Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Conyza canadensis
(Canadian fleabane)



Conyza canadensis (Canadian fleabane)


  • Last modified
  • 06 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Natural Enemy
  • Host Plant
  • Preferred Scientific Name
  • Conyza canadensis
  • Preferred Common Name
  • Canadian fleabane
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • C. canadensis is now a common weed in temperate to tropical regions. It is a mainly annual herbaceous weed spreading by producing high numbers of wind-dispersed seeds. It prefers undisturbed sites and is a particular problem in low-tillage systems su...
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C. canadensis vegetative stage.
TitleVegetative stage
CaptionC. canadensis vegetative stage.
Copyright©Chris Parker/Bristol, UK
C. canadensis vegetative stage.
Vegetative stageC. canadensis vegetative stage.©Chris Parker/Bristol, UK
C. canadensis inflorescence.
CaptionC. canadensis inflorescence.
Copyright©Chris Parker/Bristol, UK
C. canadensis inflorescence.
InflorescenceC. canadensis inflorescence.©Chris Parker/Bristol, UK
Leaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).
TitleLeaf comparison
CaptionLeaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).
Copyright©Chris Parker/Bristol, UK
Leaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).
Leaf comparisonLeaves of C. canadensis (left), C. sumatrensis (middle) and C. bonariensis (right).©Chris Parker/Bristol, UK


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Preferred Scientific Name

  • Conyza canadensis (L.) Cronq. (1943)

Preferred Common Name

  • Canadian fleabane

Other Scientific Names

  • Erigeron canadensis L.
  • Erigeron pusillus Nutt.
  • Trimorpha canadensis (L.) Lindm.

International Common Names

  • English: horseweed
  • Spanish: erigeron del Canada; escoba dura; hierba de caballo; hierba impia; olivarda
  • French: erigéron du Canada; vergerette du Canada
  • Portuguese: avoadinha

Local Common Names

  • Canada: mare's tail
  • Colombia: cvenadillo
  • Cuba: conyza; zancarana
  • France: erigeron de Canada; vergerette de Canada
  • Germany: Kanadischer berufkraut; Kanadisher katzenschweif
  • India: jarayupriya
  • Iraq: thail el-faras
  • Italy: impi; saeppola
  • Japan: himemukashiyomogi
  • Madagascar: sarijamala
  • Mauritius: herbe gandi
  • Mexico: pegajosa
  • Netherlands: fijnstraal, Canadeese
  • Norway: canadese fijnstraal; hestehamp
  • Poland: przymiotno kasnadyjskie
  • Puerto Rico: pascueta; rozuz
  • South Africa: armoedskruid; kanadese skraalhans
  • Spain: altabaca; canem bord; erigeron de Canada; zamarraga
  • Sweden: kanadabinka
  • Turkey: sifa out
  • USA: butterweed; Canada horseweed; fireweed; hogweed
  • Yugoslavia (Serbia and Montenegro): repusnjaca

EPPO code

  • ERICA (Erigeron canadensis)
  • ERIPS (Erigeron pusillus)

Summary of Invasiveness

Top of page C. canadensis is now a common weed in temperate to tropical regions. It is a mainly annual herbaceous weed spreading by producing high numbers of wind-dispersed seeds. It prefers undisturbed sites and is a particular problem in low-tillage systems such as orchards, plantations but also in some agricultural crops. It may be controlled by tillage at a suitable growth stage, but otherwise, it has developed resistance to many herbicides in a large number of countries. It has been introduced internationally as a contaminant of cereals, forage seeds and cotton, and there is a risk of further similar introduction to countries where it is not yet established. It could become a problem invasive in protected areas, though may be controlled naturally as it is an early-successional species, often being replaced by perennial grasses.

