by seed or vegetative organs, such as rhizomes, tubers or stem fragments that can take root. Transportation of all propagating parts is illegal except as allowed by state law, and the plants may not be propagated or sold in the state. Sixteen species are on the list, including three kinds of knotweed featured in a previous post, Are Psyllids the Solution to Invasive Knotweeds? No new species were added in 2023. 

·        Restricted: These plants are widespread in Minnesota. The only practical way to manage them is to restrict their importation, sale and transportation in the state, except as allowed by state law. Two plants are new to the list this year: lesser celandine (Ficaria verna), an aggressive spring ephemeral and garden escapee, and salt cedar (Tamarix ramosissima), also called tamarisk, a shrub first introduced in the West for landscape use, windbreaks and erosion control. 

·        Specially Regulated: These are native or nonnative weeds that are economically valuable but potentially harmful if not controlled. Three plants are new to the list this year. Amur corktree (Phellodendron amurense) now must be removed wherever females have been planted or escaped, or their fruits and seeds must be prevented from spreading. Only male cultivars are legal to sell. Production of Callery pear (Pyrus calleryana) is being phased out over the next three years, after which the tree will be moved to the Restricted category. Tatarian maple (Acer tataricum) and its cultivars can be sold only if a label is attached advising that they should be planted only where the seedlings can be controlled, and ideally at least 100 yards away from any natural area.

 

References

EDDMapS. 2023. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed March 20, 2023.

USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 03/23/2023). National Plant Data Team, Greensboro, NC USA.

Johnsongrass

Wisconsin Department of Natural Resources

Minnesota Department of Agriculture

USDA Fire Effects Information System

University of Missouri

Pale swallow-wort

Minnesota Department of Agriculture

USDA Forest Service

Wisconsin Department of Natural Resources

Michigan Department of Natural Resources

Red hailstone

Minnesota Department of Agriculture

University of Minnesota Extension Service

Minnesota Wildflowers

Phenology and Citizen Science

Catkins of quaking aspen, Populus tremuloides, emerging in mid-March in southern Minnesota.

Although winter is hanging on this year, spring is coming. Signs are clear now: Aspen buds are breaking, maple sap is flowing and sandhill cranes are flying north, to name a few milestones.

These observations are examples of phenology, the study of seasonal changes in plants, animals and other life. The practice of tracking these events crosses many cultures and millions of years. From Ice Age cave paintings to the Cherry Blossom Festival in Washington, D.C., people have long recorded nature’s timing to know when to hunt, plant, harvest or look for their favorite species.

This old practice has gained new importance with climate change. Spring-casting maps from the USA National Phenology Network show that first leaf-out and first bloom are happening later this year than average for cloned dogwood and honeysuckles, the plants the NPN monitors to create the maps. So far, that’s especially true in the Southeast, Lower Midwest and Atlantic states up to New York. Data for northern areas are still a few weeks out, but as spring comes, those observations will build out the maps.

This shift toward earlier leaf-out and flowering is part of a world-wide trend attributed to a warmer climate. Potential consequences include increased risk of frost damage to early-emerging plants, an earlier allergy season, and mismatched timing between plants and insect pollinators.

Data collected by phenologists can help us understand this trend, and it also has practical uses. For example, phenoforecasts – predictions of when certain phenophases, or life stage events, will occur – can determine when to control invasive plants, such as buffelgrass, or how to estimate wildfire risk, as with red brome.

Phenology in Minnesota – And How to Help

In Minnesota, volunteer phenologists are observing plants and animals to help with many lines of research. In one project, Pesky Plant Trackers observed wild parsnip (Pastinaca sativa) and Japanese, giant, and bohemian knotweeds (Polygonum species) to learn more about the life cycles of these invasive plants. Although the formal project ended in 2022, observations are still welcome through Nature’s Notebook, the NPN’s data collection site.

Several other Nature’s Notebook campaigns also need volunteers. Here are a few that include the Upper Midwest in their study areas. Visit the campaigns page for more information about these and other projects.

• Nectar Connectors observe the phenology of food sources for Monarchs and other pollinators.

• Lilac observers report on cloned or common lilacs to add to decades of phenological data that track climate change.

