Plant Profile: Bunchberry

 Cornus canadensis L.

Bunchberry, Cornus canadensis, flowering in northern Minnesota in mid-June 2022.

Also called Canada dogwood or creeping dogwood, bunchberry is a patch-forming, herbaceous plant of cool, moist forests. Although it’s related to red osier dogwood (C. sericea), gray dogwood (C. racemosa) and similar shrubs, this plant has no aboveground woody growth. Mature plants are just 3-6 inches tall, their short stems tipped by four to six, arc-veined leaves that are so closely spaced they appear whorled.

In late spring or early summer, mature plants produce a cluster of 12-40 small flowers surrounded by four white bracts. The petals of the flowers are just 1-2 millimeters long (1) and fused along their edges until they open.

The stamens of the flowers grow quickly, faster than the petals. As they mature, their anthers, the pollen-producing tips of the stamens, are trapped inside the closed flowers, but their lengthening filaments bend outward between the petals. Eventually, a trigger – a visiting bumblebee, for example, or the building pressure within the flower– causes the flowers to open explosively. As the petals flip back, the stamens spring outward, and pollen is catapulted into the air (2,3). The grains can be lofted as high as 2.5 centimeters (25 millimeters) above the flower, ten times the height of the flower itself (3).

A single bunchberry flower opens explosively -- in about half a millisecond (3). When the petals are flung back, stamens are released and catapult their pollen. Illustration based on photographs in Whitaker, et al. (2). 

[Watch a video of an exploding flower here.]

If they’re launched at high enough speed, some of the pollen may catch in the hairs of flying insects, which then carry it to other plants. Other pollen rides the wind. Unlike plants that are pollinated only by insects, bunchberry pollen grains are smooth instead of sticky, and so more easily carried by a breeze (2).

A dual system of pollination is an advantage for bunchberry. These low-growing plants are self-incompatible, so they need pollen from other plants to form seeds. If insect pollination isn’t successful, wind pollination might be, but for the latter to work, pollen must be launched high enough to be wafted over a patch of the plants.

If either method of pollination succeeds, the plants will produce bunches of red drupes, fruits with single, stony seeds. The fruits look like berries, inspiring the name bunchberry.

Where to find bunchberry

Bunchberry typically grows in cool, moist broadleaf, coniferous or mixed forests. In North America, its range is primarily the northern tier of states, all of Canada, and Greenland (4). This circumboreal plant is also found at northern latitudes in Asia.

More information

For photos and more information about bunchberry, see the Minnesota Wildflowers page for this species.

References

(1) Flora of North America, efloras.org. Accessed online on June 27, 2022. Formal citation: eFloras (2008). Published on the Internet http://www.efloras.org [accessed 27 June 2022]. Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.

(2) Whitaker, D., Webster, L., and Edwards, J. (2007). The biomechanics of Cornus canadensis stamens are ideal for catapulting pollen vertically. Functional Ecology 21. 219-225. DOI:10.1111/j.1365-2435.2007.01249.x

(3) Edwards, J., Whitaker, D., Klionsky, S. et al. 2005. A record-breaking pollen catapult. Nature 435, 164 (2005). https://doi.org/10.1038/435164a

(4) USDA, NRCS. 2022. The PLANTS Database (http://plants.usda.gov, 06/27/2022). National Plant Data Team, Greensboro, NC USA.

Does Garlic Mustard Eventually Decline?

One study found that it does, but it takes many years.

Garlic mustard, Alliaria petiolata, with flowers and maturing fruits in late May 2022.

Garlic mustard, the aromatic invader of forest understories and edges, has been here a long time. It first arrived in North America in the 1800s, when colonists likely carried it onshore to use as a medicine or potherb. Since then, it has spread from east to west, and now its invasive habits have landed it on many weed lists.

The lists inspire -- or require -- action, so each spring parties gather to pull out, cut off, or otherwise get rid of garlic mustard. It's easy where they've barely made inroads, but where populations are large and dense, removal takes many hours of stooping, kneeling, reaching and pulling. Then there's the additional commitment: Because garlic mustard seeds can remain viable in the soil for ten years or more, it's necessary to return year after year for monitoring and control. 

A less laborious solution would be welcome, and it may be emerging. A study led by Bernd Blossey at Cornell University found that although populations of garlic mustard initially increase, they eventually decrease (1). Is it best, then, to let nature take its course? Could garlic mustard's long residence be its downfall? Possibly, but many questions remain. 

Garlic Mustard Biology and Ecology

Garlic mustard is a biennial that produces rosettes the first year and flowering stems the second. Terminal clusters of small, white flowers bloom in April and May. Fruits are one- to two-inch long siliques -- narrow, linear pod-like structures that bear small seeds. One plant can produce hundreds to thousands of seeds. They are released in summer and germinate the following spring. 

At least on the leading edge of the population, growth is thick. Dense mats of seedlings grow into rosettes of similar cover, and after they overwinter, the plants bolt into a crowded stand of flowering stems. Fruits and seeds follow, and the cycle repeats as garlic mustard advances.

From left: A cluster of garlic mustard seedlings, a vigorous rosette, and a second-year, flowering stem.

Not every community is susceptible to a garlic mustard takeover. Although it's generally recognized as harmful, the magnitude of garlic mustard impacts depends on what else is present in the community -- which plants, animals, microbes and soils, for example, and how climate interacts with all of these. 

Vikki Rodgers, Sara Scanga and their team reviewed research since 2008 and teased out of that complexity a likely scenario for a successful garlic mustard invasion (2). First, earthworms and deer deplete populations of native plants. Less competition then gives garlic mustard an edge, and as it grows it further harms native plants through allelopathy, the release of compounds into the soil that harms other plants. Specifically, allelopathic compounds from garlic mustard disrupt the establishment of mycorrhizae, the associations between fungi and roots that help many plants absorb water and nutrients.  

