First reported in the wild
Invasive species - risk of Medium Impact
Occasionally present, casual, vagrant, migratory
Introduction pathways - 1
Introduction pathways subclass - 1
NAPRA Ireland risk assessed
Tall erect, green, annual, up to 1m in height, green leaves (Stace, 1997).
Considered a weed in its native range, it has a negative impact on species abundance and diversity when grown with species from outside its native range (Shah et al., 2014). Native species richness is inversely correlated with invasibility of Conyza spp suggesting it will have greater impacts on communities already subject to disturbance (Prieur-Richard et al., 2000).
Inland unvegetated or sparsely vegetated habitats; Regularly or recently cultivated agricultural, horticultural or domestic habitat; Constructed, industrial or other artificial habitats
Seed production is related to height of plant: a plant 40 cm tall produces about 2000 seeds, while a plant 1.5 m tall plant can produce as much as 230 000 seeds (Weaver, 2001).
Pathway and vector description
Repeatedly reintroduced as contaminants in various seed mixtures, generally found on waste ground (Reynolds, 2002).
Mechanism of impact
Competition, Disease transmission
Typically found on waste ground, roadsides, pavement edges, dunes and walls (Reynolds, 2002; Stace, 1997).
Occasional - Infrequent casual, localised (Reynolds, 2002). Rosslare Harbour is the only site that has persistent records for this species.
Native to North America, found in all parts of Canada (C. candensis) except Newfoundland (Weaver, 2001).
Date of first record category
Fifty year date category
Records submitted to Data Centre in 2020
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How can you help
Report any sightings to the National Biodiversity Data Centre.
Reynolds, S.C.P. (2002) A catalogue of alien plants in Ireland. National Botanic Gardens. Glasnevin, Dublin. Stace, C. (1997). New Flora of the British Isles 2nd Edition. Cambridge University Press, Cambridge. Weaver, S. E. (2001). The biology of Canadian weeds. 115. Conyza canadensis. Canadian Journal of Plant Science, 81(4), 867-875. Shah, M. A., Callaway, R. M., Shah, T., Houseman, G. R., Pal, R. W., Xiao, S., Luo, W., Rosche, C., Reshi, Z.A., Khas, D.P. & Chen, S. (2014). Conyza canadensis suppresses plant diversity in its nonnative ranges but not at home: a transcontinental comparison. New Phytologist, 202(4), 1286-1296. Prieur-Richard, A. H., Lavorel, S., Grigulis, K., & Dos Santos, A. (2000). Plant community diversity and invasibility by exotics: invasion of Mediterranean old fields by Conyza bonariensis and Conyza canadensis. Ecology Letters, 3(5), 412-422.