• Quercus Quest volunteers watch oak trees to help researchers understand how hybridization affects their ecosystems.

Another resource is Season Watch, a collaboration of the University of Minnesota and Northern Community Radio. Visitors to the website can explore phenology by place, time of year, species and other themes. There are also resources for educators, information about indigenous phenology and links to articles that investigate the connection between climate change and phenology.

Plant watchers play an important role in learning how nature is responding to environmental change and what those responses could mean. It takes only a little training, a desire to see how and when plants grow, and for the time being, some patience to wait out the cold.

The Boon of Biological Nitrogen Fixation

White Clover, Trifolium repens.

White Clover is so common and modest that it’s often ignored. It’s like background noise: always there but barely noticed, at least until it flowers. Beneath its ordinary appearance, though, is an extraordinary ability: It can capture atmospheric nitrogen, N₂, and convert it to ammonia, NH₃, a first step in making nitrogen usable.

Called biological nitrogen fixation, this process is billions of years old and vital to life as we know it. Although nitrogen gas composes about 78% of the atmosphere by volume, most living things can’t use it. We humans, for example, can’t simply take a deep breath and get the nitrogen we need. We don’t have the molecular machinery to do that.

But some kinds of bacteria do. They possess nitrogenase, a complex enzyme that can break the strong bonds in nitrogen molecules and attach the atoms to hydrogen, making ammonia. Ammonia then goes on to participate in other reactions that make proteins, DNA and other biomolecules. When these compounds decay, or when some of the captured nitrogen leaks into the soil, other plants absorb it. We eat these plants or the animals that graze on them to get our supply of nitrogen. We can’t live without it.

Clover and other legumes house nitrogen-fixing bacteria in nodules on their roots. This symbiosis is of mutual benefit: The plants receive nitrogen from the bacteria, and the bacteria receive energy and carbon compounds from the plants. The nodules also provide a low-oxygen environment for nitrogenase to work. A kind of hemoglobin called leghemoglobin scavenges oxygen that would otherwise disable the enzyme. At the same time, leghemoglobin provides oxygen for cell respiration, the set of reactions that produces the energy to drive nitrogen fixation and other processes.

Nodules on the roots of White Clover hold bacteria that fix    

nitrogen.

Legumes are the primary biological nitrogen fixers, but a few plants in other families can do the same. Speckled Alder (Alnus incana), Silver Buffaloberry (Shepherdia argentea) and New Jersey Tea (Ceanothus americanus), for example, are non-legumes that also house nitrogen-fixing bacteria in root nodules. Called actinorhizal plants, they are mostly trees and shrubs from temperate regions. They are adapted to nutrient-poor soils, so some of them have been used to restore land degraded by mining, logging, wildfires or other disturbances.

Other fixers live freely in soil, or they live in close association with roots but not inside nodules. The latter includes bacteria that live in the rhizosphere (the near-root environment) of many grasses, including wheat and corn. Some researchers are trying to develop nodulating cereal crops that capture more of the nitrogen they need naturally instead of absorbing it from manufactured, energy-intensive fertilizer, which now supplies most of the nitrogen needed for agriculture. If they succeed, it could be part of the answer to mitigating climate change – and to feeding a hungry world.

Sources

Wagner, S. C. (2011) Biological Nitrogen Fixation. Nature Education Knowledge 3(10):15

Bernhard, A. (2010) The Nitrogen Cycle: Processes, Players, and Human Impact. Nature Education Knowledge 3(10):25

Diagne, N., Arumugam, K., Ngom, M., Nambiar-Veetil, M., Franche, C., Narayanan, K. K., & Laplaze, L. (2013). Use of Frankia and actinorhizal plants for degraded lands reclamation. BioMed Research International, 2013, 948258. https://doi.org/10.1155/2013/948258

Bakum, J. (2022) Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems. International Maize and Wheat Improvement Center (CIMMYT). 

Plant Profile: Black Locust

 Robinia pseudoacacia L.

Black Locust flowering in mid-June at Baker Park Reserve, Maple Plain, MN.

