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)

Oriental Bittersweet in Winter

The yellow capsules and scarlet arils of Oriental Bittersweet stand out in winter.

Oriental, Asian or Asiatic Bittersweet (Celastrus orbiculatus) is an introduced vine that can girdle or smother its hosts. Efforts to remove this invasive plant are complicated by its similarity to native American Bittersweet (C. scandens).

Fortunately, mature female vines of each species are noticeably different, especially in fall and winter. Open this updated post from January 2021 to learn how to tell them apart. A two-page ID guide is free to download. 

What is Tar Spot?

 

The black scabs on these Silver Maple leaves are signs of tar spot, a common fungal disease that also affects Norway, Red and other maples as well as willows, holly and sycamores.

The spots, called stromata, are the overwintering form of the fungus. When infected leaves fall, they often land with their upper surfaces, and so the stromata, facing up. That puts them in a good position to release wind-borne or rain-splashed spores next spring. The freed spores then infect new leaves, eventually producing light green to yellow spots on the blades that enlarge as the season progresses. The spots turn dark and tarry-looking in late summer and early fall. 

Although it looks bad, tar spot is rarely a serious disease. It can cause early leaf drop, but otherwise it's just unsightly. Raking and destroying infected leaves can prevent reinfection in ­­spring. In most cases, fungicides are not recommended.

Inside a Stroma

If you slice through a stroma in spring and examine it with a microscope, you'll find its lower surface covered with tiny, bowl-shaped structures called apothecia. Inside each apothecium are hundreds of  sacs called asci ("as-eye" or "ask-eye," singular ascus). Each ascus contains eight needle-shaped spores that are released through cracks in the overlying stroma. 

Left: An apothecium of Rhytisma acerinum with the dark, overlying stroma ruptured. A clear layer of asci covers the bottom of the apothecium. Right: A closeup of the asci with emerging needle-like spores. Both photos by Bruce Watt, University of Maine, Bugwood.org. 

Because tar spot fungi need living tissue to survive, spores are not released in fall. Instead, they are released in spring, when leaves are emerging from their buds. Blown by wind or launched by splashes of water, many of the spores won't land on a susceptible host. With a good measure of luck, some will, and the life cycle begins again.  

The "Womb" Fungi

Tar spot fungi are in the genus Rhytisma. Three species typically infect maples: native R. americanum and R. punctatum and introduced R. acerinum.

All three species belong to a large group of fungi called Ascomycetes, or sac fungi. The apothecia of sac fungi typically are open at the top, but those of Rhytisma are covered by a layer of fungal tissue in the stromata. 

Because the protected apothecia resemble wombs, they're also called hysterothecia, from the Greek root words "hystero," meaning uterus or womb, and "theca," meaning case or cup.

References

Leaf Diseases of Hardwoods: Tar Spots. Dr. Robert Blanchette, College of Food, Agriculture and Natural Resources, University of Minnesota.

Tar Spot of Trees and Shrubs. Brian Hudelson, University of Wisconsin-Madison Plant Pathology. Last Revised 12/18/2018. 

Rhytisma acerinum and Rhytisma punctatum, two causes of Tar Spot of maple. Heather Hallen Adams and Tom Volk, University of Wisconsin-LaCrosse. Fungus of the Month, October 2007. 

 

Are Psyllids the Solution to Invasive Knotweeds?

Help is on the horizon to manage these aggressive plants.

Fallopia japonica  (Houtt.) Ronse Decr., Japanese Knotweed
Fallopia sachalinensis (F. Schmidt) Ronse Decr., Giant Knotweed
Fallopia x bohemica (Chrtek & Chrtková) J.P. Bailey, Bohemian Knotweed

A stand of Japanese Knotweed in October. Although these plants are green, others are starting to yellow. The spiky growths are inflorescences that have shed their flowers and fruits.

In Minnesota and around the world, invasive knotweeds are some of the most difficult plants to manage.

Aided by vigorous rhizomes that can extend several meters from a parent plant (1, 2), Japanese Knotweed, Giant Knotweed and their hybrid, Bohemian Knotweed, can displace native plants, change soil chemistry, alter microbe and invertebrate assemblages, and enhance soil erosion, especially on riverbanks (3, 4, 5). In addition, they are potentially allelopathic, meaning they can release compounds from living or decaying tissues that inhibit the growth of some plants (6, 7).

Knotweeds are spreading in North America and elsewhere, and as they do, so is the realization that controlling them is no easy task. (See EDDMapS for their distributions in North America; use the genus Reynoutria, a synonym.) Although a variety of chemical and mechanical control methods can be effective, all require years of repeated efforts and monitoring for renewed growth (8). Mechanical methods – digging and cutting – also demand meticulous attention, because fragments of rhizomes or stems left behind can regenerate the plants (9).

This large stand of Knotweed covers about 2,000 square feet along a property edge. The property is adjacent to a lake, offering the riparian habitat where knotweeds thrive and spread. 

Biocontrol – the use of a plant’s natural enemies – now offers some hope for an efficient alternative. Small, sap-sucking insects called Knotweed Psyllids, Aphalara itadori, have been collected from Japan, part of knotweeds’ native range. After years of study to determine that the insects would effectively target only knotweeds, scientists released the psyllids into several field locations in North America.  

According to the North American Invasive Species Management Association (NAISMA, 10), one race of the psyllids, called the Kyushu line, was released on Japanese and Bohemian Knotweed populations in several states in 2020 and 2021. (Minnesota was not one of them.) Another race, called the Hokkaido line, was released on Giant Knotweed populations in 2021. The nymphs and adults feed on knotweed stems and leaves, removing sap from the plants and causing the leaves to curl. The insects leave behind lerp, a flaky or stringy crystallized form of honeydew, a sugary solution they excrete.

