Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Ageratum houstonianum
(Blue billygoatweed)

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Datasheet

Ageratum houstonianum (Blue billygoatweed)

Summary

  • Last modified
  • 21 November 2019
  • Datasheet Type(s)
  • Invasive Species
  • Host Plant
  • Preferred Scientific Name
  • Ageratum houstonianum
  • Preferred Common Name
  • Blue billygoatweed
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • A. houstonianum is an annual herb native to Mexico and Central America. It was brought to Europe shortly after its discovery, where its use as an ornamental started (

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Pictures

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PictureTitleCaptionCopyright
A. houstonianum; foliage and flowers.
TitleFoliage and flowers
CaptionA. houstonianum; foliage and flowers.
Copyright©Chris Parker/Bristol, UK
A. houstonianum; foliage and flowers.
Foliage and flowersA. houstonianum; foliage and flowers.©Chris Parker/Bristol, UK
Leaves of Ageratum conyzoides (a) and A. houstonianum (b).|Leaves of Ageratum conyzoides (a) and A. houstonianum (b). The leaves of A. conyzoides are opposite, 20-100 mm long, 5-50 mm wide, on hairy petioles 5-75 mm long, broadly ovate, with a rounded or narrowed acute base and an acute or obtuse or sometimes acuminate tip and toothed margins.
TitleLeaves
CaptionLeaves of Ageratum conyzoides (a) and A. houstonianum (b).|Leaves of Ageratum conyzoides (a) and A. houstonianum (b). The leaves of A. conyzoides are opposite, 20-100 mm long, 5-50 mm wide, on hairy petioles 5-75 mm long, broadly ovate, with a rounded or narrowed acute base and an acute or obtuse or sometimes acuminate tip and toothed margins.
Copyright©Chris Parker/Bristol, UK
Leaves of Ageratum conyzoides (a) and A. houstonianum (b).|Leaves of Ageratum conyzoides (a) and A. houstonianum (b). The leaves of A. conyzoides are opposite, 20-100 mm long, 5-50 mm wide, on hairy petioles 5-75 mm long, broadly ovate, with a rounded or narrowed acute base and an acute or obtuse or sometimes acuminate tip and toothed margins.
LeavesLeaves of Ageratum conyzoides (a) and A. houstonianum (b).|Leaves of Ageratum conyzoides (a) and A. houstonianum (b). The leaves of A. conyzoides are opposite, 20-100 mm long, 5-50 mm wide, on hairy petioles 5-75 mm long, broadly ovate, with a rounded or narrowed acute base and an acute or obtuse or sometimes acuminate tip and toothed margins.©Chris Parker/Bristol, UK
Inflorescences of Ageratum conyzoides (a) and A. houstonianum (b).|Inflorescences of Ageratum conyzoides (a) and A. houstonianum (b). Individual flower heads of A. conyzoides are light blue, white or violet, carried on 50-150 mm long peduncles, and are 5 mm across, 4-6 mm long with 60-75 tubular flowers. The flower head is surrounded by two or three rows of oblong bracts which are green with pale or reddish-violet tops.
TitleInflorescences
CaptionInflorescences of Ageratum conyzoides (a) and A. houstonianum (b).|Inflorescences of Ageratum conyzoides (a) and A. houstonianum (b). Individual flower heads of A. conyzoides are light blue, white or violet, carried on 50-150 mm long peduncles, and are 5 mm across, 4-6 mm long with 60-75 tubular flowers. The flower head is surrounded by two or three rows of oblong bracts which are green with pale or reddish-violet tops.
Copyright©Chris Parker/Bristol, UK
Inflorescences of Ageratum conyzoides (a) and A. houstonianum (b).|Inflorescences of Ageratum conyzoides (a) and A. houstonianum (b). Individual flower heads of A. conyzoides are light blue, white or violet, carried on 50-150 mm long peduncles, and are 5 mm across, 4-6 mm long with 60-75 tubular flowers. The flower head is surrounded by two or three rows of oblong bracts which are green with pale or reddish-violet tops.
InflorescencesInflorescences of Ageratum conyzoides (a) and A. houstonianum (b).|Inflorescences of Ageratum conyzoides (a) and A. houstonianum (b). Individual flower heads of A. conyzoides are light blue, white or violet, carried on 50-150 mm long peduncles, and are 5 mm across, 4-6 mm long with 60-75 tubular flowers. The flower head is surrounded by two or three rows of oblong bracts which are green with pale or reddish-violet tops. ©Chris Parker/Bristol, UK

Identity

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Preferred Scientific Name

  • Ageratum houstonianum Mill.

Preferred Common Name

  • Blue billygoatweed

Other Scientific Names

  • Ageratum mexicanum

International Common Names

  • English: Blue maudlin
  • French: Agerate bleu

Local Common Names

  • Cuba: celestina azul
  • Germany: Mexikanischer Leberbalsam
  • Japan: Murasakikakkoazami; Ookakkoazami

EPPO code

  • AGEHO (Ageratum houstonianum)

Summary of Invasiveness

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A. houstonianum is an annual herb native to Mexico and Central America. It was brought to Europe shortly after its discovery, where its use as an ornamental started (Johnson, 1971). The species is reported to be a weed and invasive outside its native range, due to its high production of small seeds with easy dispersal by wind or water (BioNet-EAFRINET, 2016). It is also dispersed by animals, clothing, vehicles, contaminated soils and agricultural produce (Johnson, 1971; BioNet-EAFRINET, 2016). Mainly as a garden escape, it has naturalized on farmlands, wastelands, roadsides, forest trails, crops, riverbanks and wetlands (BioNet-EAFRINET, 2016). The species is reported as invasive in China, Taiwan (PIER, 2016), Mozambique, Swaziland, Tanzania, Zimbabwe, USA (Hawaii), Cuba, Peru, Australia, Fiji, French Polynesia, New Zealand, and as a declared weed and alien invader plant with the highest category of invasiveness in South Africa (Oviedo Prieto et al., 2012; BioNet-EAFRINET, 2016; PIER, 2016; SANBI, 2016). It is also reported as invasive in Kenya, Malawi and Rwanda. Barua et al. (2013) report the species as an invasive which has affected ecosystems, causing the decline of native species in Assam, India. It can be particularly invasive along waterways and in riparian vegetation (Weeds of Australia, 2016).