Black Locust, also called False Acacia or Yellow Acacia, is a medium to large tree originally from the Appalachian and Ozark mountains. Intentionally planted for its strong wood, stabilizing roots and attractive flowers, it has spread widely from its native range. It’s now found in all lower 48 states, several Canadian provinces and every continent around the globe (1, 2).

Black Locust History and Habitat

Although it isn’t native to the Minnesota, Black Locust has been here a while. The earliest herbarium record in the Minnesota Biodiversity Atlas (3) is an 1887 specimen collected in Lake City, along the Mississippi River in the southeast part of the state.

Black Locust thrives in full sun. It reproduces quickly      

from  root suckers.       

In later years, collectors found the plant farther north and west. The Atlas documents the plant in Duluth in the 1940s and Mankato and Pipestone in the1960s. Records increased in the following decades: 11 in the 70s and 80s, 18 in the 90s. Then collections taper off, perhaps because the plant became so abundant that it was no longer a novelty.

It’s unsettling, this march across the state. Black Locust has escaped plantings to become naturalized, and where this adaptable plant finds sun and anything but waterlogged soils, it survives and even thrives. Old fields, rights of way and degraded woods are now part of its expanded habitat. So are prairies, savannas and open forests, where single species stands of Black Locust can challenge restoration (4, 5, 6).

Adapted for Colonization

Black Locust is aided in its spread by its pioneer habit, a set of adaptations for quick colonization of canopy gaps and other disturbances. It grows fast and produces abundant seeds carried by wind, water and gravity. Although seeds may drop and germinate below their parent plants, within a stand the trees reproduce mainly by vigorous root suckers and stump sprouts, shoots that arise from roots and trunks. That’s especially true if the trees are damaged, such as from a storm or from cutting, and explains why Black Locust is so resilient and persistent.

What’s more, Black Locust is a legume, a member of the bean family, Fabaceae. Like other plants in that family, it can fix nitrogen. Nodules on its roots hold bacteria that can convert nitrogen gas in the atmosphere to ammonia, a usable form. That’s an advantage in nutrient-poor soils, and not just for Black Locust. Soils enriched with this captured nitrogen support other plants, including non-native ones that may further displace native species (7).

The Neonative Debate

For these reasons, even inside its native range Black Locust is known to be weedy. Outside its range it’s often called invasive, but some biologists stop short of calling it non-native. Because its historical range is in North America, they prefer to call it a neonative, a species that isn’t native (here before Europeans) but also isn’t non-native (from another continent).

Black Locust's distribution is in green, from EDDMapS (17). Approximate native range is added and circled in red, based on a map from the USDA Southern Research Station (18).

As it was originally defined, a neonative species is one that moves to a new area in response to an environmental change, such as a warming climate (8). Such species arrive without direct human intervention, and because they come on their own, they challenge our ideas about which species belong in an area and which don’t – and therefore which should stay, and which shouldn’t (9).

Black Locust doesn’t quite fit that definition. Humans brought it here, so it’s not a neonative in the original sense of the word. The label stuck, though, and now Black Locust, the “nuisance neonative,” has a mixed reputation. In this area it’s bad for biodiversity, especially in natural areas, but in its home range it’s good for native insects and other animals (10). In the Midwest the nitrogen it adds to soils can alter natural communities in undesirable ways, but where mine lands need to be reclaimed, nitrogen enrichment aids recovery. And where forests and prairies need protection, Black Locust’s suckering growth is a problem, but where soils are eroding, the trees’ clutching roots are a solution (19). 

Overseas, a Similar Story

The debate is just as vigorous in Europe. Black Locust was introduced there in the 1600s and now is found in more than 40 countries. It has been widely planted, and although it’s considered one of the worst invasive plants in the continent, it’s also valued for biomass production, erosion control and honey-making (11, 12).  Biologists there recommend a “tiered approach” to managing Black Locust: Remove it from natural areas but keep it in commercial forests, in urban plantings and in selected forests where succession, the gradual change in plant community composition, is allowed to play out  (12).

Winter Identification

Mature Black Locust hold 3- to 4-inch-long, brown pods through winter. Each contains 4-8 seeds. Buds are alternate, but they’re under the leaf scars and barely visible. Nodes often have a pair of stout thorns, ½ to 1 ½ inches long. Branches and young trunks may also have thorns. Mature bark is dark gray or brown with deep furrows and flat-topped ridges.

