Conyza canadensis: Towering Green Pillars of Canadian Horseweed

Summary: Canadian horseweed (Conyza canadensis) is a New World native plant that may grow fast and tall in a variety of habitats.

young Canadian horseweed (Conyza canadensis) prior to flowering: Rasbak, CC BY SA 3.0, via Wikimedia Commons

As a New World annual forb in the sunflower family, Asteraceae, Canadian horseweed claims homelands across North America, from Canada southward to Mexico and Central America. Canadian horseweed is found in all of the United States’ Lower 48. Canadian horseweed also occurs in two U.S. territories: Puerto Rico and the U.S. Virgin Islands.

In Canada, Canadian horseweed thrives in all ten provinces. In the easternmost province of Newfoundland and Labrador, Canadian horseweed occurs natively on the island of Newfoundland but not in Labrador, the mainland’s portion of the province. Canadian horseweed is found natively in Canada’s Northwest Territories. It does not occur natively in Canada’s other two territories, Nunavut and Yukon.

Canadian horseweed also claims homelands on the Overseas Collectivity of Saint Pierre and Miquelon (Collectivité d’Outre-mer de Saint Pierre et Miquelon). The small northwestern Atlantic archipelago of eight islands lies at the entrance of Fortune Bay, off the southwestern coast of the island of Newfoundland.

Canadian horseweed’s scientific name is Conyza canadensis. Another accepted scientific synonym is Erigeron canadensis.

Alternative common names for Canadian horseweed include Canadian fleabane, coltstail and marestail. The common name of fleabane acknowledges the plant’s presumed repugnance to fleas. Common names of coltstail and marestail reflect its presumed resemblance to a horse’s tail.

Canadian horseweed favors well-drained, sunny habitats, with neutral to alkaline soils, in grasslands as well as in such disturbed moist sites as abandoned agricultural lands; floodplains; riparian, or riverbank, areas; urban locales; wetlands; woodland paths. Habitats that feature Canadian horseweed include old fields, orchards, railways, roadsides and vineyards.

The New World native, however, adapts to a variety of environments beyond its comfort zones, including non-wetlands, poor soils and semi-shade. Canadian horseweed is able to tolerate drought. It also appears as an early successional species after forest clearcutting.

Synecology (frequent plant community associations) reveals floral camaraderie with New World peppercresses (Lepidium spp.). Canadian horseweed also appears in communities of such naturalized Old World annuals and biennials as shepherd’s purse (Capsella bursa pastoris), henbit deadnettle (Lamium amplexicaule) and prickly lettuce (Lactuca serriola).

As a herbaceous flowering plant that lacks grass-like features, Canadian horseweed is considered botanically as a forb. Canadian horseweed is described botanically as both a summer and a winter annual forb. Summer Canadian horseweeds germinate in spring while winter forms germinate in the autumn.

The New World annual displays a range of heights, depending upon soil poorness or richness. In natively favored soils, Conyza canadensis may soar to heights in a dramatic range from 5 feet (1.5 meters) to 10 feet (3.048 meters).

Narrow, unstalked leaves whorl alternately around the forb’s slender stem. Leaves have a lance-like shape.

Flowering occurs in mid to late summer as loose, branching clusters known as panicles. Flower heads measure a diminutive height of 0.08 to 0.16 inches (2 to 4 millimeters) and a miniature width of 0.12 to 0.28 inches (3 to 7 millimeters).

Canadian horseweed’s small flower heads consist of numerous, ray flowers. The tiny rays display purplish or white colors. Each ray flower is remarkably small, with lengths measuring no more than 0.02 to 0.04 inches (0.5 to 1.0 millimeters).

Towering green pillars of Canadian horseweed define summer’s landscapes. Delicately colored white and purplish flowers wink from clustered flower heads at the top of perfectly postured, slender, tall stems.

The solitary, wind-seeded Canadian horseweed in the garden strip along the northeastern extent of my front porch stands straight and tall as it outlives this year’s series of grape hyacinths, narcissus and tiger lilies.

flowering Canadian horseweed, Rock Creek Park, Washington DC: Fritzflohrreynolds, CC BY SA 3.0, via Wikimedia Commons

Acknowledgment

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

Image credits:

Canadian horseweed prior to flowering: Rasbak, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Canadese_fijnstraal_plant_Conyza_canadensis.jpg

flowering Canadian horseweed, Rock Creek Park, Washington DC: Fritzflohrreynolds, CC BY SA 3.0, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Conyza_canadensis_-_Canadian_horseweed.jpg?uselang=fr

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

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

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

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 Engingeering/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