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Asterales
  •                         Family: Asteraceae
  •                             Genus: Ageratum
  •                                 Species: Ageratum houstonianum

Notes on Taxonomy and Nomenclature

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Ageratum is a Linnaean name derived from the Greek a (not), and geras (old age), referring to the long-lasting nature of the flowers. The majority of species are native to the New World. A. houstonianum was first collected in 1731, at Veracruz, Mexico, and is now a popular garden plant. The cultivated varieties include tetraploids and hybrids (Johnson, 1971). The species epithet comes from the English botanist Dr William Houston who first collected A. houstonianum and sent seeds back to the UK.

Description

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The following description is from Johnson, 1971:

Annual, (2.5-)3-7(-9) dm tall; roots fibrous, adventitious at lower nodes if stem decumbent; stems simple or branched, especially above, erect or decumbent, reddish to green, glandular-villous to lanate above, hairs white to yellowish; leaves opposite, at times alternate above, ovate to deltoid, 2.4-8.6(-9.5) cm long, (1.7-)2.9-6.5(-8) cm wide, apex rounded or acute, base cordate to truncate, margin crenate or rarely dentate, more or less ciliate, upper surface dark green, pilose, hairs scattered or dense, especially over the veins, lower surface pale green, penninerved, densely pilose, especially over veins, to nearly glabrous; petioles 0.6-3.5 mm long, densely white pilose, especially on upper ones; inflorescences terminal, the 5-15 heads borne in tight or open cymose clusters on densely pilose-puberulent (at times glandular) bracteolate peduncles; involucres campanulate, bracts biseriate, narrowly lanceolate, (3.75-)4-5 mm high, outer ones 0.5-0.75(-0.95) mm wide, green or brownish, 2-ribbed, densely pilose to nearly glabrous, apex acuminate, glandular-ciliate, at times deep red, margin entire, herbaceous to scarious, glandular-ciliate especially above; corolla (2.15-) 2.5-3.5 mm long, funnelform, tube white, glandular to glabrous, throat blue, lilac, lavender, rarely white, the 5 lobes upright or spreading, acute, externally puberulous, pigmented as the throat; achenes 5-angled, black, (1.15-)1.5-1.75 mm long, scabrous on angles, carpopodium white, prominent; pappus of 5 free oblong, scarious scales (1.5-)2-3(-3.4) mm long, margin pectinate, apex setiferous, setae scabrous, at times the scales apically truncate and without setae, then scales 0.1-0.15 mm long. 

Plant Type

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Annual
Herbaceous
Seed propagated

Distribution

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A. houstonianum is native to Mexico and Central America (Johnson, 1971). It has spread mostly as a garden escape and it is naturalized in North America, the Caribbean, Africa, Asia, Europe and Oceania (see Distribution Table for details).

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.

Last updated: 14 Dec 2020
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes

Africa

CameroonPresentIntroduced
Congo, Democratic Republic of thePresentIntroduced
EgyptPresentIntroduced2004Reported as a new record for the flora of Egypt
EswatiniPresentIntroducedInvasive
EthiopiaPresentIntroduced
GuineaPresentNzérékoré
KenyaPresentIntroducedInvasive
MadagascarPresentIntroduced
MalawiPresentIntroducedInvasive
MozambiquePresentIntroducedInvasive
RwandaPresentIntroducedInvasive
Saint HelenaPresentIntroducedWeed of cultivation
South AfricaPresentIntroducedInvasiveAs a declared weed and an alien invader plant.
SudanPresentIntroduced
TanzaniaPresentIntroducedInvasiveArusha, Iringa, Tanga
ZambiaPresentIntroduced
ZimbabwePresentIntroducedInvasiveWidely grown as a border plant

Asia

ChinaPresentIntroducedInvasiveGrasslands, roadsides, slopes in valleys, 100-1500m. Also cultivated.
-AnhuiPresentIntroducedInvasive
-FujianPresentIntroducedInvasive
-GuangdongPresentIntroducedInvasive
-GuangxiPresentIntroducedInvasive
-GuizhouPresentIntroducedInvasive
-HainanPresentIntroducedInvasive
-HebeiPresentIntroducedInvasive
-HeilongjiangPresentIntroduced
-JiangsuPresentIntroducedInvasive
-ShandongPresentIntroducedInvasive
-SichuanPresentIntroducedInvasive
-YunnanPresentIntroducedInvasive
-ZhejiangPresentIntroducedInvasive
GeorgiaPresentIntroduced
Hong KongPresentIntroducedInvasive
IndiaPresentIntroduced1883Common weed of wastelands and fallow fields. Common, cultivated in gardens, moist and shaded situations and around settlements
-AssamPresentIntroducedInvasive
IndonesiaPresentIntroduced
-JavaPresentIntroduced1875NaturalizedReported as one of the naturalized American species collected by O. Kuntze in 1875
JapanPresentIntroducedOkinawa
MyanmarPresentIntroduced
PhilippinesPresentIntroducedMountains
TaiwanPresent, WidespreadIntroducedInvasiveCommon weed from low to middle elevations
ThailandPresentIntroduced
VietnamPresentIntroducedEscaped from cultivation