Black Locust pods hang on mature trees through winter. Each pod is 3-4 inches long.

Black Locust buds are alternate but lie under the leaf scars, which may be cracked on their surfaces.

A pair of stout thorns is found at most nodes. 

Mature bark is dark brown or gray with deep furrows

and flat-topped ridges. Smaller trunks bear thorns.

Look-alikes

Prickly Ash (Zanthoxylum americanum) is a shrub or small tree that also has pairs of thorns at the nodes, but the thorns are only ¼ to ½ inch long. Buds are red, fuzzy and clearly visible. Prickly Ash does not produce pods. 

From a distance, Kentucky Coffee Tree (Gymnocladus dioicus) resembles Black Locust. Pods on mature females are larger than those of Black Locust and filled with a green mash. The tree has no thorns.

Honey Locust (Gleditsia triacanthos), likely introduced to Minnesota as cultivated varieties, also has persistent pods, but they’re much longer than Black Locust – about 16 inches. Some trees have large, three-parted thorns on their trunks (13).

Prickly Ash buds are red and fuzzy. The pair of thorns at the nodes are smaller

than those of Black Locust

Female Kentucky Coffee Trees carry pods through winter, but they're much larger than those of Black Locust. Seeds are embedded in a sticky, green mash.

Regulation

The Minnesota Department of Agriculture includes Black Locust on its list of Restricted noxious weeds, meaning the plant can’t be imported, sold or transported except as allowed by state law (14).  The rules are similar in Wisconsin (15).

Black Locust Toxicity

Leaves, bark and seeds contain robin and phasin, compounds that interfere with protein synthesis and can kill cells. Horses are especially sensitive to these toxins, but humans can also get sick with nausea, weakness, flushing and lethargy. If any part of the plant is ingested, seek medical help immediately (16). 

References

1)      USDA, NRCS. 2023. The PLANTS Database (http://plants.usda.gov, 02/02/2023). National Plant Data Team, Greensboro, NC USA.

2)      Robinia pseudoacacia (black locust). CABI. CABI Compendium, https://doi.org/10.1079/cabicompendium.47698. Accessed Feb. 2, 2023.

3)      University of Minnesota Bell Museum Minnesota Biodiversity Atlas. https://www.bellmuseum.umn.edu/atlas/ Accessed Feb. 2, 2023.

4)      Minnesota Department of Natural Resources. Black locust (Robinia pseudoacacia). Accessed Feb. 2, 2023.

5)      Minnesota Department of Agriculture. Black Locust. Accessed Feb. 2, 2023.

6)      Woody Invasives of the Great Lakes Collaborative. Black locust. Accessed Feb. 2, 2023.

7)      Von Holle, B., Neill, C., Largay, E.F., et al. 2013. Ecosystem legacy of the introduced N₂-fixing tree Robinia pseudoacacia in a coastal forest. Oecologia 172: 915-924. https://doi.org/10.1007/s00442-012-2543-1

8)      Essl, F. et al. 2019. A conceptual framework for range-expanding species that track human-induced environmental change. Bioscience 69 (11): 908-919. https://doi.org/10.1093/biosci/biz101

9)      Shah, S. 2020. Native Species or Invasive? The Distinction Blurs as the World Warms. Yale E360.

10)   Jaffe, D. Rethinking Black Locust. Posted in 2019 on the website for the Ecological Landscape Alliance.

11)   Sitzia, T., Cierjacks, A., de Rigo, D., and Caudullo, G. 2016. Robinia pseudoacacia in Europe: distribution, habitat, usage and threats. In European Atlas of Forest Tree Species. Ed: San-Miguel-Avanz, J., de Rigo, D., Caudullo, G., Durrant, T., and Mauri, A. Publication Office of the European Union, Luxembourg.

12)   Vítková^, M., Müllerová, J., Sádlo, J., Pergl, J., Pyšek, P. 2017. Black locust (Robinia pseudoacacia) beloved and despised: A story of an invasive tree in Central Europe. Forest Ecology and Management 384: 287-302. DOI: 10.1016/j.foreco.2016.10.057

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

14)   Minnesota Department of Agriculture. Minnesota Noxious Weed List. Accessed Feb. 6, 2023.