Taxonomic Tree

Top of page
  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Asterales
  •                         Family: Asteraceae
  •                             Genus: Conyza
  •                                 Species: Conyza canadensis

Notes on Taxonomy and Nomenclature

Top of page The species was first described by Linnaeus as Erigeron canadensis in 1753, and transferred to the genus Conyza in 1943 by Cronquist. C. canadensis is a clearly defined species and is not nomenclaturally confused with any other related species, unlike several others within the genus (see datasheets on C. bonariensis and C. sumatrensis). It is, however, still widely referred to by its older name, Erigeron canadensis. Thebauld and Abbott (1995) noted that C. canadensis was the only diploid species of five invasive European species tested, and was more closely related to the genus Erigeron than the other taxa. This supports a hypothesis that C. canadensis is older in evolutionary terms.


Top of page C. canadensis is an erect annual with a long taproot and one or more stems arising from a basal rosette, it is usually about 1 m high but may be much taller. Leaves are up to 10 cm long and about 1 cm wide with some shallow teeth, clear green (not greyish as in other common Conyza species), almost glabrous on the surfaces, but with some scattered hairs. Leaf margins ciliate and with longer conspicuous hairs towards the leaf base. Flower heads are very numerous on short pedicels, only 2-3 mm in diameter when fresh (broader in pressed specimens), involucral bracts about 5 mm long, glabrous. Disc florets yellow, contrasting with distinct white ray florets which are 0.5 to 1 mm long, the latter distinguishing C. canadensis from other common weedy Conyza species. Seeds 1.0-1.3 mm long with 10-25 off-white pappus hairs, 2-4 mm long (Holm et al., 1997).

Plant Type

Top of page Annual
Seed propagated


Top of page C. canadensis is native to North America (USA and Canada) but spread to Europe as early as the 1600s (Michael, 1977) and later to much of Asia and Australia, including tropical regions such as in the Americas. However, in Africa it is so far restricted to north and south subtropical regions (Holm et al., 1997). In Bhutan, it is restricted to higher elevations, over 2000 m (Parker, 1992) but is apparently not so restricted in Central America. The native range elsewhere in the Americas remains obscure, but it is considered as exotic in Central America and the Caribbean in this datasheet. Weaver (2001) reported that the species was native to all provinces of Canada except Newfoundland, whereas USDA-ARS (2004) included Newfoundland in the native range.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes


BhutanPresentIntroducedParker, 1992
ChinaPresentIntroducedHolm et al., 1979
-HebeiPresentIntroducedWang et al., 1990
-HenanPresentIntroducedWang et al., 1990
-JiangxiPresentIntroducedWang et al., 1990
-ShaanxiPresentIntroducedWang et al., 1990
-ShandongPresentIntroducedWang et al., 1990
-ShanxiPresentIntroducedWang et al., 1990
-SichuanPresentIntroducedWang et al., 1990
-YunnanPresentIntroducedMissouri Botanical Garden, 2004
-ZhejiangPresentIntroducedWang et al., 1990
IndiaWidespreadIntroducedHolm et al., 1979
-HaryanaPresentIntroducedDahiya et al., 1988
-Jammu and KashmirPresentIntroducedSiddiq et al., 1987
IndonesiaPresentIntroducedHolm et al., 1997
IranPresentIntroducedHolm et al., 1979
IraqPresentIntroducedHolm et al., 1979
IsraelPresentIntroducedHolm et al., 1979
JapanPresentIntroducedHolm et al., 1979
-HokkaidoWidespreadIntroducedMorita, 1997
-HonshuWidespreadIntroducedMorita, 1997
-KyushuWidespreadIntroducedMorita, 1997
-Ryukyu ArchipelagoWidespreadIntroducedMorita, 1997
-ShikokuWidespreadIntroducedMorita, 1997
JordanPresentIntroducedHolm et al., 1979
Korea, DPRWidespreadIntroducedHolm et al., 1979
Korea, Republic ofWidespreadIntroducedHolm et al., 1979
PakistanPresentIntroducedHolm et al., 1979
PhilippinesWidespreadIntroducedHolm et al., 1979
TaiwanWidespreadIntroducedWang et al., 1990; Holm et al., 1979
ThailandPresentIntroducedHolm et al., 1979
TurkeyPresentIntroducedHolm et al., 1979
VietnamPresentIntroducedHolm et al., 1979


