Canadian Horseweed is a native erect annual forb, growing on slightly hairy ridged stems from 2 feet to 7 feet high. Stems are not branched below the inflorescence. If seed germinates in autumn, a rosette is formed which overwinters. Spring germination also produces a small rosette but the flower stalk soon appears.
The leaves are dense on the stem, alternate, linear in shape with the lower leaves usually toothed and the upper leaves becoming smaller and without teeth. They have fine white hair on the margins and usually on the veins. Longer lower leaves are about 8x as long as wide, generally no more than 1/2 inch wide.
The floral array is a pyramidal shaped branched panicle with many small flowers individually stalked.
The flower heads are no more than 3/16 inch wide but with a green cylindric to bell shaped head. There are two types of florets: An outer ring of 20 to 30 fertile pistillate ray florets that have white corollas and white rays. The rays however are extremely short, 0.3 to 1 mm, but they still spread outward a little, and are longer than the style of the floret. These ray florets surround 8 to 30+ disc florets that have yellow tubular corollas whose tip lobes can be erect to spreading. The disc florets are bisexual, fertile, with 5 stamens surrounding a branched style. Stamens and style are exserted from the floret throat when it opens. The flower head is surrounded by several series of phyllaries where the outer series is greenish, lanceolate to linear in shape and shorter than the inner series which are usually linear and may be reddish. The phyllaries are usually smooth, but sometimes with sparse short stiff hair.
Seed: Fertile florets produce a dry pale tan to light gray-brown cypsela with a fluffy pappus and 2 to 3 mm long white bristles attached for wind dispersion.
Habitat: Canadian Horseweed is a favorite of disturbed sites or path edges, growing in all kinds of soil in full sun to partial shade, moist to dry conditions - just what most adventive plants do. With adequate moisture the plant grows to great height. The root is a branching short taproot. The root photo below is from a 7 foot plant.
Names: The genus Conyza is an old name said to be derived from the Greek konops, meaning 'flea' and said to be used by Pliny the Elder to name a type of Fleabane, which may be why one of the alternate common names is Canadian Fleabane. The species canadensis, means 'of Canada'. Before being assigned to the Conyza genus, the plant was listed as Erigeron canadensis. Erigeron contains many plants commonly called 'fleabanes'. The author names for the plant classification are: First to classify, as Erigeron canadensis in 1753, was '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was amended and the name changed in 1943 by ‘Cronquist’ which refers to Arthur Cronquist (1919-1992) American botanist, specialist on composites, whose major work was on development of the “Cronquist System”, a taxonomic system for flowering plants, now widely adopted.
Comparisons: While the panicle may look like a heavily flowered Wild Lettuce, the leaves are much different. The second species of Horseweed found in Minnesota, C. ramosissima, is a shorter many-branched plant with linear leaves.
The genus Cronza contains the Horseweeds of which there are 4 in North America. There are separated from the Fleabanes (Erigeron by have phyllaries of unequal length, usually 2 to 20 times as many ray florets as disc florets and rays on the ray florets extremely short or absent.
Above: 1st photo - the stem is unbranched below the inflorescence. 2nd photo - Plants are frequently found in large clusters.
Below: 1st photo - There are two types of florets - outer pistillate ray florets with the very short white rays and bisexual inner disc florets with the spreading yellow corolla lobes. Both are fertile. 2nd photo - The dry cypsela has pappus and fine bristles for wind dispersion.
Below: 1st photo - the root system. 2nd photo - - the underside has a very prominent vein with whitish hairs on the margins and along the veins.
Below: Leaf Comparison - An upper stem leaf on top and a lower stem leaf below it.
Below left: 1st photo - The floral array is a pyramidal shaped branched panicle. 2nd photo - individual heads have hair on the stalk, some fine hair on the outer series of phyllaries. Note the leaf like green bract. 3rd photo - Stems are also hairy and slightly ridged.
Notes: Canadian Horseweed is considered indigenous to the Garden area. Eloise Butler noted it in her Garden Log on May 31, 1907 using the older name Erigeron canadensis. Canadian Horseweed is found throughout North America except for several far north Canadian Provinces. It is believed to be native to North America, but has now spread to South America, Africa, Asia and Europe. Within Minnesota it is found in most counties with only a few exceptions, most of those in the agricultural SW. One other species of Conyza is found in Minnesota - C. ramosissima, Spreading Fleabane or Dwarf Horseweed. It is usually found in the SW counties where C. canadensis is absent and not in the metro area. Several varieties or subspecies have been described for our main species but neither the Minnesota authorities nor Flora of North America recognize them.
Uses and problems: Canadian Horseweed is an agricultural pest. It produces certain chemicals which inhibit germination and seedling growth in several species. Resistance to certain herbicides was first reported in 1980. The common herbicide used today in agriculture - Glyphosate - is no longer effective by itself. The first report of such resistance to Glyphosate was in 2000. In recent years herbicide resistant populations have become common in the farm belt and in other parts of the world including resistance to paraquat, atrazine, diuron and ALS-inhibiting herbicides. In addition, the plant serves as a host for the tarnished plant bug, Lygus lineolaris, and can also act a host for aster yellows. One of the aromatics in the plant is a terpene which is irritating to certain animals.
Nevertheless, Horseweed has seen use as a medicinal herb after it was brought to Europe in the 17th century. Grieve (Ref.#7) reports that the whole plant was gathered and dried. One of the constituents of value was a volatile oil, which when distilled was called 'Oil of Erigeron'. It was used to treat hemorrhage from the lungs or intestinal tract, inflamed tonsils and inflammation of the throat - although it was said to have a odor and a bitter taste.
The plant made its way to Europe from North America quite early - perhaps with the fur trade. Grieve (Ref.#7) reports that by 1653 it was growing in the Botanic Gardens of Paris and soon after it was a weed all over Paris. By 1669 it was in England.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"