Monday, May 13, 2024

Plant Profile: Wild Ginger

A single flower emerges between a pair of leaves of wild ginger, Asarum canadense.

In spring, wild ginger is one of the first plants to emerge on the deciduous forest floor. Softly hairy leaves grow in pairs from shallow rhizomes, eventually expanding into heart- or kidney-shaped blades 3-5 inches wide. Mature petioles, or leaf stalks, are several inches long, and like the leaves, they are finely white-hairy. To an imaginative observer, they resemble pipe cleaners.

A single, reddish-brown, tubular flower develops in the axil of each pair of leaves. The flower is close to the ground on a slightly bent peduncle, or flower stalk. The flower has no petals, but its three, long-pointed sepal resemble petals and curve back over an open floral cup.

Inside the cup, the stigmas – the parts that receive pollen – mature first. They’re in the center of the flower, supported by their styles and surrounded by 12 stamens. Initially the stamens bend down and away from the stigmas, their pollen-bearing anthers lying parallel with the bottom of the cup. Over several days, the stamens straighten.


This dissected flower shows a central column of several upright stamens (solid arrow) surrounding stigmas and styles, which are hidden. The whitish dust around the top of the column is pollen. Several anthers still rest on the bottom of the cup (dashed arrow). 


Left: A flower with most stamens upright and a few still lying on the bottom of the floral cup.
Right: An older flower with all stamens upright.


A Pollination Puzzle

Pollination is a bit of a mystery. Many general references say the flowers are pollinated by flies and ground beetles attracted to the flowers’ fleshy color and supposed rotting-meat odor. As it turns out, that’s an assumption passed from one reference to the next, but it’s easy to see why it persisted.

Wild ginger doesn’t look like something pollinated by bees or butterflies. Although the flowers are beautiful in their details, they’re generally drab and mostly hidden under the leaves. They don’t look anything like the brightly colored, conspicuous flowers typically pollinated by bees or butterflies. Instead, their maroon to brown color matches that of animal flesh, like the flowers of some other plants pollinated by flies.


The bright yellow flowers of marsh marigold (Caltha palustris), left, are typical those pollinated by bees and other insects. The flower structure of skunk cabbage, center, looks and smells like decaying animal flesh and is pollinated by flies. Wild ginger, right, more closely resembles a fly-pollinated flower, at least in color. Photos not to scale. Skunk cabbage photo © 2009 Katy Chayka at Minnesota Wildflowers, used with permission granted on the website. 

Skunk cabbage (Symplocarpus foetidus), another Minnesota native, is a good example. It emerges in late winter or very early spring, even while snow covers the ground. Its flowering structure is a reddish-brown, leaf-like spathe enclosing a club of flowers called a spadix. As the plant’s name suggests, the structure has a fetid, dead-skunk smell. The small flowers on the spadix are pollinated by flies and beetles drawn to the plant’s carrion-like color and odor.

Other Evidence

Wild ginger doesn’t smell that bad. A sniff test finds that, at worst, the flowers can have a slightly unpleasant odor, but they don’t smell so strongly of rotting carcass that you would recoil. “Earthy” might be the best word to describe it. Some even say the flowers have a sweet smell. In any case, they aren’t obvious fly bait.

Early studies of wild ginger find other contradictions with the fly-pollination hypothesis. In the late 1940s, Harvey E. Wildman of the University of West Virginia experimented with wild ginger flowers to answer the question of how they’re pollinated (1). He removed the stamens from one group of flowers and left another group intact. In each group, he covered some of the flowers in wax paper bags (after ensuring no insects were inside the flowers) and left others uncovered. After several weeks, he checked the flowers for seed development.

None of the flowers with stamens removed, even those that were uncovered, developed seeds. In fact, all such flowers he checked had either fallen off or withered. In contrast, most of the flowers left intact developed “sound seeds.” That includes the ones that were covered. Wildman also reported that few insects were found inside any of the flowers.

If the flowers were strictly cross-pollinated by flies or other insects, at least some of the uncovered ones without stamens would have developed seeds, because something would have brought pollen to their stigmas. At the same time, the intact, covered flowers would not have developed seeds, because insects did not have access. Wildman concluded that wild ginger is primarily self-pollinated, not cross-pollinated.

Timed for Cross-Pollination?