LesothoPresentIntroducedWells et al., 1986
MadagascarPresentIntroducedHolm et al., 1997
MauritiusPresentIntroducedHolm et al., 1979
MozambiquePresentIntroducedHolm et al., 1979
NigeriaPresentIntroducedHolm et al., 1979
South AfricaPresentIntroducedHolm et al., 1979
-Canary IslandsPresentIntroduced Invasive Cronquist, 1976
SudanPresentIntroducedBebawi and Neugebohrn, 1991
SwazilandPresentIntroducedWells et al., 1986
TunisiaPresentIntroducedHolm et al., 1979
ZimbabwePresentIntroducedHolm et al., 1979

North America

CanadaWidespreadNativeHolm et al., 1979
-AlbertaPresentNativeUSDA-ARS, 2004
-British ColumbiaPresentNativeUSDA-ARS, 2004
-ManitobaPresentNativeUSDA-ARS, 2004
-Newfoundland and LabradorPresentNativeUSDA-ARS, 2004
-Northwest TerritoriesPresentNativeUSDA-ARS, 2004
-Nova ScotiaPresentNativeUSDA-ARS, 2004
-NunavutPresentNativeUSDA-ARS, 2004
-OntarioPresentNativeWhittle et al., 1997
-Prince Edward IslandPresentNativeUSDA-ARS, 2004
-QuebecPresentNativeLeroux et al., 1996
-SaskatchewanPresentNativeUSDA-ARS, 2004
-Yukon TerritoryPresentNativeUSDA-ARS, 2004
MexicoPresentIntroducedHolm et al., 1979; USDA-ARS, 2004
USAPresentPresent based on regional distribution.
-AlabamaWidespreadNativeLorenzi and Jeffery, 1987
-ArizonaWidespreadNativeLorenzi and Jeffery, 1987
-ArkansasWidespreadNativeLorenzi and Jeffery, 1987
-CaliforniaWidespreadNativeLorenzi and Jeffery, 1987
-ColoradoWidespreadNativeLorenzi and Jeffery, 1987
-ConnecticutWidespreadNativeLorenzi and Jeffery, 1987
-DelawareWidespreadNativeLorenzi and Jeffery, 1987
-FloridaWidespreadNativeLorenzi and Jeffery, 1987
-GeorgiaWidespreadNativeLorenzi and Jeffery, 1987
-HawaiiWidespreadIntroducedHolm et al., 1979
-IdahoWidespreadNativeLorenzi and Jeffery, 1987
-IllinoisWidespreadNativeLorenzi and Jeffery, 1987
-IndianaWidespreadNativeLorenzi and Jeffery, 1987
-IowaWidespreadNativeLorenzi and Jeffery, 1987
-KansasWidespreadNativeLorenzi and Jeffery, 1987
-KentuckyWidespreadNativeLorenzi and Jeffery, 1987
-LouisianaWidespreadNativeLorenzi and Jeffery, 1987
-MaineWidespreadNativeLorenzi and Jeffery, 1987
-MarylandWidespreadNativeLorenzi and Jeffery, 1987
-MassachusettsWidespreadNativeLorenzi and Jeffery, 1987
-MichiganWidespreadNativeLorenzi and Jeffery, 1987
-MinnesotaWidespreadNativeLorenzi and Jeffery, 1987
-MississippiWidespreadNativeLorenzi and Jeffery, 1987
-MissouriWidespreadNativeLorenzi and Jeffery, 1987
-MontanaWidespreadNativeLorenzi and Jeffery, 1987
-NebraskaWidespreadNativeLorenzi and Jeffery, 1987
-NevadaWidespreadNativeLorenzi and Jeffery, 1987
-New HampshireWidespreadNativeLorenzi and Jeffery, 1987
-New JerseyWidespreadNativeLorenzi and Jeffery, 1987
-New MexicoWidespreadNativeLorenzi and Jeffery, 1987
-New YorkWidespreadNativeLorenzi and Jeffery, 1987
-North CarolinaWidespreadNativeLorenzi and Jeffery, 1987
-North DakotaWidespreadNativeLorenzi and Jeffery, 1987
-OhioWidespreadNativeLorenzi and Jeffery, 1987
-OklahomaWidespreadNativeLorenzi and Jeffery, 1987
-OregonWidespreadNativeLorenzi and Jeffery, 1987
-PennsylvaniaWidespreadNativeLorenzi and Jeffery, 1987
-Rhode IslandWidespreadNativeLorenzi and Jeffery, 1987
-South CarolinaWidespreadNativeLorenzi and Jeffery, 1987
-South DakotaWidespreadNativeLorenzi and Jeffery, 1987
-TennesseeWidespreadNativeLorenzi and Jeffery, 1987
-TexasWidespreadNativeLorenzi and Jeffery, 1987
-UtahWidespreadNativeLorenzi and Jeffery, 1987
-VermontWidespreadNativeLorenzi and Jeffery, 1987
-VirginiaWidespreadNativeLorenzi and Jeffery, 1987
-WashingtonWidespreadNativeLorenzi and Jeffery, 1987
-West VirginiaWidespreadNativeLorenzi and Jeffery, 1987
-WisconsinWidespreadNativeLorenzi and Jeffery, 1987
-WyomingWidespreadNativeLorenzi and Jeffery, 1987

