Erigeron canadensis

open flower cluster with tiny daisy-like flowers
East Basin south side, Santa Carina revegetation area | June 2022

How can one admire a plant so common that it has been described as “the plant equivalent of grey noise”?293

Horseweed (Erigeron canadensis, also called Conyza canadensis) is native across much of North America. In our area, it is a frequent inhabitant of disturbed areas, but, unlike most weeds, it is rarely found in undisturbed, native vegetation. Horseweed is not unattractive. It’s erect habit and green foliage gives it a statuesque appearance that contrasts with the grayish, shrubby plants of the sage scrub. Admittedly, the numerous, daisy-like flowers are inconspicuous, but if horseweed were less common, or if it had a showy flower display, it might be a welcome sight, not a bit of “grey noise”.

Other Common Names:

Mare's tail, Canadian horseweed, Canadian fleabane

Description 4,11,67,340

Horseweed is a spring-sprouting annual or biennial herbaceous plant with a tap root and strong lateral root system. A basal rosette of leaves produces a single stem that grows rapidly to 2 to 5 feet or more (60 – 150 cm). The stem is generally unbranched below the midpoint. The green foliage is dense from the base to the lower branches with linear to oblanceolate leaves to 4 inches (10 cm) in length with smooth or irregularly, coarsely-toothed margins. Petioles are short or absent. Foliage may be smooth or with hairs.

Tiny flowers occur in a loose, multibranched cluster toward the top of the plant. The flowers are tiny, about 0.1 inches across (0.3 cm) and resemble a typical daisy flower (a composite  flower head), with two types of florets. There are 20-40 short, asymmetrical, white ray florets surrounding 10-15 symmetrical, yellow disk florets. Ray florets are female, with a two-branched pistil. Disk florets are bisexual with five stamens in a column around the pistil. The horseweed pollen ripens and begins to disperse before the floret is fully opened, so that flowers are largely self-pollinated. 541 Below each compound flower head, an urn-shaped receptacle is covered with several rows of unequal, overlapping phyllaries. Flowers bloom primarily from June into October.1

Seeds are approximately 1/16 inch long (0.1-0.15 cm), slim, each with a tawny pappus which forms a parachute that disperses the seed in the wind.

horseweed in the Santa Carina revegitation area

East Basin south side, Santa Carina revegetation area | June 2022

close-up of a tiny daisy-like flower

A flower head composed of two kinds of florets | East Basin south side, Santa Carina revegetation area | June 2022

flower cluster with fluffy seed heads

East Basin south side, Santa Carina revegetation area | July 2022

Distribution 67,144,306  

Horseweed is thought to be native across most of North America and to have been accidentally introduced to France in the mid 17th century, 67 perhaps in a shipment of Canadian furs. It has since spread throughout Europe, Asia, Africa and Australia.  It is a designated weed in over 70 countries.

In California, it occurs widely below about 6600 feet (2000 m), especially in areas where the natural vegetation has been disturbed: agricultural fields and landscaped areas. “The plant is so common in some localities that ….. it is the plant equivalent of grey noise.293 Never-the-less, horseweed does not appear to invade established native vegetation.290

In the Reserve, horseweed is found in our active restoration areas and disturbed areas along trails. It is frequent (and tolerated by restoration managers) in newly revegetated areas, but disappears as the new plants fill in. In 2022, it was one of the first plants to appear along the upper berm of the newly-widened Interstate 5.


Horseweed is a dicot angiosperm in the sunflower family, the Asteraceae.2
This is the largest family of vascular plants in the Northern Hemisphere.143 “Flowers” of Asteraceae are made up of one or both of two types of flowers (florets): symmetrical disk florets and strapped-shaped ray florets. These are crowded onto a common base (receptacle), and together are often assumed to be a single flower, which we call a flower head.11,44,49

Many other Asteraceae are common in the Reserve; these include bush sunflower (Encelia californica), with both ray and disk florets, twiggy wreath plant (Stephanomeria spp.), with all ray florets, and coyote brush (Baccharis pilularis), and California sagebrush (Artemisia californica) that both have all disk florets

As originally described by Linnaeus, horseweed was placed in the genus  Erigeron.  In 1943, horseweed was moved into the genus Conyza, the genus now accepted by many current sources. 67,144,202,306,340,540

As of 2022, the Jepson eFlora, 2 the standard for Plant Guide, uses the original genus name, and this name is used by many local references. 4,7,8,59,468 Curiously, the 1993 Jepson Manual (Hickman, reprinted in 1996) placed horseweed in Conyza, so it is unclear (to me at least) whether the current Jepson eFlora treatment is a return to the original name, or a lack of acceptance of the affiliation with Conyza.