Europe

AustriaPresentIntroducedIn disturbed areas. Common Intentional introduction
BelgiumPresentIntroduced
CzechiaPresentIntroduced
DenmarkPresent, Few occurrencesIntroducedRare, in urban areas. Seeds sold from catalogues in late 1800's. Intentional and unintentional introduction
EstoniaPresentIntroduced
FrancePresentIntroducedEscaped from cultivation
GermanyPresent, Only in captivity/cultivationIntroducedSeeds and plants available at nurseries
HungaryPresentIntroduced
ItalyPresentIntroduced
NorwayPresentIntroduced2003In Norway the plant is unable to reproduce and it is mostly confined to garden centres and greenhouses
PortugalPresentIntroduced
-AzoresPresentIntroduced
-MadeiraPresentIntroduced"Established"
RussiaPresent, Only in captivity/cultivationIntroduced
SpainPresentIntroduced"Established"
SwedenPresent, Few occurrencesIntroduced1933Disturbed areas, rare. In Upland in 2002, no longer in Skåne
UkrainePresentIntroduced
United KingdomPresent, Few occurrencesIntroducedAn occasional garden escape

North America

BelizePresentNativeCayo
BermudaPresentIntroduced
CanadaPresent, Only in captivity/cultivationIntroduced
Costa RicaPresentGuanacaste, Heredia, Limón, Puntarenas
CubaPresentIntroducedInvasiveReported as of one of the species of most concern, as a transformer and an invasive in other countries
Dominican RepublicPresentIntroducedIn pastures, Loma del Puerto Pedro García, Puerto Plata Prov., 800 m on limestone. Roadside, La Cumbre, Altamira, 300 m
El SalvadorPresentNativeSan Salvador
GuatemalaPresentIntroducedAlta Verapaz, Baja Verapaz, Huehuetenango, Petén, San Marcos
HondurasPresentNativeComayagua, Copán, Cortés, Francisco Morazán, Yoro
JamaicaPresentIntroduced1888
MartiniquePresent
MexicoPresent, WidespreadNativeType locality at Veracruz. Also at Chiapas, Colima, Guerrero, Hidalgo, Jalisco, Morelos, Nayarit, Nuevo León, Oaxaca, Puebla, Querétaro, Quintana Roo, Tabasco, Yucatán
NicaraguaPresentNativeChinandega
PanamaPresentNativeBocas del Toro, Canal Area, Chiriquí, Panamá, Amambay
Puerto RicoPresent, Only in captivity/cultivationIntroduced
United StatesPresentPresent based on regional distribution.
-AlabamaPresentIntroduced
-ConnecticutPresentIntroduced
-FloridaPresentIntroduced
-GeorgiaPresentIntroduced
-HawaiiPresentIntroduced1937InvasiveBig Island, Kauai, Maui and Oahu
-MassachusettsPresent, Few occurrencesIntroduced1880“Waif” (sporadically), no habitat given.
-New YorkPresent, Only in captivity/cultivationIntroducedCold Spring Harbor
-North CarolinaPresentIntroduced
-PennsylvaniaPresent, Only in captivity/cultivationIntroducedOutdoor, cultivated and becoming weedy in cultivated grounds.
-South CarolinaPresentIntroduced
-TexasPresentIntroduced

Oceania

AustraliaPresentIntroducedInvasive
-QueenslandPresentIntroducedInvasive
FijiPresentIntroducedInvasiveViti Levu Island. Also cultivated
French PolynesiaPresentIntroducedInvasiveNuku Hiva Island, Raivavae. Also cultivated on Huahine and Tahiti Islands
New CaledoniaPresentIntroducedIle Grande Terre
New ZealandPresentIntroducedInvasiveInvasive in Raoul Island. Also cultivated
Papua New GuineaPresentIntroducedInvasive

South America

BoliviaPresentIntroducedLa Paz, Santa Cruz
ColombiaPresentIntroducedNueva Granada, Antioquia, Medellín, Chocó, Meta, Nariño, Valle del Cauca
EcuadorPresentIntroducedAzuay, Morona-Santiago, Tungurahua, La Libertad, La Paz
PeruPresentIntroducedInvasive
SurinamePresent, Only in captivity/cultivationIntroducedNaturalizedWidely cultivated, possibly naturalized
VenezuelaPresentIntroducedAragua, Monagas, Distrito Federal, Lara, Mérida, Táchira

History of Introduction and Spread

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A. houstonianum seeds were sent by Dr. William Houston to England from Veracruz, Mexico, where he first collected the species. The species naturalized and became a weed in hotbeds in England prior to 1768. It was cultivated in Europe by 1822, and reported in Salzburg (Austria) by 1860 (Johnson, 1971). It is cited as introduced to India in the early nineteenth century and reported as occupying the niches of upland herbaceous species, including Ageratum conyzoides (Sahu, 1982; Barua et al., 2013). Johnson (1971) reports the species as escaping from gardens and becoming established in central Florida, Cuba, Jamaica, and Hawaii. He also reports the species having escaped from cultivation in Africa, China, India, France, Java, Okinawa (Japan), the Philippines, and North Vietnam. First reports of the species in the USA and the Caribbean are from the late 1800's as an ornamental and escaping from cultivation (Sorrie, 2005; Missouri Botanical Garden, 2016). Although Palmer (2004) cites it as recently introduced in Hawaii, Fosberg (1943) collected the species at Honolulu in 1937, reporting it as escaped from cultivation.