Central America and Caribbean

BelizePresentIntroducedMissouri Botanical Garden, 2004; USDA-ARS, 2004
Costa RicaPresentIntroducedMissouri Botanical Garden, 2004; USDA-ARS, 2004
CubaPresentIntroduced Invasive Oviedo Prieto et al., 2012
El SalvadorPresentIntroducedMissouri Botanical Garden, 2004; USDA-ARS, 2004
GuatemalaPresentIntroducedMissouri Botanical Garden, 2004; USDA-ARS, 2004
HondurasPresentIntroducedHolm et al., 1979
JamaicaPresentIntroducedHolm et al., 1979
NicaraguaPresentIntroducedMissouri Botanical Garden, 2004; USDA-ARS, 2004
PanamaPresentIntroducedMissouri Botanical Garden, 2004; USDA-ARS, 2004
Puerto RicoPresentIntroducedHolm et al., 1979
Trinidad and TobagoPresentIntroducedHolm et al., 1979
Turks and Caicos IslandsPresentIntroducedMissouri Botanical Garden, 2004

South America

BrazilPresentIntroducedHolm et al., 1997
ChilePresentIntroducedHolm et al., 1979
ColombiaPresentIntroducedHolm et al., 1979
EcuadorPresentIntroducedHolm et al., 1979
GuyanaPresentIntroducedHolm et al., 1997
PeruPresentIntroducedHolm et al., 1997
SurinamePresentIntroducedHolm et al., 1979
VenezuelaPresentIntroducedHolm et al., 1997