15)   Wisconsin Department of Natural Resources. Black Locust. Accessed Feb. 6, 2023.

16)   Poison Control. Are Black Locust Trees Toxic? National Capital Poison Center. Accessed Feb. 9, 2023.

17)   EDDMapS. 2023. Early Detection & Distribution Mapping System. The University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed February 11, 2023.

18)   Huntley, J.C. No date. Black Locust. USDA Southern Research Station.

19)   Farmer, S. 2020. Black Locust & Drought. CompassLive, USDA Southern Research Station.

What Is a False Terminal Bud?

On trees and shrubs, a false terminal bud is a lateral bud at the end of a twig. Unlike a true terminal bud, it doesn’t enclose the growth at the tip of a stem or branch.

This happens in some species when growing shoots don’t harden off in fall. Instead, their stems die back, leaving the end-most lateral buds to take over as the last buds on the twigs.  

According to several keys, false terminal buds are found only on woody plants with alternate lateral buds. The stems of plants with opposite buds may also wither or die back before winter, but the end-most pair of buds are not called false terminal buds. 

This difference may seem trivial, but some winter keys use false and true terminal buds to separate species into groups or to confirm a plant’s identity. Fortunately, with a hand lens and some practice, there are reliable ways to recognize each.

True terminal buds

Consider a stem of a deciduous (leaf-shedding) tree or shrub with alternate leaves along its length and a growing shoot at its tip. By mid-summer, the plant begins forming buds in leaf axils, the angles between leaves and stems. These are axillary or lateral buds, also called leaf buds. When leaves are shed in fall, their petioles (leaf stalks) leave marks below these buds, called leaf scars.

At the same time, growth stops at the tip of the stem and the young shoot is enclosed in either bud scales or protective leaves. This is a true terminal bud (c). It has no leaf scar (b) below it, and it is often larger than lateral buds (a). 

I

False terminal buds

Now consider a stem that, instead of forming a terminal bud, dies back to the closest lateral bud. This is a false terminal bud. It has a leaf scar below it and is about the same size as other lateral buds.

The stub of the withered stem may project above the base of the bud on the side opposite the leaf scar. The mark it leaves, called a branch scar (b), can be mistaken for a leaf scar, but it won’t have vascular bundle scars. This can be hard to see without a hand lens. Also, in some species with false terminal buds, the bud is noticeably angled toward or away from the twig (a).

Examples

Black Walnut (Juglans nigra) is a tree with alternate buds (a) and heart-shaped or three-lobed leaf scars. The true terminal bud (b) is larger than the lateral buds and does not have a leaf scar. The last lateral bud (c) sits just below the terminal bud. 

American Elm (Ulmus americana), another tree with alternate buds, has false terminal buds. In the photos below, a branch scar is evident above the base of the bud (a) and a leaf scar (b) is on the opposite side, below the bud. Like some other species with false terminal buds, the bud is angled. 

Silver Maple (Acer saccharinum) is a tree with opposite or paired lateral buds (a) and a true terminal bud (b) that is larger than the lateral buds and has no leaf scar (c). The last pair of buds (d) sits just below the terminal bud. 

Basswood (Tilia americana) is a tree with alternate lateral buds and false terminal buds. Below, the false terminal bud has a leaf scar (a) and is about the same size as the lateral bud below it. The branch scar (b) is smaller and darker than the leaf scar. 

Highbush Cranberry (Viburnum trilobum, V. opulus) is a shrub with opposite buds. It does not form terminal buds of any kind. Instead, in fall, the ends of the stems (a) wither back to the last pair of opposite buds (b), which will resume growth in spring. As this pattern repeats, the shrub branches in a Y-shaped form called sympodial growth (c).

Some Winter ID Guides

The following guides can help with winter ID of trees and shrubs. When using any guide or key, look at several twigs and buds to see what’s typical. Variations are common.

The LEAF Program from UW-Stevens Point is a K-12 forestry education initiative that offers many online resources. Under Curriculum & Resources, choose LEAF Tree Identification Tools. The LEAF Winter Tree ID Key is available there as a downloadable PDF.