Introductions

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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
Austria 1861 Ornamental purposes (pathway cause) Yes Johnson (1971) Cultivated at Salzburg
Cuba 1900 Yes Robinson (1913) In tobacco field
Hawaii 1937 Ornamental purposes (pathway cause) Yes Fosberg (1943)
Jamaica 1888 Yes Johnson (1921) In disturbed area
Norway 2003 Ornamental purposes (pathway cause) No NOBANIS (2016) Unintentional; not reproducing
Sweden 1933 Ornamental purposes (pathway cause) Yes NOBANIS (2016) Intentional introduction, in disturbed areas, rare.
UK 1768 Breeding and propagation (pathway cause) Yes Miller (1768) Brought to England by Dr. William Houston. Reported as a weed in hotbeds

Risk of Introduction

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The worldwide use of this species as a flower bed plant and the ease with which its small seeds are easily transported by wind and water makes A. houstonianum of high risk of introduction into open areas mainly in temperate zones. It is reported as a naturalized garden escape in many countries and a good colonizer (Johnson, 1921; Johnson, 1971; BioNet-EAFRINET, 2016; PIER, 2016; Missouri Botanical Garden, 2016). Seeds can spread unintentionally by being carried by animals, in clothes, vehicles, produce or soil (BioNet-EAFRINET, 2016). It is listed as a declared weed and alien invader plant by the government of South Africa (SANBI, 2016). 

Habitat

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A. houstonianum is reported as growing in pastures, disturbed sites, crops, roadsides, savannas, openings in forests, humid areas and riparian zones; from sea level to 1300 m elevation (BioNet-EAFRINET, 2016; PIER, 2016). In Australia it is also reported from gardens, pastures, wetlands and waterways (Weeds of Australia, 2016). It has been in cultivation since being brought to England a few years after its discovery (Johnson, 1971; Cornell Garden-Based Learning, 2016).  

Habitat List

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CategorySub-CategoryHabitatPresenceStatus
Terrestrial ManagedCultivated / agricultural land Present, no further details Harmful (pest or invasive)
Terrestrial ManagedCultivated / agricultural land Present, no further details Natural
Terrestrial ManagedCultivated / agricultural land Present, no further details Productive/non-natural
Terrestrial ManagedManaged forests, plantations and orchards Present, no further details Natural
Terrestrial ManagedManaged forests, plantations and orchards Present, no further details Productive/non-natural
Terrestrial ManagedDisturbed areas Present, no further details Natural
Terrestrial ManagedRail / roadsides Present, no further details Natural
Terrestrial ManagedUrban / peri-urban areas Present, no further details Natural
Terrestrial ManagedUrban / peri-urban areas Present, no further details Productive/non-natural
Terrestrial Natural / Semi-naturalNatural forests Present, no further details Natural
Terrestrial Natural / Semi-naturalNatural grasslands Present, no further details Natural
Terrestrial Natural / Semi-naturalRiverbanks Present, no further details Natural
Terrestrial Natural / Semi-naturalWetlands Present, no further details Natural

Hosts/Species Affected

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A. houstonianum is a common weed of sugarcane crops in Queensland (Weeds of Australia, 2016).

Host Plants and Other Plants Affected

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Plant nameFamilyContextReferences
Nicotiana tabacum (tobacco)SolanaceaeMain

    Biology and Ecology

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    Genetics

    Chromosome number is reported as n=10, 20 (Johnson, 1971).

    Reproductive Biology

    The species propagates by seeds and vegetatively by the prostrate portion of the stem (Barua et al., 2013). It can be reproduced from cuttings (Cornell Garden-Based Learning, 2016).

    Physiology and Phenology

    Flowering and fruiting occur all year long (Flora of China Editorial Committee, 2016). Optimal germination reported is between 6.0-7.0 pH; seeds will not germinate on more acidic soils (Shoemaker and Carlson, 1990).

    Environmental Requirements

    Wielgolaski (1966) report 8°C as the minimum temperature for the optimal growth of the species. Nursery websites report the plant does not tolerate frost, although some state that the plants might survive, but with reduced to no flowering. It is reported as unable to reproduce outside in Norway, most probably due to low temperatures (NOBANIS, 2016). A. houstonianum grows well in free draining soils, and in full sun to partial shade (Cornell Garden-Based Learning, 2016).  

    Climate

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    ClimateStatusDescriptionRemark
    Af - Tropical rainforest climate Preferred > 60mm precipitation per month
    Am - Tropical monsoon climate Preferred Tropical monsoon climate ( < 60mm precipitation driest month but > (100 - [total annual precipitation(mm}/25]))
    Aw - Tropical wet and dry savanna climate Preferred < 60mm precipitation driest month (in winter) and < (100 - [total annual precipitation{mm}/25])
    BS - Steppe climate Tolerated > 430mm and < 860mm annual precipitation
    BW - Desert climate Tolerated < 430mm annual precipitation
    Cf - Warm temperate climate, wet all year Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, wet all year
    Cs - Warm temperate climate with dry summer Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers
    Cw - Warm temperate climate with dry winter Preferred Warm temperate climate with dry winter (Warm average temp. > 10°C, Cold average temp. > 0°C, dry winters)
    Ds - Continental climate with dry summer Tolerated Continental climate with dry summer (Warm average temp. > 10°C, coldest month < 0°C, dry summers)

    Latitude/Altitude Ranges

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    Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
    62 44

    Air Temperature

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    Parameter Lower limit Upper limit
    Absolute minimum temperature (ºC) -5

    Rainfall

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    ParameterLower limitUpper limitDescription
    Mean annual rainfall4301500mm; lower/upper limits

    Soil Tolerances

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    Soil drainage

    • free

    Soil reaction

    • acid
    • alkaline
    • neutral

    Soil texture

    • heavy
    • light
    • medium

    Natural enemies

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    Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
    Frankliniella schultzei Herbivore Leaves/Stems not specific
    Microcephalothrips abdominalis Herbivore Leaves/Stems not specific
    Pythium mamillatum Pathogen Roots not specific
    Trialeurodes vaporariorum Herbivore Leaves/Stems not specific

    Notes on Natural Enemies

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    The oomycote Pythium mamillatum [Globisporangium mamillatum], which causes root rot, is reported to parasitize A. houstonianum (Middleton, 1943).