AlbaniaPresentIntroduced Invasive Cronquist, 1976
AndorraPresentIntroduced Invasive Cronquist, 1976
AustriaPresentIntroduced Invasive Cronquist, 1976
BelarusPresentIntroduced Invasive Cronquist, 1976
BelgiumPresentIntroduced Invasive Cronquist, 1976
Bosnia-HercegovinaPresentIntroduced Invasive Cronquist, 1976
BulgariaPresentIntroduced Invasive Cronquist, 1976
CroatiaPresentIntroduced Invasive Cronquist, 1976
Czechoslovakia (former)PresentIntroduced Invasive Cronquist, 1976
DenmarkPresentIntroduced Invasive Cronquist, 1976
EstoniaPresentIntroduced Invasive Cronquist, 1976
FinlandPresentIntroduced Invasive Cronquist, 1976
FrancePresentIntroduced Invasive Cronquist, 1976
GermanyPresentIntroduced Invasive Cronquist, 1976
GibraltarPresentIntroduced Invasive Cronquist, 1976
GreecePresentIntroduced Invasive Cronquist, 1976; Baliousis, 2014
HungaryPresentIntroduced Invasive Cronquist, 1976
ItalyPresentIntroduced Invasive Cronquist, 1976
LatviaPresentIntroduced Invasive Cronquist, 1976
LiechtensteinPresentIntroduced Invasive Cronquist, 1976
LithuaniaPresentIntroduced Invasive Cronquist, 1976
LuxembourgPresentIntroduced Invasive Cronquist, 1976
MacedoniaPresentIntroduced Invasive Cronquist, 1976
MoldovaPresentIntroduced Invasive Cronquist, 1976
MonacoPresentIntroduced Invasive Cronquist, 1976
NetherlandsPresentIntroduced Invasive Cronquist, 1976
NorwayPresentIntroduced Invasive Cronquist, 1976
PolandPresentIntroduced Invasive Cronquist, 1976
PortugalPresentIntroduced Invasive Cronquist, 1976
-AzoresPresentCronquist, 1976
RomaniaPresentIntroduced Invasive Cronquist, 1976
Russian FederationPresentPresent based on regional distribution.
-Central RussiaPresentIntroduced Invasive Royal Botanic Garden Edinburgh, 2003
-Southern RussiaPresentIntroduced Invasive Royal Botanic Garden Edinburgh, 2003
San MarinoPresentIntroduced Invasive Cronquist, 1976
SlovakiaPresentIntroduced Invasive Cronquist, 1976
SloveniaPresentIntroduced Invasive Cronquist, 1976
SpainPresentIntroduced Invasive Cronquist, 1976
SwedenPresentIntroduced Invasive Cronquist, 1976
SwitzerlandPresentIntroduced Invasive Cronquist, 1976
UKPresentIntroduced Invasive Cronquist, 1976
UkrainePresentIntroduced Invasive Cronquist, 1976
Yugoslavia (former)PresentIntroduced Invasive Cronquist, 1976
Yugoslavia (Serbia and Montenegro)PresentIntroduced Invasive Cronquist, 1976


AustraliaPresentIntroducedHolm et al., 1979
-QueenslandPresentIntroducedHolm et al., 1997
-South AustraliaPresentIntroducedHolm et al., 1997
-TasmaniaPresentIntroducedHolm et al., 1997
-Western AustraliaPresentIntroducedHolm et al., 1997
FijiWidespreadIntroducedParham, 1958
New ZealandPresentIntroducedHolm et al., 1979
Papua New GuineaWidespreadIntroducedHenty and Pritchard, 1975

Risk of Introduction

Top of page Although there is a risk of accidental introduction of C. canadensis, the fact that it is already so widespread means that the phytosanitary risk is relatively low. It is present in many regions where it is able to survive and grow and exclusion from other areas where it is not yet present may prove impossible. However, certain quarantine measures may ensure that it does not spread into certain specified areas within a country or region, such as protected areas. Being a weed of mainly undisturbed ground, the potential risks to such sites may be considerable.


Top of page C. canadensis is a weed of agriculture and forestry in temperate and subtropical climates and at higher elevations in some parts of the tropics. It is associated with perennial crops, fallows and field borders and, in annual crops, is favoured by reduced tillage. It is sometimes associated with sandy soils and irrigation, but is not limited to these conditions. It is also a weed of roadsides, old fields and is also associated with recently disturbed land, as an early successional species (Thebaud et al., 1996).

