Conyza canadensis (L.) Cronq.
Common Name: HORSEWEED
Coefficient of Conservatism: 0
Coefficient of Wetness: 3
Wetness Index: FACU
Physiognomy: Nt A-Forb

Conyza canadensis A. A. Reznicek

Now considered a nearly cosmopolitan weed, but apparently native to North America, widely naturalized in Europe and elsewhere. Especially characteristic of recently cleared, burned, abandoned, or otherwise disturbed sites, including rebuilt roadsides, railroads, fields, gravel pits, dumps, parking areas, shores, gardens; also in prairie-like areas.

The rudimentary rays are white to pink. Stature of plants varies from a few centimeters (flowering as short as 3 cm, with heads solitary or few) to well over a meter tall (with hundreds of heads). The stem is usually erect and unbranched till near its summit. The prominent stiff hairs nearly or quite at right angles to the stem and leaf margins (which appear boldly ciliate) usually make it an easy species to recognize, even vegetatively. Though typically considered an annual, it can also be a biennial under some circumstances.

Some plants (particularly from the Upper Peninsula) are small, with ascending branches from near or below the middle overtopping the stem apex. These and some others may have very sparse or more appressed pubescence, but most have the widely spreading bristly hairs of C. canadensis. None, however, have the diffuse, decumbent branches at the base that characterize the more delicate and narrower-leaved C. ramosissima Cronquist, which also has only antrorse or appressed hairs. That species has been collected in Ohio not too far from the Michigan border and should be sought here. Whether the Michigan plants result from seasonal or genetic variation, or other factors deserves investigation.

Locations

Alcona County
Alger County
Allegan County
Alpena County
Antrim County
Arenac County
Baraga County
Barry County
Bay County
Benzie County
Berrien County
Branch County
Calhoun County
Cass County
Charlevoix County
    Including Beaver Island
Cheboygan County
Chippewa County
    Including Drummond Island
Clare County
Clinton County
Crawford County
Delta County
Dickinson County
Emmet County
Gladwin County
Gogebic County
Grand Traverse County
Gratiot County
Hillsdale County
Houghton County
Ingham County
Iosco County
Iron County
Isabella County
Jackson County
Kalamazoo County
Kalkaska County
Kent County
Keweenaw County
    Including Isle Royale
Lake County
Lapeer County
Leelanau County
    Including Fox Islands
    Including Manitou Islands
Lenawee County
Livingston County
Luce County
Mackinac County
    Including Bois Blanc, Mackinac, Round Islands
Macomb County
Manistee County
Marquette County
Mason County
Mecosta County
Menominee County
Missaukee County
Monroe County
Montcalm County
Montmorency County
Muskegon County
Newaygo County
Oakland County
Oceana County
Ogemaw County
Ontonagon County
Oscoda County
Ottawa County
Saginaw County
Sanilac County
Schoolcraft County
St. Clair County
St. Joseph County
Tuscola County
Van Buren County
Washtenaw County
Wayne County

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 18, 2017. http://www.michiganflora.net/species.aspx?id=294.