Seedy Conyza canadensis: Prolific Seeds Produced by Canadian Horseweed

Summary: Canadian horseweed (Conyza Canadensis), a New World native annual plant with summer and winter forms, easily thrives via prolific seed production.

flowering, seeding Canadian horseweed, Karlsruhe, southwestern Germany: H. Zell, CC BY SA 3.0, via Wikimedia Commons

Canadian horseweed produces an abundance of seeds. The mean number of seeds per flower head covers a range of 60 to 70.

The stem height determines the number of flower heads per plant and, consequently, the total seed production per plant. A plant with a height of around 15 inches (40 centimeters) produces about 2,000 seeds, whereas a plant that reaches almost 5 feet (1.5 meters) may produce about 230,000 seeds.

Canadian horseweed’s morphology, or form, is conducive to wind dispersal of the plant’s seeds. A 1979 study by David L. Regehr and Fakhri A. Bazzaz of the University of Illinois at Urbana finds successful deposition of Canadian horseweed seeds onto a corn field located over 400 feet (122 meters) downwind from a thick stand of the towering green plants. A research article in 2007 by Joseph T. Dauer and David A. Mortensen at Pennsylvania State University and Mark J. Vangessel at University of Delaware reports travel distances of at least 1,640 feet (500 meters) by Canadian horseweed seeds from their source populations.

Water serves as another successful medium for dispersal of Canadian horseweed seeds. According to a 1975 study by A. D. Kelly and Victor Friedrich Bruns of the USDA Irrigated Agriculture Research and Extension Center in Prosser, Washington, field populations adjacent to irrigation canals and rivers disperse huge quantities of seeds, with a germination of 80 percent, into the waters.

Native North American ethnobotany reveals extensive uses of Canadian horseweed in medicinal remedies. For example, the Chippewa of Canada and the United States, from Lake Superior to the St. Lawrence River, decoct leaves and roots to treat stomach pain. The Hopi of northeastern Arizona apply a poultice to the temples as an analgesic remedy for headaches. The Iroquois of northeastern North America make an infusion of the whole plant and roots as an anticonvulsive and febrifuge to treat children with convulsions and fevers. The Navajo of the southwestern United States make a cold infusion or a poultice of crushed leaves as a dermatological aid for pimples and also apply a cold infusion as an anti-snake bite lotion. The Zuni of the southwestern United States crush the plants’ flowers for insertion in nostrils as a remedy that relieves rhinitis (inflammation of nasal mucous membrane) by causing sneezes.

Oil of erigeron is the essential oil that is distilled from the plant’s leaves. In American folk medicine, oil of erigeron is valued as an astringent, a diuretic and a tonic. Of the oil’s 47 volatile components, 91 percent are terpenoids, which are aromatic antioxidants. Plant-based terpenoids have traditional applications in the chemical, food and pharmaceutical industries and recent involvement in the development of biofuel products.

Canadian horseweed also has culinary applications. The Miwok of northern California pulverize raw leaves and tender tops for enjoyment as a delicacy. Modern-day foragers enjoy cooked seedlings, especially as a crunchy accompaniment to rice dishes or in salads. Crushed leaves and stems contribute a carroty scent to salads.

My yard’s claim to Canadian horseweed is a solitary plant at the northern edge of the garden bed that adorns the northwestern section of my front porch. The plant serendipitously appeared at the beginning of the summer in a propitious location next to the white 5 gallon bucket that collects rainwater from the north porch downspout.

The plant has grown unostentatiously and steadfastly throughout the summer, while its bedmates to the south have leafed, flowered and succumbed successively to their annual demises. My Canadian horseweed has patiently yielded the spotlight to the resplendent colors and forms of grape hyacinths (Muscari armeniacum), narcissus (Narcissus spp.) and tiger lilies (Hemerocallis fulva).

Now in August my Canadian horseweed measures a height halfway between 5 and 6 feet (1.52 and 1.82 meters). Great quantities of delicate white flowers adorn the plant’s top quarters. Soon the seeds, which are contained within dry fruits known as achenes, will be conspicuously topped with the fluffy pappus that serve as the secret of the plant’s successful expansion by water and wind dispersal.

In late spring, I often admired a vibrant green stand of tall Canadian horseweed in a small public garden about a mile northeast of my house. Then in June I realized that I am the proud owner of a serendipitously rooted Canadian horseweed. All summer I have wondered about the plant’s origin. Last week as I tended to Virginia juniper (Juniperus virginiana) saplings on the banks between the ephemeral creek and vernal pool along my yard’s northwestern border, I espied several Canadian horseweed plants. They were vividly green, and they were about half the height of the porch’s Canadian horseweed.