Habitat List

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Terrestrial – ManagedCultivated / agricultural land Present, no further details Harmful (pest or invasive)
Managed forests, plantations and orchards Present, no further details Harmful (pest or invasive)
Managed grasslands (grazing systems) Present, no further details Harmful (pest or invasive)
Disturbed areas Present, no further details Harmful (pest or invasive)
Rail / roadsides Present, no further details Harmful (pest or invasive)
Terrestrial ‑ Natural / Semi-naturalNatural forests Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

Top of page C. canadensis occurs in a wide range of crops, annual and perennial, especially where tillage is less intensive. It has been reported in many orchard and other tree crops, including forestry, in grassland and forage crops, and also in annual crops being managed under minimal or reduced tillage regimes. It is also found as a weed in fields producing flowering bulbs and perennials, in forest nurseries, and in ornamental situations in turfgrass.

Biology and Ecology

Top of page Genetics

C. canadensis is a diploid species, with a chromosome number of 2n=18. This is in contrast with several other species of Conyza, which were ascertained as allopolyploids (Thebaud and Abbot, 1995). As there is a tendency in plants to increasing ploidy levels, it may be assumed that C. canadensis is an ancestral member of the genus, and other species may have arisen via hybridization events. Chromosome size has been studied by Gosteva (1998).

Physiology and Phenology

C. canadensis is predominantly an annual plant, germinating in autumn and persisting as a rosette of leaves over the winter before bolting and flowering the following spring. It may, however, behave as a biennial in temperate climates, but rarely, if ever, persists for a second season after flowering. Seeds need a temperature of 10-25°C and require light for germination (Zinzolker et al., 1985). Establishment occurs mainly in occasionally disturbed situations. Intensive cultivations for annual crops apparently bury most of the seed and greatly reduce emergence, while in completely uncultivated situations other vegetation tends to interfere with its establishment (Escarré et al., 1998; Németh et al., 1998). After establishment as a rosette, elongation of the stem is inhibited by short days but occurs rapidly under longer day conditions (Zinzolker et al., 1985). For further information on the biology of C. canadensis, refer to Weaver (2001).

Reproductive Biology

Seed production can be immense, up to 250,000 seeds per plant (Holm et al., 1997; Weaver, 2001), and seed dispersal by wind is made highly efficient by the pappus (Weaver, 2001). Seed size is small, but also variable, with Fenner (1983) describing the rather complex relation between seed size, seedling establishment and vigour.

Environmental Requirements

C. canadensis is native to an area with a broad climatic amplitude, though is most common in temperate and mediterannean zones, it is found from tundra and taiga to the sub-tropics. Where introduced, it can even be found in tropical regions, though generally at higher altitudes.

There is little evidence for preference regarding soil type, with C. canadensis apparently able to grow in a wide range of soil types.


Prieur-Richard et al. (2002) studied in detail the invasion of C. bonariensis in Mediterranean annual plant communities (see the datasheet on C. bonariensis), though comparisons with C. canadensis may be invalid, as Conyza species are observed to have significantly different characteristics in terms of their position in plant succession. Nonetheless, Conyza spp. including C. canadensis are generally early successional species (e.g. Escarré et al., 1998), though Thebaud et al. (1996) noted that C. canadensis was restricted to recently disturbed areas, whereas C. sumatrensis colonized early- to mid-successional old fields. In Japan, they were noted as dominant in two-year-old fields, being able to grow as shade-tolerant rosettes under the canopy of other annuals, and being gradually succeeded by perennial grasses in later years (Ohtsuka, 1998).

Soil Tolerances

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Soil drainage

  • free
  • impeded

Soil reaction

  • acid
  • neutral

Soil texture

  • heavy
  • light
  • medium

Special soil tolerances

  • infertile
  • saline
  • shallow
  • sodic

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Basidiophora entospora Pathogen Leaves
Engyaulus pulchellus Herbivore Roots/Stems
Procecidochares australis Herbivore Stems
Uroleucon behuri Herbivore Leaves/Stems
Uroleucon erigeronense Herbivore Leaves/Stems

Means of Movement and Dispersal

Top of page Natural Dispersal (Non-Biotic)

C. canadensis is principally a wind-dispersed species (e.g. Ohtsuka, 1998), facilitated by light seed accompanied by a pappus which aids flight.