    The species is affected by Botrytis sp., spider mites, thrips, whiteflies, plant lice, fungus gnats, parasitic nematodes and powdery mildew. Excessive humidity can cause oedema, as reported by the Cornell Garden-Based Learning resources (2016) and plant nursery websites.

    Means of Movement and Dispersal

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    The plant reproduces by seed, and wind and water are the main means of dispersal for the small, light seeds of this and other species of Ageratum (Johnson, 1971; BioNet-EAFRINET, 2016). Seeds also attach easily to animals (BioNet-EAFRINET, 2016), and are reported as attaching to clothes, vehicles and to spread through contaminated agricultural produce. Plant spread by escaping from cultivation (Johnson, 1971; BioNet-EAFRINET, 2016).

    The species has been introduced as an ornamental outside its native range (Johnson, 1971; BioNet-EAFRINET, 2016).

    Pathway Causes

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    CauseNotesLong DistanceLocalReferences
    Breeding and propagationSold as a flower bed plant. Yes Yes
    Crop productionCan spread in contaminated agricultural produce Yes Yes BioNET-EAFRINET, 2016
    Cut flower tradeTaller varieties are used for cut flowers Yes Yes Cornell Garden-Based Learning, 2016
    DisturbanceReported to be a colonizer in disturbed sites Yes Johnson, 1921
    Escape from confinement or garden escapeCited as escaping from gardens and naturalizing in nearby areas. Yes PFAF, 2016
    Garden waste disposalEstablishes itself easily in open soil where cultivated. Yes Johnson, 1971
    HitchhikerCan attach to animals, vehicles, clothes. Yes Yes Johnson, 1971
    Horticulture Yes Yes
    Internet salesSeeds sold over the internet. Yes Yes
    Nursery tradePlants sold for gardens. Yes
    Ornamental purposesUsed as ornamental and for landscaping, sold at nurseries. Yes Thoden et al., 2009
    Seed trade Yes Yes

    Pathway Vectors

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    VectorNotesLong DistanceLocalReferences
    Clothing, footwear and possessionsSeeds attach easily to clothes. Yes Yes Johnson, 1971
    Land vehicles Yes Cornell Garden-Based Learning, 2016
    Soil, sand and gravelEstablishes easily in open soil in cultivated land. Yes Johnson, 1971
    Water Yes Johnson, 1971
    Wind Yes Johnson, 1971

    Impact Summary

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    CategoryImpact
    Economic/livelihood Positive and negative
    Environment (generally) Positive and negative
    Human health Positive

    Economic Impact

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    A. houstonianum is one of the host plants of the Tobacco Streak Virus (TSV) and the Ageratum Latent Virus (ALV), transmitted by pollen of the species or by the thrips Frankliniella schultzei and Microcephalothrips abdominalis, causing crop losses around the world (Sharman et al., 2015). It is also a host plant for Ageratum Enation Virus (AEV) and the Radish Leaf Curl Virus (RaLV), transmitted by whiteflies (Bemisia sp.) and causing yellow vein disease in several crops and ornamentals (Srivastava et al., 2015).

    Zebu cattle have been reported to suffer from intoxication by grazing on A. houstonianum; symptoms include acute internal haemorrhages and a sub-acute photodynamic dermatitis. The plant toxicosis has been so severe to even cause the death of cattle (Noa et al., 2004).

    A. houstonianum is a very common weed of sugar cane crops in Queensland, and is also a weed of other cropping systems in the coastal areas of southern and central Queensland (Weeds of Australia, 2016).

    Environmental Impact

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    Impact on Habitats

    Barua et al. (2013) report that the species can produce between 30,000 to 40,000 seedlings and 8-20 mature plants per m2 in Assam, India. The seeds germinate almost simultaneously, creating a dense ground cover and colonies, which have a smothering effect over other species. Weeds of Australia (2016) reports that A. houstonianum is one of only a few introduced species which can colonise undisturbed or relatively intact open forest vegetation. It also forms dense patches on coastal dunes and headlands in New South Wales.

    Impact on Biodiversity

    The dense ground cover and colonies of A. houstonianum can exclude the growth of other species, causing changes in the species composition and reducing species richness (Barua et al., 2013). BioNet-EAFRINET (2016) also suggests that A. houstonianum has allelopathic effects on the germination and growth of other species.

    Risk and Impact Factors

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    Invasiveness
    • Proved invasive outside its native range
    • Abundant in its native range
    • Pioneering in disturbed areas
    • Tolerant of shade
    • Fast growing
    • Gregarious
    • Has propagules that can remain viable for more than one year
    • Reproduces asexually
    Impact outcomes
    • Ecosystem change/ habitat alteration
    • Monoculture formation
    • Negatively impacts agriculture
    • Negatively impacts animal health
    • Negatively impacts livelihoods
    • Reduced native biodiversity
    • Threat to/ loss of native species
    Impact mechanisms
    • Competition - monopolizing resources
    • Competition - smothering
    • Pest and disease transmission
    • Hybridization
    • Rapid growth
    Likelihood of entry/control
    • Highly likely to be transported internationally accidentally
    • Highly likely to be transported internationally deliberately
    • Difficult to identify/detect in the field

    Uses

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    Economic Value

    A. houstonianum has been cultivated as a flower garden plant since the early 1800's, and several varieties have been developed (Johnson, 1971). It is valued for providing the uncommon blue colour to gardens and landscapes (Cornell Garden-Based Learning, 2016). Its use as a garden plant is encouraged to attract butterflies and deter deer (Cornell Garden-Based Learning, 2016).

    The species contains secondary metabolites that suppress the development of the juveniles of Meloidogyne hapla, a nematode that causes significant damage and crop losses in temperate zones (Thoden et al., 2009). Intercropping of A. houstonianum and other aromatic species in apple orchards had reduced the abundance of the pests Aphis citricola [A. spiraecola] and Lepidoptera, increasing the density of beneficial insects of Trichogrammatidae, Ichneumonidae and Braconidae (Song et al., 2013; 2014). It has fungicidal activity against Phytophthora infestans, reducing the disease severity in tomato crops (Goufo et al., 2010).