Pocket Reference for Winter Tree Identification. Champaign County Forest Preserves, Mahomet, IL.

Fruit and Twig Key to Trees and Shrubs, by William M. Harlow, PhD. Reprint edition, 1959. Dover Publications, Inc., New York. ISBN 0-486-20511-8. 

Winter Identification of Deciduous Trees and Shrubs

In winter, trees and shrubs can be identified using twigs, bark, overwintering fruit and sometimes leaves. This post offers some tips and terms for winter ID. A printable version is available for free through the Downloads tab.  

Tip #1: If they're within reach, look at twigs. 

As in this photograph of Green Ash (Fraxinus pennsylvanica), look for the size, color, shape and texture of terminal, or end, buds and lateral, or side, buds (b). Lateral buds are attached at nodes (a) and are arranged in one of four patterns:

• Alternate: One bud per node   
• Opposite: Paired, or two buds per node
• Subopposite: Paired but not quite opposite
• Whorled: Three buds per node

Green Ash has opposite buds (d). 

The size and shape of leaf scars (c) can also help identify a species. These scars are left by petioles, or leaf stalks, when they fall from the tree. Green Ash typically has light, semicircular leaf scars.

    

Tip #2: Within leaf scars, look for vascular bundle scars.

These scars are made when strands of water- and food-conducting cells are severed in fall. Their size, number and arrangement are typical for a species. Some are easier to see with a magnifying lens. 

Above left: Green Ash bundle scars are small, brown dots arranged in a semicircle.

Center: The bundle scars of Red Elderberry (Sambucus racemosa) are raised, irregular shapes arranged at the points and along the sides of a triangular leaf scar. 

Right: The bundle scars of Northern Catalpa (Catalpa speciosa) are light brown dots arranged in an oval.

Tip #3: Look at bark. 

Bark color and texture are helpful clues but can change with age. Also look for lenticels, spots or irregular shapes on the bark of younger trees or shrubs. In the photographs of Green Ash below, lenticels are the white spots on the reddish-brown bark of the sapling shown on the left (arrows).

On the right is a mature Green Ash showing the typical honeycomb-like pattern of ridges and furrows of its bark.

Tip #4: Look for overwintering fruit.

Some species retain their fruits, or parts of them, well into winter. Also look under the shrub or tree for fruits that may be on the ground or on top of the snow.

Clockwise from top left: Red capsule walls of Winged Burning Bush, Euonymus alatus; samaras of Box Elder, Acer negundo; pods of Kentucky Coffee Tree, Gymnocladus dioicus, each 3 to 4 inches (7-10 cm) long; and the fruits of Amur Cork Tree, Phellodendron amurense. 

Tip #5: A few species hold on to their leaves.

Some trees are marcescent -- their leaves turn brown but aren't shed in fall. In this region, oaks (Quercus), Ironwood (Ostrya virginiana) and Blue Beech (Carpinus caroliniana) are among the few trees that are marcescent. 

Below is Ironwood, an understory tree that retains its leaves through winter. 

Tip #6: Remember MAD Cap Buck Horse.

Bud arrangement -- alternate, opposite, subopposite or whorled -- can quickly narrow choices for identification. One way to remember which species have an opposite arrangement is the mnemonic MAD Cap Buck Horse:

M            Maples (Acer)

A            Ash (Fraxinus)

D            Dogwoods (Cornus, except for alternate-leaved dogwood, C. alternifolia)

Cap        Plants that are or were in the family Caprifoliaceae, including honeysuckles (Lonicera), wolfberry or snowberry (Symphoricarpos), elderberry (Sambucus) and viburnums (Viburnum). 

Buck Horse    Ohio Buckeye (Aesculus glabra) and Horse Chestnut (Aesculus hippocastanum)

Although this mnemonic is helpful, it doesn't include all trees and shrubs with opposite buds. For example, Wahoo and Burning Bush, genus Euonymus, also have opposite buds. So does Bladdernut, Staphylea trifolia, an understory shrub.

Most remaining species have alternate buds. Common Buckthorn, Rhamnus cathartica, is unusual in having subopposite buds. Catalpa, another unusual species, has whorled buds. 