Vector Transmission

No information is available on the possibility of spread by animals, but if it occurs, it is likely to be only of minor significance in comparison to wind-dispersal.

Agricultural Practices

C. canadensis is encouraged by irrigation, perhaps partly because of the distribution of seed by irrigation water (Holm et al., 1997). Mowing along roadsides, especially during seed production, is also likely to increase spread. Also, late tillage or other practices at such inappropriate times will also facilitate seed dispersal.

Accidental Introduction

Seed of several Conyza species now widely present as weeds outside of their native ranges were probably introduced to most of their introduced ranges accidentally as contaminants in cotton, cereals or forage grains/seed. The first appearance of C. bonariensis around textile mills in Europe and elsewhere where exotic means it may have been a contaminant of cotton, and seeds of other Conyza species may have been introduced via this pathway also. C. canadensis was considered by Park et al. (2001) to have been introduced into Korea Republic with imported cereals for concentrate feed or within seeds for forage production. This is a likely pathway, considering how the species is a weed in forage seed production stands, e.g. in lucerne (Dimitrova and Milanova, 2003).

Also a weed in nurseries, Conyza spp. may be spread as seed present in the soil in pots or other planting containers that accompany nursery stock, either as ornamentals (Gallitano and Skroch, 1993) or for establishing forest plantations (Prach et al., 1995). The spread of C. canadensis, along with numerous other weeds in central European forests, was thought to have been assisted by seeds in tree containers (Prach et al., 1995), and thus, presence in soil must be considered as a potential pathway.

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Soil, sand and gravelSeeds in potting compost. Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Growing medium accompanying plants seeds Yes Pest or symptoms usually visible to the naked eye
True seeds (inc. grain) seeds Yes Pest or symptoms usually visible to the naked eye
Plant parts not known to carry the pest in trade/transport
Fruits (inc. pods)
Seedlings/Micropropagated plants
Stems (above ground)/Shoots/Trunks/Branches

Wood Packaging

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Wood Packaging not known to carry the pest in trade/transport
Loose wood packing material
Processed or treated wood
Solid wood packing material with bark
Solid wood packing material without bark

Impact Summary

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Animal/plant collections None
Animal/plant products None
Biodiversity (generally) None
Crop production None
Environment (generally) None
Fisheries / aquaculture None
Forestry production None
Human health None
Livestock production None
Native fauna None
Native flora None
Rare/protected species None
Tourism None
Trade/international relations None
Transport/travel None


Top of page Holm et al. (1997) record that C. canadensis occurs as a weed in 70 countries, in more than 40 crops. It is most common in the less intensively cultivated perennial crops, such as fruit orchards, vines, forestry, tea, grassland, sugarcane, pineapple and ornamental nurseries. It has also become increasingly important in zero-tilled annual crops, such as cotton and soyabean, where the seeds are left on the soil surface and are exposed to the light necessary for germination, and are not controlled by normal pre-planting tillage (Buhler and Owen, 1997). Having germinated in the autumn it is often well grown by the time of planting and thus quite difficult to control. Its importance is increased by its widespread resistance to paraquat and triazines (see Chemical Control). The are few estimates of its competitive effect, but in Romania yield reductions of 64% have been recorded in sugar beet (Sarpe and Torge, 1980) and Holm et al. (1997) note that vegetative growth of vines can be reduced by 28%. There are some reports of damaging allelopathic effects, for example, to Trifolium repens and lettuce (Souto et al., 1990) It may cause additional economic damage as a result of its aromatic oils contaminating those harvested from Mentha spp. (Ogg et al., 1975).

It is recorded as an alternative host of Tomato bushy stunt virus (Grbelja et al., 1988), and as a host for numerous other serious plant viruses including Tomato spotted wilt virus and Cucumber mosaic virus, nematodes such as Meloidogyne javanica (Dahiya et al., 1998) and Rotylenchulus reniformis (Wang et al., 2003), fungal pathogens such as Sclerotinia minor, also aster yellows phytoplasma, and a range of insect pests (e.g. Weaver, 2001).

