What are the sun requirements for Conyza canadensis?

Conyza canadensis (syn. Erigeron canadensis) is an erect summer or winter annual in the daisy family (Asteraceae spp.) reaching 12–72 inches (30–180 cm) tall and 6–18 inches (15–45 cm) wide. Stems are erect, usually unbranched below the inflorescence, hairy, and densely leafy. Basal rosette leaves are oblanceolate, 2–4 inches (5–10 cm) long, coarsely toothed, and often withering by flowering time. Stem leaves are numerous and crowded, linear, 1–4 inches (2.5–10 cm) long, entire to slightly toothed, progressively smaller upward. The inflorescence is a large elongated terminal panicle 4–12 inches (10–30 cm) long carrying hundreds to thousands of small flower heads. Each head is 0.1–0.2 inch (3–5 mm) across with inconspicuous white ray florets barely exceeding the involucre and a yellow disc. A single plant produces 50,000–200,000 seeds bearing a hair-like pappus for long-distance wind dispersal; seeds have been detected at altitudes above 3,000 feet (900 m) in air column sampling. Seeds germinate in both fall and spring, producing winter annual and summer annual cohorts. The species was the first weed to evolve widespread resistance to glyphosate, and glyphosate-resistant populations are now documented in more than 25 U.S. states.

Native Range

Conyza canadensis is native to North America, occurring across all 50 U.S. states, all Canadian provinces, and into Mexico and Central America. The species is found in cultivated fields, no-till cropland, orchards, vineyards, roadsides, construction sites, sidewalk cracks, and virtually any disturbed ground from sea level to approximately 8,000 feet (2,400 m). Now naturalized on every continent except Antarctica.

Suggested Uses

Used in weed science education as a primary example of glyphosate resistance evolution. Studied in aerobiology and seed dispersal ecology because of the long-distance wind transport of pappus-bearing seeds. Included in agricultural weed identification curricula. Native to North America and historically used in indigenous ethnobotany for various medicinal applications. Intentional garden planting is not a standard use.

How to Identify

Identified by an erect densely leafy stem carrying numerous crowded linear leaves and a large terminal panicle of hundreds of tiny flower heads. Separated from Conyza bonariensis (hairy fleabane) by narrower leaves, taller stature, and panicle shape (elongated versus rounded in C. bonariensis). Separated from Erigeron spp. species with visible ray florets by the tiny inconspicuous rays that barely exceed the involucre. Stems are hairy; pulling a stem through a closed hand reveals the rough texture of the hairs.

Appearance

Size & Dimensions

Height1' - 6'

Width/Spread6" - 1'6"

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~8 weeks

Summer Fall

Flowers from July through October. Individual flower heads bloom for 3–5 days. The panicle produces new heads from the bottom upward over 6–8 weeks. In the Pacific Northwest, peak flowering occurs in August and September. Seeds mature 2 weeks after flowering and are immediately wind-dispersed. Self-pollinating; insect visitation is minimal.

Detailed Descriptions

Flower Description

White to pale yellow; tiny flower heads 0.1–0.2 inch (3–5 mm) across with inconspicuous ray florets barely exceeding the involucre and a yellow disc; massed in a 4–12 inch (10–30 cm) terminal panicle

Foliage Description

Medium green; basal leaves oblanceolate, coarsely toothed; stem leaves linear, numerous and crowded, entire to slightly toothed

Growing Conditions

Sun Requirements

Full Sun Partial Shade

Requires 6-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.0 - 8.0(Neutral)

357912

Soil Types

loam clay sand silt rocky

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Management focuses on removal before seed dispersal, because the very high seed production (50,000–200,000 per plant) and long-distance wind dispersal make containment difficult. Fall-germinating rosettes are hand-pulled or hoed in late winter before bolting. Spring-germinating plants are removed before the inflorescence develops. Mulching with 2–3 inches (5–8 cm) of organic material suppresses germination. Plants bolt rapidly and can produce mature seed within 60–90 days of germination. Glyphosate-resistant populations are widespread in agricultural settings; resistance testing before herbicide application is standard practice in affected regions. In no-till farming systems, horseweed is the primary weed management challenge across large portions of the United States.

Pruning

Pruning is not applicable. Plants are removed by pulling or hoeing before the inflorescence develops seed. Cutting stems after panicle development but before seed maturation prevents dispersal only if all cut material is bagged and removed from the site. Seeds on cut stems can mature after removal if panicles are partially developed.