Tip #7: Look at the pith.

The pith is the center of a branchlet or twig. The appearance of the pith -- hollow or solid, color, texture -- can help confirm the identity of a tree or shrub. For example, honeysuckle shrubs (Lonicera, left photo below) have hollow piths, and Red Elderberry (Sambucus racemosa, right photo below) has a soft, yellow pith.

Tip #8: Try these guides.

The LEAF Program from UW-Stevens Point is a K-12 forestry education initiative that offers many online resources. Under Curriculum & Resources, choose LEAF Tree Identification Tools. The LEAF Winter Tree ID Key is available there as a downloadable PDF.

Pocket Reference for Winter Tree Identification. Champaign County Forest Preserves, Mahomet, IL.

Fruit and Twig Key to Trees and Shrubs, by William M. Harlow, PhD. Reprint edition, 1959. Dover Publications, Inc., New York. ISBN 0-486-20511-8. 

Progress in Buckthorn Management

Seedlings of Common Buckthorn rise from the seedbank after an area is cleared of larger plants. Suppressing reinvasion
is a major challenge of Buckthorn management. 

Researchers at the University of Minnesota rank Buckthorn high on their list of problem invaders. They’ve been studying Common Buckthorn, Rhamnus cathartica, and Glossy Buckthorn, Frangula alnus, for years, and last summer they shared some of their findings.

Their white paper, Managing Invasive Buckthorn, focuses on two areas of research: goat browsing to manage Buckthorn growth and native plant cover to suppress reinvasion. Here are highlights from their work.

Goat Browsing Has Potential – and Pitfalls

On steep hillsides or other inaccessible places, goats are an alternative for removing buckthorn. Their browsing and trampling can reduce Buckhorn abundance and open the canopy, allowing more light to reach other plants.

Goats are most effective on small Buckthorn within the animals’ reach. To limit damage to other plants, fall is the best time to release the animals, but that’s also when they’re most at risk of acquiring meningeal worms. These brain parasites can infest snails and slugs that the goats also consume while browsing. Co-grazing ducks and geese with goats can lessen the risk, as waterfowl can eat infested snails and reduce their numbers but not get sick.  

Goats don’t eat just Buckthorn; they’ll eat other plants, too, including desirable ones. Some plants may rebound the following year, but they’ll be competing with Buckthorn that also resurges under the newly opened canopy. As explained in the next section, that’s why establishing cover is important after Buckthorn is removed, whether by goats or other means. A good way to suppress Buckthorn’s return, the researchers found, is to increase competition.

Part of this Buckthorn thicket was cleared with a forestry mower. Just a few 

years later, it has regrown to become as dense as the unmowed portion.

Native Cover Suppresses Reinvasion and Rebuilds Communities

Buckthorn management doesn’t end when the plants are cleared from an area. Reinvasion and a return to dominance are common, because Buckthorn can regrow from the seed bank or from cut stumps that weren’t treated with herbicide.

Fortunately, the researchers discovered a way to suppress reinvasion. They experimented with dense plantings or seedings of trees, shrubs, grasses and forbs and found that if light availability under plant cover drops below 3-4%, Buckthorn regrowth is limited. In closed forests with less light, planted trees and shrubs worked best to establish that cover. In more open areas, such as oak woods, both planted stock and seeded grasses and wildflowers were effective. The scientists are now experimenting with less dense planting and seeding.

Even after plant cover is introduced, it is important to monitor a site for germinating or sprouting Buckthorn.  As the native planting matures and casts more shade, removing Buckthorn should become easier as fewer plants survive. It’s a years-long effort, but with the right combination of techniques, Buckthorn should recede as the native plant community returns.

More Resources

For help identifying Buckthorn in winter, see this updated post from January 2021 or download this free, two-page guide. Additional Buckthorn information is available from the Minnesota Department of Natural Resources and the Minnesota Department of Agriculture.

Reference

Bernhardt, C., et al. 2022. Managing Invasive Buckthorn. University of Minnesota College of Food, Agricultural and Natural Resource Sciences and the Minnesota Invasive Terrestrial Plants and Pests Center.  CFANS-Buckthorn-White-Paper-June-2022.pdf (umn.edu)