Canadian fleabane
Invasive Species Information

Canadian-fleabane Biodiversity Medium Risk Invasive Species 14

What Is Canadian fleabane - (Conyza canadensis, Erigeron canadensis)?

Habitat: Terrestrial
Distribution in Ireland:

Status: Established

Family name: Asteraceae

Common name/s: Horseweed, Canadian horseweed, Canadian fleabane, Coltstail, Marestail, and Butterweed.

Reproduction:

Each flower head (capitulum) is 3 to 5 mm in diameter and is composed of many outer, white pistillate ray florets and central yellowish-green perfect disk florets. Conyza canadensis is self-compatible. Pollen is released before the capitula have fully opened, suggesting that it is primarily self-pollinating,

Canadian-fleabane - Conyza canadensis 4

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Canadian-fleabane - Conyza canadensis 4

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Canadian fleabane images

Canadian fleabane is an annual plant growing to 1.5m tall, with sparsely hairy stems. The leaves are unstalked, slender, 2–10 centimetres long, and up to 1 cm across, with a coarsely toothed margin. They grow in an alternate spiral up the stem and the lower ones wither early.

Flowers are produced in dense inflorescences 1 cm in diameter. Each individual flower has a ring of white or pale purple ray florets and a centre of yellow disc florets. The fruit is a cypsela tipped with dirty white down.

Canadian fleabane seeds

Canadian fleabane can easily be confused with Guernsey fleabane (Conyza sumatrensis), which may grow to a height of 2 m, and the more hairy flax-leaf fleabane (Erigeron bonariensis), which does not exceed 1 m .

Canadian fleabane is distinguished by bracts that have a brownish inner surface and no red dot at the tip, and are free (or nearly free) of the hairs found on the bracts of the other species.

Originally from North America Canadian fleabane has spread to inhabited areas of most temperate zones of Asia, Europe, and Australia. It can commonly be found in Ireland, growing as a weed along roadsides and wasteland.

is an annual and does not survive more than one year. Most individuals germinate in autumn and overwinter as rosettes. A small fraction of the population will emerge in spring, produce seeds and die in the same year.

How To Identify Canadian fleabane?

ID Guide (coming soon)

Leaf: Green, glabrous or with limited hairs at margins / inital part of the midrib; lightly dentate margins. Leaves at base are oblong to oblanceolate; upper leaves are thinner, elliptc to linear.

Flower: Arranged in numerous capitula, with involucral bracts are glabrous or nearly glabrous. White or pinkish ligules, very visible.

Smell: faint odour suggestive of carrots when crushed

Fruit: Fatened cypselas with a pappus of 1-3 mm diameter, yellowish-white. Capitula (open) from 4-8 mm diameter

Root

Leaf

Stem

Why Is Canadian fleabane A Problem?

Canadian fleabane is an alien (non-native) invasive plant, meaning it out-competes crowds-out and displaces beneficial native plants that have been naturally growing in Ireland for centuries.

Ecological impacts

It forms dense areas that prevent the growth of natve vegetation.

Economic impacts

High costs in applying control methodologies, mainly in crop areas where it reduces productvity in crop felds.

Other impacts

Due to the high producton of pollen, it is considered an allergenic plant.