Garlic mustard has the additional advantages of an extended growing season -- it begins in early spring, before most other species -- and prolific seed production. In a favorable location, all these characteristics appear to be an unbeatable combination. Time, however, could work against its dominance. 

Residence Time and Population Growth

The age of populations can also affect their staying power, and this is where Blossey's research comes in. From 2000 to 2006, he and his colleagues established 16 long term, permanent monitoring sites along garlic mustard's invasion trail, from states in the Northeast, where populations are older, to the Midwest, where they are younger. Each site was monitored for 5 to 15 years. 

At each location they set up quadrats (four-sided sampling plots) where twice a year they recorded stem or rosette density, stem height and percent cover. At the end of the study, they concluded that as residence time increases, garlic mustard populations become less able to sustain themselves. Although they are initially abundant, populations eventually decline until their growth rate falls below a level needed to maintain steady or increasing numbers. Overall, it took just over ten years to reach that point. 

According to Blossey, there are several possible reasons why this happens. Increasing residence time may help communities develop local biotic resistance, such as the buildup of parasites, diseases and insect herbivores that target garlic mustard. Garlic mustard decline was faster and greater in eastern study sites, which might be explained by regional differences in climate, soil and vegetation, three additional factors that could affect garlic mustard performance.

Blossey's team thinks that negative plant-soil feedback also plays an important role in garlic mustard decline. In a separate experiment, they found that the survival of garlic mustard rosettes was greater in soils that were not yet invaded or recently invaded, compared to soils that had "old" invasions of more than five years. In this experiment, at least, garlic mustard evidently creates or fosters soil conditions that work against its vigor in the long run. 

Call Off the Garlic Gangs?

Blossey points out that his research was possible only in areas where garlic mustard was not actively managed. Declines were observed in populations that were allowed to run their natural course and any interference could delay the development of biotic resistance, feedback mechanisms or other causes of garlic mustard's eventual decline. 

However, he also states that further long-term research is needed to answer questions that could determine if a hands-off approach is best. If garlic mustard eventually peters out, is the decline permanent, or will populations rebound? How fast can native communities recover after garlic mustard declines? Where garlic mustard is established, does it alone explain the impacts on native plants, or could other, more persistent, factors, such as deer or earthworms, contribute to the harm? 

Although not discussed in this research, questions of patience and acceptance are also important. Can people tolerate garlic mustard on their properties or in public parks, where they may be fostering or expecting to observe native communities? Are they willing to accept the advancing, dense growth of garlic mustard populations while they wait ten or more years for them to subside? Are there places where garlic mustard should be allowed to play out, and places where it shouldn't? 

The questions go on. Depending on the answers, it might be too soon to retire efforts to manage garlic mustard. More research is needed, especially long-term studies that include both land managers and research scientists. State or local laws may also have to be changed or exceptions granted to allow garlic mustard to grow unhindered. 

In the meantime, garlic mustard gangs have their work cut out for them. Some organizations offer contests and prizes for the most plants pulled. They might be the only occasions when a lot of garlic mustard is a good thing. 

References

(1) Blossey, B. et al. 2020. Residence time determines invasiveness and performance of garlic mustard (Alliaria petiolata) in North America. Ecology Letters 24(2): 327-336. https://doi.org/10.1111/ele.13649

(2) Vikki L Rodgers, Sara E Scanga, Mary Beth Kolozsvary, Danielle E Garneau, Jason S Kilgore, Laurel J Anderson, Kristine N Hopfensperger, Anna G Aguilera, Rebecca A Urban, Kevyn J Juneau, Where Is Garlic Mustard? Understanding the Ecological Context for Invasions of Alliaria petiolata, BioScience, 2022; biac012, https://doi.org/10.1093/biosci/biac012

Plant Profile: Wild Lupine

Wild lupine, Lupinus perennis

Wild lupine, Lupinus perennis, is at or just after its peak season of flowering at Crow Hassan Park Reserve. It's also called sundial lupine because its leaves are said to orient themselves to the sun.

This native of oak savannas and sandy prairies is a larval food source for the Karner blue butterfly, Lycaeides melissa samuelis, a federally endangered species. Wild lupine also supports at least seven other moths or butterflies as well as bumble bees, carpenter bees, mining bees and mason bees. See the reference to Heather Holm's book below.

In northeast Minnesota and parts of Wisconsin, bigleaf lupine, Lupinus polyphyllus, has become abundant and even invasive. Also called garden lupine, it was introduced from western states for ornamental use, and it is still available at many nurseries. Although it's valued for its colorful flowering scapes, its aggressive growth can displace native plants and pollinators. Bigleaf lupine does not support the Karner blue butterfly. 

The name "lupine" comes from lupus, the Latin word for "wolf." Lupine was once thought to deplete or "wolf" soils of minerals, but it does the opposite. Bacteria inside small nodules on its roots convert atmospheric nitrogen gas to usable form. When plant parts decompose, the soil is then enriched. 

References

Pollinators of Native Plants, by Heather Holm. Pollination Press LLC, Minnetonka, Minnesota. ISBN 978-0-9913563-0-0.

Oak Savanna Restoration for Karner Blue Butterfly. Minnesota Department of Natural Resources. Accessed June 12, 2022. 

Lupinus perennis (Wild Lupine). Minnesota Wildflowers Info. Accessed June 12, 2022. 

For the love of (wild) lupine. Tufts Pollinator Initiative, Tufts University. Accessed June 12, 2022.

Wild Lupine, Lupinus perennis. Illinois Wildflowers. Accessed June 12, 2022.

Lupinus perennis. Flora of Wisconsin, Wisconsin State Herbarium, UW-Madison. Accessed June 12, 2022.