So which arrived first: the porch plant or the Creekside plants?

closeup of seeds' pappus, the secret to Canadian horseweed's easy, widespread expansion via water and wind dispersal: Patrick J. Alexander, CC BY 2.0, via USDA NRCS PLANTS Database

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

flowering, seeding Canadian horseweed, Karlsruhe, southwestern Germany: H. Zell, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Conyza_canadense_001.JPG

essential oil of erigeron, Canadian horseweed, also known as Canadian fleabane: Itineranttrader, Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:CanadianFleabaneEssentialOil.png

closeup of seeds' pappus, the secret to Canadian horseweed's easy, widespread expansion via water and wind dispersal: Patrick J. Alexander, CC BY 2.0, via USDA NRCS PLANTS Database @ http://plants.usda.gov/java/largeImage?imageID=coca5_004_ahp.jpg

For further information:

“Canadian horseweed.” University of Florida Natural Area Teaching Laboratory > COCAN.
Available @ http://natl.ifas.ufl.edu/docs/COCAN.pdf

“Conyza – (L.) Cronquist.” Plants For A Future.
Available @ http://pfaf.org/user/Plant.aspx?LatinName=Conyza

"Conyza canadensis." Native American Ethnobotany.
Available @ http://naeb.brit.org/uses/search/?string=conyza+canadensis

“Conyza canadensis (L.) Cronquist Native Status.” USDA National Resources Conservation Service Plants Database > Plant Profile.
Available @ http://plants.usda.gov/core/profile?symbol=coca5

Dauer, Joseph T.; David A. Mortensen; Mark J. Vangessel. “Temporal and Spatial Dynamics of Long-Distance Conyza canadensis Seed Dispersal.” Journal of Applied Ecology, vol. 44, issue 1 (February 2007): 105-114.
Available @ http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2664.2006.01256.x/abstract

Dickinson, Richard, and France Royer. Weeds of North America. Chicago IL: The University of Chicago Press, 2014.

Fine, Timothy; Sean C. McKenzie; Chengci Chen; Fabian D. Menalled. “Biology, Identification, & Management of Glyphosate-Resistant Horseweed (Marestail, Conyza canadensis).” Montana State University Extension > MontGuide.
Available @ http://store.msuextension.org/publications/AgandNaturalResources/MT201608AG.pdf

“Horseweed (Conyza canadensis).” University of California Agriculture & Natural Resources Statewide Integrated Pest Management Program > Weed Gallery > Sunflower Family: Asteraceae.
Available @ http://ipm.ucanr.edu/PMG/WEEDS/horseweed.html

Kelley, A.D., and V.F. Bruns. “Dissemination of Weed Seeds by Irrigation Water.” Weed Science, vol. 23, no. 6 (November 1975): 486-493.
Available @ https://www.jstor.org/stable/4042395

Marriner, Derdriu. "Conyza canadensis: Towering Green Pillars of Canadian Horseweed." Earth and Space News. Saturday, Aug. 20, 2016.
Available @ https://earth-and-space-news.blogspot.com/2016/08/conyza-canadensis-towering-green.html

Regehr, D.L.; F.A. Bazzaz. “The Population Dynamics of Erigeron canadensis, A Successional Winter Annual.” Journal of Ecology, vol. 67 (1979): 923-933.
Available @ https://www.jstor.org/stable/2259221

Royer, France, and Richard Dickinson. Weeds of the Northern U.S. and Canada: A Guide for Identification. Edmonton Canada: The University of Alberta Press, 1999.

Tholl, Dorothea. “Biosynthesis and Biological Functions of Terpenoids in Plants.” Advances in Biochemical Engineering/Biotechnology, vol. 148 (2015): 63-106. DOI: 10.1007/10_2014_295
Available @ http://www.ncbi.nlm.nih.gov/pubmed/25583224

Tilley, Derek. “Plant Guide for Canadian horseweed (Conyza canadensis).” USDA Natural Resources Conservation Service Plants Database > Plant Guide. October 2012.
Available @ https://plants.usda.gov/plantguide/pdf/pg_coca5.pdf

Weaver, Susan E. “The Biology of Canadian Weeds. 115. Conyza canadensis.” Canadian Journal of Plant Science 81: 867-875.
Available @ http://www.nrcresearchpress.com/doi/pdf/10.4141/P00-196