When winter arrives, the adult psyllids take shelter and enter diapause, a time of suspended activity. Because the insects are native to a similar climate in Japan, it is hoped the adults will survive winters here and resume activity in spring, as they do in their original range. However, according to NAISMA, no sustained populations have been confirmed. Additional races or populations are being studied for future release.  

Although psyllids are not yet a sure solution to knotweed invasions, the research is promising. It also highlights the importance of accurate plant identification, because different races of psyllids are effective on different knotweeds. The Kyushu race is more effective on Japanese and Bohemian Knotweed, whereas the Hokkaido race is better for Giant Knotweed. A third race, called the Murakami line, is being studied for release on Bohemian knotweed in Canada.

In Minnesota, Japanese, Giant and Bohemian Knotweed are on the state Department of Agriculture’s noxious weed list. They are classified as “prohibited-control,” meaning all propagating parts, including seeds and vegetative parts, must be prevented from spreading. In practice, that means treating at least the above-ground growth before the stems flower in late summer, and if the stems are cut, collecting and disposing of the cuttings so they don’t escape and establish knotweed stands elsewhere.

In other words, it’s a lot of work, especially in large patches. With some additional research and field tests, psyllids may someday make that work easier.

Resources for Knotweed Identification and Management

Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Fallopia spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. [Accessed online at 23198.pdf (bugwoodcloud.org)]

Dusz M-A, Martin F-M, Dommanget F, Petit A, Dechaume-Moncharmont C, Evette A. Review of Existing Knowledge and Practices of Tarping for the Control of Invasive Knotweeds. Plants. 2021; 10(10):2152. https://doi.org/10.3390/plants10102152.

Minnesota Department of Natural Resources. Non-native knotweeds: Japanese, Bohemian, and Giant knotweed. Website accessed October 10, 2022.

Mary H. Meyer, University of Minnesota Extension Service. Japanese knotweed, a major noxious weed. October 6, 2020.

Angela Gupta, Amy Rager and Megan M. Weber, University of Minnesota Extension Service. Japanese knotweed. Reviewed in 2019.

Minnesota Department of Agriculture. Knotweeds. (Brochure.) No date.

Minnesota Department of Agriculture. Japanese Knotweed. Website accessed October 10, 2022.

Minnesota Department of Transportation. Minnesota Noxious Weeds. February 5, 2020.

Linda M. Wilson. British Columbia Ministry of Forests and Range. Key to Identification of Invasive Knotweeds in British Columbia

References

1) Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Fallopia spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. [Accessed online at 23198.pdf (bugwoodcloud.org)]

2) Fennell M, Wade M, Bacon KL. 2018. Japanese knotweed (Fallopia japonica): an analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension. PeerJ 6:e5246 https://doi.org/10.7717/peerj.5246

3) Lavoie, C. 2017. The impact of invasive knotweed species (Reynoutria spp.) on the environment: review and research perspectives. Biological Invasions 19: 2319-2337.

4) Matte, R., Boivin, M., and Lavoie, C. 2021. Japanese knotweed increases soil erosion on riverbanks. River Research and Applications 38 (3): 561-572. DOI: 10.1002.rra.3918

5) Colleran, B., Lacy, S.N., and Retamal, M.R. 2020. Invasive Japanese knotweed (Reynoutria japonica Houtt.) and related knotweeds as catalysis for streambank erosion. River Research and Applications 36 (9): 1962-1969. DOI: 10.1002/rra.3725.

6) Kato-Noguchi, H. Allelopathy of Knotweeds as Invasive Plants. Plants 2022, 11, 3. DOI: 10.3390/plants11010003.

7) Drazan, D., Smith, A.G., Anderson, N.O., Becker, R., and Clark, M. 2021. History of knotweed (Fallopia spp.) invasiveness. Weed Science 69(6): 617-623. DOI: 10.1017/wsc.2021.62.

8) Grevstad, F.S., J.E. Andreas, R.S. Bourchier, R. Shaw, R.L. Winston, and C.B. Randall. 2018. Biology and Biological Control of Knotweeds. USDA Forest Service, Forest Health Assessment and Applied Sciences Team, Morgantown, West Virginia. FHTET-2017-03. [Accessed online at https://bugwoodcloud.org/resource/pdf/Biology_&_Biological_Control_Series/FHTET-2017-03_Biological_Control_of_Knotweeds_508.pdf.]

9) Lawson JW, Fennell M, Smith MW, Bacon KL. 2021. Regeneration and growth in crowns and rhizome fragments of Japanese knotweed (Reynoutria japonica) and desiccation as a potential control strategy. PeerJ 9:e11783 https://doi.org/10.7717/peerj.11783

10) USDA. Field Release of the Knotweed Psyllid Aphalara itadori (Hemiptera: Psyllidae) for Classical Biological Control of Japanese, Giant, and Bohemian Knotweeds, Fallopia japonica, F. sachalinensis, and F. x bohemica (Polygonaceae), in the Contiguous United States. Environmental Assessment, January 2020.

11) Grevstad, F.S., J.E. Andreas, R.S. Bourchier, and R. Shaw. 2022. Knotweeds (Fallopia spp.): History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2022-19-KNOTWEEDS-P. [Accessed online at 23198.pdf (bugwoodcloud.org)]