Although Wildman’s experiment is illuminating, pollination is still a head-scratcher. One way plants foster cross-pollination is by staggering the development of stigmas and anthers inside a single flower, and wild ginger does exactly that. As mentioned above, the stigmas mature first, Eventually the filaments and anthers straighten and approach the stigmas, but not before the flowers have had a chance to receive pollen from another plant. This suggests that self-pollination is a back-up rather than a primary means of fertilization. Are we missing a pollinator? 

Whether self-pollinated or somehow cross-pollinated, fertilized flowers later develop seeds within capsules. When the capsules open in mid-summer, they expose small seeds with tiny fat bodies attached. The bodies, called elaiosomes (e-LY-oh-somes or e-LAY-oh-somes) attract ants, which carry the seeds back to a nest, eat the elaiosomes or feed them to their young, and leave the seeds to germinate, safely out of reach of seed predators. Seeds can also fall next to the parent plant and germinate there.


Left: The swollen ovary at the base of the flower indicates that this flower has been fertilized. Center: The same flower viewed from above. Each of the twelve dots around the center is what remains of a stamen. Right: Seeds are released in mid-summer. Each is just a few millimeters wide and long, with a golden-brown elaiosome attached. 

Rhizomes for Spread, Not for Spice

A rhizome of wild ginger (arrow).
If its seeds don’t succeed in helping wild ginger reproduce, its rhizomes can. (See the previous post for more about rhizomes.) The plant is almost aggressive in its vegetative spread, quickly filling suitable habitat, especially where it has limited competition.

Many say the rhizomes are aromatic and ginger-y in smell and taste. Although they have a long history of use as medicine and flavoring, ingesting them in any form is discouraged now. Wild ginger rhizomes and other parts have been found to contain variable amounts of aristolochic acid, a compound known to damage kidneys and perhaps cause cancer (2, 3). Handling the plants can also cause dermatitis.

This isn’t true of ginger roots (rhizomes) or ginger spice found in grocery stores. Culinary ginger is “true” ginger, Zingiber officinale, a tropical plant. It is not related to Asarum canadense.


Where to Find Wild Ginger

Wild ginger is native to deciduous and mixed deciduous-coniferous forests. It prefers full to part shade and moist, humus-rich soils. It wilts in prolonged drought. 


Wild ginger range in the upper Midwest and North America. Maps from USDA NRCS Plants Database (4).

Cited References

1)      Wildman, Harvey E. 1950. Pollination of Asarum Canadense L. Science 111 (2890): 551. http://www.jstor.org/stable/1676584.

2)      McMillin, D.L., Nelson, C.D., Richards, D.G., and Mein, E.A. 2003. Research Report: Determination of Aristolochic Acid in Asarum canadense (Wild Ginger). Meridian Institute.

3)      Qingqing Zhou, et al. 2023. Overview of aristolochic acid nephropathy: an update. Kidney Res Clin Pract 42 (5): 579-590.

4)      USDA, NRCS. 2024. The PLANTS Database (http://plants.usda.gov, 05/01/2024). National Plant Data Team, Greensboro, NC USA.

Other References and More Information

Anderson, M.K. Ed. 2000, 2003 and 2006. Plant Guide: Canadian Wildginger. USDA NRCS National Plant Data Center, Davis, California.

Baskin, J. M., & Baskin, C. C. 1986. Seed Germination Ecophysiology of the Woodland Herb Asarum canadense. The American Midland Naturalist, 116 (1), 132–139. https://doi.org/10.2307/2425945

Dunphy, S.A. Meadley, K. M. Prior, and M.E. Frederickson. 2016. An invasive slug exploits an ant-seed dispersal mutualism. Oecologia 181: 149-159. DOI 10.1007/s00442-015-3530-0 .

Hayden, W. John. 2010. Don't Judge a Book by its Cover: The Curious Case of Wild Ginger Pollination. Bulletin of the Virginia Native Plant Society 29 (1): 1, 6.

Schultz, K. 2014. Using shade to propagate Canadian wild ginger (Asarum canadense L.) and other woodland forbs. Native Plants Journal 15 (3): 231-235. DOI: https://doi.org/10.3368/npj.15.3.231.

Stritch, L. No date. Plant of the Week: Wild Ginger (Asarum canadense L.). USDA, US Forest Service.


 

Plant Profile: Wild Ginger

A single flower emerges between a pair of leaves of wild ginger, Asarum canadense . In spring, wild ginger is one of the first plants to eme...