Conyza canadensis (L.) Cron. var canadensis

Horseweed

Conyza canadensis plant

Family - Asteraceae

Habit - Taprooted annual forb, usually robust.

Stems - Strongly ascending to erect, highly variable (5 cm to 2.5 m), ridged, usually branched above the midpoint, usually pubescent with spreading white hairs.

Conyza canadensis stem2Stem and leaves.

Conyza canadensis stem

Leaves - Alternate, numerous, simple, entire, 0.5-10.0 cm long, sessile or short-petiolate. Blades linear to oblong-lanceolate or oblanceolate, glabrous or more commonly sparsely to moderately hairy, mostly along the margins and midvein.

Conyza canadensis leaf1Leaves adaxial.

Conyza canadensis leaf2Leaf abaxial.

Inflorescences - Panicles at branch tips, usually dense and well developed at apex of larger plants. Shorter plants with correspondingly smaller inflorescences, sometimes appearing as short racemes.

Conyza canadensis inflorescence2Inflorescence (partial).

Heads - Radiate. Involucre 2.5-4.0 mm long, cup-shaped or slightly bell-shaped or urn-shaped. Involucral bracts in 2-4 unequal, overlapping series, narrowly lanceolate to linear, the tip green or white and ascending, with a slender to relatively broad, green or brown central stripe, and with relatively slender, thin, pale margins, glabrous. Receptacle flat or nearly so, relatively smooth.

Conyza canadensis inflorescence3Heads.

Conyza canadensis involucreInvolucres.

Ray flowers - 20-40 in 1-3 series, pistillate, the corolla inconspicuous, the strap-shaped portion 0.2-0.8 mm long, sometimes reduced to a short fringe at the tip of the slender tube, white, shed before fruiting.

Conyza canadensis headFlorets.

Disk flowers - Relatively few (8-28), perfect, the corolla 1.5-2.5 mm long, yellow, shed before fruiting. Pappus of the ray and disc florets similar, of numerous capillary bristles, 2-3 mm long, usually white.

Fruits - Achenes 1.0-1.5 mm long, narrowly oblong in outline, flattened, the angles usually with inconspicuous nerves, the surface usually sparsely to moderately and minutely hairy, light tan to pale grayish brown.

Conyza canadensis fruitsFruits.

Flowering - June - November.

Habitat - Fields, disturbed sites, waste ground, roadsides, railroads.

Origin - Native to U.S.

Lookalikes - Distantly, Erechtites hieraciifolius.

Other info. - This coarse, weedy plant is found across Missouri, and everywhere else in the continental U.S. It is a common sight in pastures and fallow fields, and is easily recognized by general appearance. However, it is highly variable, with flowering specimens ranging in size from a few inches to eight feet or more tall. The degree of branching is also highly variable. The flowering heads are quite small and must be examined closely to observe the ray florets.

In Missouri, two fairly distinct varieties of Conyza canadensis are currently recognized. Shown on this page is var. canadensis, which is by far the more common and weedy. The other, var. pusilla, is uncommon in Missouri and found in sand prairies and dry forests of the Bootheel region. It differs from the common variety in having glabrous stems and involucral bracts with dark purple tips. The species is also known as Erigeron canadensis (L.) Cronquist.

The plant was traditionally used to treat many ailments such as diarrhea, kidney stones, diabetes, and nosebleeds. Many people are allergic to its pollen and can get a reaction just by handling the plant. It is one of the leading causes of fall allergies. Livestock seem to ignore the plant because of its bitter taste. Conyza canadensis was the first wild plant known to develop resistance to glyphosate herbicides (such as Roundup®), as reported in 2001. It is also the preferred plant stem for used in friction generation of fire.

Photographs taken off Hwy 60, Carter County, MO., 7-19-03 (DETenaglia); also at Shaw Nature Reserve, Franklin County, MO, 8-13-2007, Holly Ridge Conservation Area, Stoddard County, MO, 8-15-2009, and near Washington, Franklin County, MO, 8-25-2016 (SRTurner).


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