    Chemicals that induce the premature metamorphosis and sterilization of some insect species were isolated from A. houstonianum and are marketed as Precocene I and Precocene II (Jacobson, 1982).

    Social Benefit

    The species is used in traditional medicine to treat skin infections and sore throats (Johnson, 1971; Andrade-Cetto, 2009). Leaves are applied to wounds to stop bleeding (Bodner and Gereau, 1988). Tennyson et al. (2012a) report that the species has a high antioxidant activity with potential cosmetic and medicinal uses.

    Essential oils have antimicrobial and antifungicidal properties (Pandey et al, 1984; Menut et al., 1993; Njateng et al., 2010). The plant also shows acaricidal properties against the tick Rhipicephalus lunulatus (Pamo et al., 2005). Leaf extracts have adulticidal activities, oviposition deterrent properties and repellent properties against the mosquitoes Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus; all vectors of diseases that affect humans and domestic animals (Ravindran et al., 2012; Tennyson et al., 2012b, 2012c).

    Environmental Services

    Koi (2008) list A. houstonianum as one of the nectar sources for the butterfly Eumaeus atala, which is listed as rare and vulnerable in Florida, USA. 

    Uses List

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    Environmental

    • Soil conservation

    General

    • Ornamental

    Materials

    • Essential oils
    • Oils
    • Pesticide

    Medicinal, pharmaceutical

    • Source of medicine/pharmaceutical
    • Veterinary

    Ornamental

    • Cut flower
    • Potted plant
    • Seed trade

    Similarities to Other Species/Conditions

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    A. houstonianum is closely related to Ageratum conyzoides, as is evidenced from the similarities of habit, achenes, pappus, and corollas (Johnson,1971). Both species can be difficult to differentiate in the field and in herbarium collections (Sharma, 1987). Weeds of Australia (2016) lists the following distinguishing characters:

    • A. houstonianum has numerous sticky hairs on the bracts surrounding its flower-heads (i.e. the involucral bracts are glandular pubescent). Each of the tiny flowers (i.e. florets) which make up the flower-heads have two short and narrow projections (i.e. style branches) that are about 1-2 mm long. The bases of the flower-heads are relatively small (3-6 mm across)
    • A. conyzoides has only a few hairs on the bracts surrounding its flower-heads (i.e. the involucral bracts are glabrous or sparsely pubescent). Each of the tiny flowers (i.e. florets) which make up the flower-heads have two long and narrow projections (i.e. style branches) that are about 5 mm long. The bases of the flower-heads are relatively large (5-8 mm across).

    The genus Ageratum is also closely related to Alomia. They can be differentiated by the lack of a pappus in Alomia (Johnson, 1971). In Australia, the similar native plant Vernonia (Cyanthillium cinereum) is distinguished at the seed stage by seeds being topped with about 20 relatively large bristles (Weeds of Australia, 2016).

    Prevention and Control

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    Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

    A. houstonianum is listed by the government of South Africa as a "Category 1 declared weed"; which is the strictest category, and prohibits its importation, propagation and establishment on any land or inland water surface (SANBI, 2016).

    Chemical Control

    The application of the herbicides dicamba and 2,4-D cause visible foliar injury and reduce flowering on A. houstonianum (Hatterman-Valenti and Mayland, 2005). The herbicide diphenamid, applied to the soil reduces the growth of the species (Adamson and Crossley, 1968). Chemical control is not advised when A. houstonianum is growing in crop fields; the use of a 20% salt solution is suggested instead (BioNet-EAFRINET, 2016).

    Gaps in Knowledge/Research Needs

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    More information is needed about the invasiveness status of the species in some of the countries where it is reported as a weed and invasive; especially on its effects on habitats and other species. 

    References

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    Acevedo-Rodríguez P, Strong MT, 2012. Catalogue of the Seed Plants of the West Indies. Smithsonian Contributions to Botany, 98:1192 pp. Washington DC, USA: Smithsonian Institution. http://botany.si.edu/Antilles/WestIndies/catalog.htm

    Adamson RM, Crossley JH, 1968. Annual flower plant growth as affected by soil-incorporated herbicides. Weed Science, 16(1):60-2

    Andrade-Cetto A, 2009. Ethnobotanical study of the medicinal plants from Tlanchinol, Hidalgo, México. Journal of Ethnopharmacology, 122(1):163-171. http://www.sciencedirect.com/science/journal/03788741

    Ashish Srivastava, Susheel Kumar, Raj SK, 2015. Molecular characterization of a begomovirus and betasatellite causing yellow vein net disease of Ageratum houstonianum. Plant Disease, 99(5):627-631. http://apsjournals.apsnet.org/loi/pdis

    Backer CA, 1938. A revision of Kuntze's types of new Javan species. Brittonia, 3(1):75-90

    Barua IC, Deka J, Devi M, 2013. Invasive weeds and vegetation dynamics in Assam. In: The role of weed science in supporting food security by 2020. Proceedings of the 24th Asian-Pacific Weed Science Society Conference, Bandung, Indonesia, October 22-25, 2013 [ed. by Bakar, B. H.\Kurniadie, D.\Tjitrosoedirdjo, S.]. Bandung, Indonesia: Weed Science Society of Indonesia, 166-170

    BioNET-EAFRINET, 2016. Invasive plants key and fact sheets. http://keys.lucidcentral.org/keys/v3/eafrinet/index.htm

    Bodner CC, Gereau RE, 1988. A contribution of Bontoc ethnobotany. Economic Botany, 42(3):307-369

    Corlett RT, 1992. The naturalized flora of Hong Kong: a comparison with Singapore. Journal of Biogeography, 19(4):421-430