Alternate Scientific Names:

Conyza canadensis

Jepson eFlora Taxon Page
tall weedy plants in a field

East Basin south side, Santa Carina trailhead | Sept. 2011

open flower cluster with tiny daisy-like flowers

Central Basin, south side - Solana Hills road | Aug. 2011

single seed

single seed | East Basin south side, Santa Carina revegetation area | June 2022


On a global basis, horseweed has been called…”one of the most problematic, noxious, invasive and widespread weeds in modern-day agriculture.”541  It has several characteristics that are shared by successful weeds. It has a prolific seed set – over 200,000 seeds/plant;542 it has an effective dispersal mechanism- the tiny, airborn seeds have been found at least 500 m (nearly 1/3 of a mile) from the plant. It has both a taproot and a strong lateral root system, which make it a good competitor for water and allows it to grow rapidly.144 A large portion of flowers are self-pollinated, which could be important in non-native habitats where the resident pollinators might not be effective pollinators of horseweed.541  However, most of our most “successful” weeds, are not native, and their success is often attributed to the absence of their native predators and parasites. Perhaps these native organisms explain the lack of horseweed in natural vegetation.

In the Reserve, we have seen a good demonstration of the exclusion of horseweed from mature sage scrub. In 2019 a small burn in October killed all above-ground vegetation in an area at the junction of Holmwood Canyon and the Rios trail. By summer of 2020,  new growth was heavy and included many dense patches of horseweed. By July 2022, no horseweed was visible; apparently overgrown by the more permanent members of the sage scrub.

horseweed in the Santa Carina revegitation area

East Basin south side, Santa Carina revegetation area | June 2022

tall weedy plants in a field

Horseweed was abundant in the Rios burn area, ten months after the fire | Aug. 2020

Human Uses

Horseweed was used by a wide variety of Native Americans, reflecting the wide geographic range of the plant. In the Southeast, the Seminoles used it as cough medicine; in the Northeast, the Iroquois mixed an infusion from horseweed and another plant for childhood fevers and convulsions. In Hawaii it was pounded into a liquid for sprains and backaches, while in the Midwest, the Meskwaki steamed it in sweatbaths and in the Southwest the Navajo applied it for earaches, stomachaches, snake bites and pimples.282

The vast majority of uses, past and present,  involve treatment of chronic congestion.293 The sole report  of horseweed used for food comes from the Miwok tribe of Central California, who pounded the leaves and young tops and ate them uncooked; they are reported to taste like onions. 34,282

Among coastal tribes of Southern California, the only record of use we found is from the Chumash of the Santa Barbara-Channel Islands area. They ground the plant into a rub for body aches, and also made a tea for kidney problems. 15

top view of young green plants
tall weedy plants in a field

Central Basin, south side, Solana Hills trail | Aug. 2011

flower cluster with fluffy seed heads

East Basin south side, Santa Carina trailhead | Sept. 2011

Interesting Facts

There are numerous definitions of the word “weed”.

Some are matter-of-fact and all-encompassing:         “a plant that is not valued where it is growing and is usually of vigorous growth, especially one that tends to overgrow or choke out more desirable plants.”    Webster

or:        “a plant that interferes with management objectives for a given area of land at a given point in time.”   Torell 240

Some are brief:          “a plant out of place”.   Wikipedia 41

or humorous:        “A weed is a plant that has mastered every survival skill except for learning how to grow in rows”   Larson 544

A few are poignant:          “Weeds are flowers too, once you get to know them.” Eeyore 543

The common denominator in all of these quotes, and others, is that a “weed” is in the eye of the beholder.

close-up of a tiny daisy-like flower

East Basin south side, Santa Carina revegetation area | June 2022

Eeyore befriends a weed

"Weeds are flowers too, once you get to know them."

close-up of fa fluffy seed head

East Basin south side, Santa Carina revegetation area | July 2022

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