Environmental Impact

Top of page The presence of C. canadensis is seen to have significant effects on the soil carbon:nitrogen ratio in a subtropical orchard (Chen et al., 2004), and as such can be expected to have similar effects on natural ecosystems if it become invasive there, though the actual importance of any changes to soil have yet to be ascertained in such situations.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Highly adaptable to different environments
  • Highly mobile locally
  • Has high reproductive potential
Impact outcomes
  • Negatively impacts agriculture
Impact mechanisms
  • Competition - monopolizing resources
  • Pest and disease transmission
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult to identify/detect as a commodity contaminant
  • Difficult/costly to control


Top of page In Japan and China, the essential oils from C. canadensis are used in the treatment of jaundice (Miyazawa et al., 1992), and the oils have been observed to have antifungal effects. Also, plant extracts have some antifeedant effects on insect species, and as such have potential use as repellents in stored grains. There are also some reports of suppression of other weeds, which could perhaps be regarded as beneficial (Varadi et al., 1987).

Uses List

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  • Essential oils

Medicinal, pharmaceutical

  • Traditional/folklore

Similarities to Other Species/Conditions

Top of page Several other Conyza spp. are common weeds, overlapping with C. canadensis in distribution, but these are generally distinguished by having more finely hairy, greyish-green foliage, without the scattered long marginal hairs, and flower heads without ray florets (see datasheets on C. bonariensis and C. sumatrensis). See also Reutelingsperger (2000) for a key differentiating these species.

Prevention and Control

Top of page Cultural Control

In traditional farming, C. canadensis is controlled satisfactorily by tillage, hand-weeding and also suitable crop rotation (e.g. Leroux et al., 1996). In more developed systems, non-chemical methods include the use of living mulches of, for example, Trifolium subterraneum (Enache and Ilnicki, 1988). Other mulches, living or inert, are observed to increase control of C. canadensis, which may be expected as seeds require light for germination. The use of cover crops may also have a similar effect, due to direct competition for light and possible water and plant nutrients, and were shown to be effective in controlling C. canadensis in apple orchards in Korea (Jung et al., 1998). It is, however, surprisingly resistant to destruction by soil solarization (Horowitz et al., 1983).

Chemical Control

C. canadensis is normally susceptible to most of the herbicides used to control annual broad-leaved weeds, including 2,4-D and dicamba. However, it has developed widespread resistance to paraquat and the triazines in Europe and the USA. The mechanisms of resistance have been studied in detail, including the dual resistance to both herbicide groups in Hungary (Polos et al., 1988; Lehoczki et al., 1992; Darko et al., 1996). Paraquat resistance has been shown to be controlled by a single dominant gene (Yamasue et al., 1992). This resistance automatically confers moderate resistance to diquat, and triazine resistance results in less susceptibility to bromacil and lenacil. Linuron resistance is also reported from France (Beuret, 1988) and terbacil resistance in Hungary (Molnar et al., 1988). One previously isolated report of resistance to glyphosate (Talbert et al., 1975) is now supported by others, e.g. Solymosi (2001) in Hungary and Van Gessel (2001) in the USA, confirming the increased resistance of this species to such herbicides normally effective along with a wide range of other alternatives depending on the particular cropping regime. These include sulfonylurea herbicides (Németh et al., 1998), cyanazine, sulfallate, glufosinate, oxyfluorfen, hexazinone, tebuthiuron, amitrole, asulam, oryzalin, clopyralid and imazapyr. Inconsistent results are reported with diuron, metribuzin, bentazon and acifluorfen, while poor results have been reported with oxadiazon and imazethapyr.

Biological Control

There has been consideration of biological control possibilities in Italy (Pecora, 1977), the insects of interest including the tephritid Procecidochares australis from USA and the coleopteran Agrilus pulchellus [Engyaulus pulchellus] which is known to attack other Conyza/Erigeron species; but there are no reports of any practical progress.


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