    Cornell Garden-Based Learning, 2016. Learn, Garden and Reflect with Cornell Garden-Based Learning. http://gardening.cce.cornell.edu/

    Cronk QCB, 1989. The past and present vegetation of St Helena. Journal of Biogeography, 16:47-64

    DAISIE, 2016. Delivering Alien Invasive Species Inventories for Europe. European Invasive Alien Species Gateway. www.europe-aliens.org/default.do

    Flora of China Editorial Committee, 2016. Flora of China. St. Louis, Missouri and Cambridge, Massachusetts, USA: Missouri Botanical Garden and Harvard University Herbaria. http://www.efloras.org/flora_page.aspx?flora_id=2

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    Grier CR, Grier NM, 1929. A list of plants growing under cultivation in the vicinity of Cold Spring Harbor, N.Y. The American Midland Naturalist, 11(8):389-434

    Hatterman-Valenti H, Mayland P, 2005. Annual flower injury from sublethal rates of dicamba, 2,4-D, and premixed 2,4-D+mecoprop+dicamba. HortScience, 40(3):680-684

    Jacobson M, 1982. Plants, insects, and man: their interrelationships. Economic Botany, 36(3):346-354

    Johnson DS, 1921. Invasion of virgin soil in the Tropics. Botanical Gazette, 72(5):305-312

    Johnson MF, 1971. A monograph of the genus Ageratum L. (Compositae-Eupatorieae). Annals Missouri Botanical Garden, 58:6-88

    Koi S, 2008. Nectar sources for Eumaeus atala (Lepidoptera: Lycaenidae: Theclinae). Florida Entomologist, 91(1):118-120. http://www.fcla.edu/FlaEnt/

    Komarov VL, 1968. Flora of the USSR, Volume 25, Compositae. Leningrad, USSR: Academiae Scientiarum

    Maroyi A, 2006. Preliminary checklist of introduced and naturalized plants in Zimbabwe. Kirkia, 18(2):177-247

    Menut C, Lamaty G, Zollo PHA, Kuiate JR, Bessière JM, 1993. Aromatic plants of tropical central Africa. Part X. Chemical composition of the essential oils of Ageratum houstonianum Mill. and Ageratum conyzoides L. from Cameroon. Flavour and Fragrance Journal, 8(1):1-4

    Middleton JT, 1943. The taxonomy, host range and geographic distribution of the genus Pythium. Memoirs of the Torrey Botanical Club, 20:1-171

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    Missouri Botanical Garden, 2016. Tropicos database. St. Louis, Missouri, USA: Missouri Botanical Garden. http://www.tropicos.org/

    Moldenke HN, 1946. A contribution to our knowledge of the wild and cultivated flora of Pennsylvania. Amer. Midl. Nat. 1946. 35 (2). (289-399). [New York Botanical Garden, New York, N.Y.]

    Nakahara S, Hilburn DJ, 1989. Annotated checklist of the whiteflies (Homoptera: Aleyrodidae) of Bermuda. Journal of the New York Entomological Society, 97(3):261-264

    Nash DL, Williams LO, 1976. Flora of Guatemala. Vernonieae, Astereae, Inuleae, Heliantheae, Anthemideae, Cynareae, Mutisieae, Cichorieae, Eupatorieae, Helenieae, Senecioneae. Fieldiana: Botany, 24(12):613 pp

    Njateng GSS, Kuiate JR, Gatsing D, Tamokou JD, Mouokeu RS, Kuete V, 2010. Antidermatophytic activity and dermal toxicity of essential oil from the leaves of Ageratum houstonianum (Asteraceae). Journal of Biological Sciences, 10(5):448-454. http://scialert.net/qredirect.php?doi=jbs.2010.448.454&linkid=pdf

    Noa M, Sánchez LM, Durand R, 2004. Ageratum houstonianum toxicosis in Zebu cattle. Veterinary and Human Toxicology, 46(4):193-194

    NOBANIS, 2016. North European and Baltic Network on Invasive Alien Species. http://www.NOBANIS.org

    Oliver RW, 1973. Annual flowers for Canadian gardens. Ottawa, Canada: Plant Research Institute

    Oviedo Prieto R, Herrera Oliver P, Caluff MG, et al. , 2012. National list of invasive and potentially invasive plants in the Republic of Cuba - 2011. (Lista nacional de especies de plantas invasoras y potencialmente invasoras en la República de Cuba - 2011). Bissea: Boletín sobre Conservación de Plantas del Jardín Botánico Nacional de Cuba, 6(Special Issue 1):22-96

    Palmer CT, 2004. The inclusion of recently introduced plants in the Hawaiian ethnopharmacopoeia. Economic Botany, 58(1(Supplement 1)):S280-S293. http://rd.springer.com/article/10.1663/0013-0001%282004%2958%5BS280%3ATIORIP%5D2.0.CO%3B2

    Pamo ET, Tendonkeng F, Kana JR, Payne VK, Boukila B, Lemoufouet J, Miegoue E, Nanda AS, 2005. A study of the acaricidal properties of an essential oil extracted from the leaves of Ageratum houstonianum. Veterinary Parasitology, 128(3/4):319-323. http://www.sciencedirect.com/science/journal/03044017

    Pandey DK, Chandra H, Tripathi NN, Dixit SN, 1984. Mycotoxicity in leaves of some higher plants with special reference to that of Ageratum houstonianum Mill. Mykosen, 26:565-573

    PFAF, 2016. Plants for a Future. http://www.pfaf.org/user/default.aspx

    PIER, 2016. Pacific Island Ecosystems at Risk. Honolulu, USA: HEAR, University of Hawaii. http://www.hear.org/pier/index.html

    Ravindran J, Samuel T, Alex E, William J, 2012. Adulticidal activity of Ageratum houstonianum Mill. (Asteraceae) leaf extracts against three vector mosquito species (Diptera: Culicidae). Asian Pacific Journal of Tropical Disease, 2(3):177-179. http://www.apjtcm.com

    Robinson BL, 1913. Revisions of Alomia, Ageratum, and Oxylobus. Contributions from the Gray Herbarium of Harvard University, 42:438-491

    Sahu TR, 1982. Taxonomic studies of the genus Ageratum L. in India. Feddes Repertorium, 93(1-2):61-65

    SANBI, 2016. South African National Biodiversity Institute (SANBI). Declared weeds & alien invader plants. http://www.plantzafrica.com/miscell/aliens1.htm

    Shaltout KH, 2004. An updated flora of Egypt. Diversity and Distributions, 10:75-79

    Sharma VS, 1987. Comments on the identity of Ageratum conyzoides L., and A. houstonianum Mill. -two naturalized weeds in India. Feddes Repertorium, 98(11-12):557-560

    Sharman M, Thomas JE, Persley DM, 2015. Natural host range, thrips and seed transmission of distinct Tobacco streak virus strains in Queensland, Australia. Annals of Applied Biology, 167(2):197-207. http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1744-7348

    Shoemaker CA, Carlson WH, 1990. pH affects seed germination of eight bedding plant species. HortScience, 25(7):762-764

    Song B, Jiao H, Tang G, Yao Y, 2014. Combining repellent and attractive aromatic plants to enhance biological control of three tortricid species (Lepidoptera: Tortricidae) in an apple orchard. The Florida Entomologist, 79(4):1679-1689

    Song BeiZhou, Tang GuangBo, Sang XuSheng, Zhang Jie, Yao YunCong, Wiggins N, 2013. Intercropping with aromatic plants hindered the occurrence of Aphis citricola in an apple orchard system by shifting predator-prey abundances. Biocontrol Science and Technology, 23(4):381-395. http://www.tandfonline.com/doi/full/10.1080/09583157.2013.763904

    Sorrie BA, 2005. Alien vascular plants in Massachusetts. Rhodora, 107(931):284-329

    Tennyson S, Balaraju K, Park KyungSeok, Ravindran KJ, Eapen A, William SJ, 2012. In vitro antioxidant activity of Ageratum houstonianum Mill. (Asteraceae). Asian Pacific Journal of Tropical Disease, No.Supplement 2:S712-S714. http://www.apjtcm.com/zz/2012s2/32.pdf

    Tennyson S, Ravindran J, Eapen A, William J, 2012. Repellent activity of Ageratum houstonianum Mill. (Asteraceae) leaf extracts against Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae). Asian Pacific Journal of Tropical Disease, 2(6):478-480. http://www.apjtcm.com/zz/20126/12.pdf

    Tennyson S, Ravindran KJ, Eapen A, William SJ, 2012. Effect of Ageratum houstonianum Mill. (Asteraceae) leaf extracts on the oviposition activity of Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae). Parasitology Research, 111(6):2295-2299. http://rd.springer.com/article/10.1007/s00436-012-3083-7/fulltext.html

    The Plant List, 2013. The Plant List: a working list of all plant species. Version 1.1. London, UK: Royal Botanic Gardens, Kew. http://www.theplantlist.org

    Thoden TC, Hallmann J, Boppré M, 2009. Effects of plants containing pyrrolizidine alkaloids on the northern root-knot nematode Meloidogyne hapla. European Journal of Plant Pathology, 123(1):27-36. http://springerlink.metapress.com/link.asp?id=100265

    Torres AM, Liogier AH, 1970. Chromosome numbers of Dominican Compositae. Brittonia, 22(3):240-245

    USDA-ARS, 2016. Germplasm Resources Information Network (GRIN). National Plant Germplasm System. Online Database. Beltsville, Maryland, USA: National Germplasm Resources Laboratory. https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysearch.aspx

    Weeds of Australia, 2016. Weeds of Australia, Biosecurity Queensland Edition. http://keyserver.lucidcentral.org/weeds/data/03030800-0b07-490a-8d04-0605030c0f01/media/Html/search.html?zoom_query=

    Wester L, Juvik JO, 1983. Roadside Plant-Communities on Mauna-Loa, Hawaii. Journal of Biogeography, 10(4):307-316

    Wielgolaski FE, 1966. The influence of air temperature on plant growth and development during the period of maximal stem elongation. Oikos, 17(2):121-141

    Witt, A., Luke, Q., 2017. Guide to the naturalized and invasive plants of Eastern Africa, [ed. by Witt, A., Luke, Q.]. Wallingford, UK: CABI.vi + 601 pp. http://www.cabi.org/cabebooks/ebook/20173158959 doi:10.1079/9781786392145.0000

    Distribution References

    Acevedo-Rodríguez P, Strong M T, 2012. Catalogue of the Seed Plants of the West Indies. Washington, DC, USA: Smithsonian Institution. 1192 pp. http://botany.si.edu/Antilles/WestIndies/catalog.htm

    Backer CA, 1938. A revision of Kuntze's types of new Javan species. In: Brittonia, 3 (1) 75-90.

    Barua I C, Deka J, Devi M, 2013. Invasive weeds and vegetation dynamics in Assam. In: The role of weed science in supporting food security by 2020. Proceedings of the 24th Asian-Pacific Weed Science Society Conference, Bandung, Indonesia, October 22-25, 2013 [The role of weed science in supporting food security by 2020. Proceedings of the 24th Asian-Pacific Weed Science Society Conference, Bandung, Indonesia, October 22-25, 2013.], [ed. by Bakar B H, Kurniadie D, Tjitrosoedirdjo S]. Bandung, Indonesia: Weed Science Society of Indonesia. 166-170.

    BioNET-EAFRINET, 2016. Invasive plants key and fact sheets., http://keys.lucidcentral.org/keys/v3/eafrinet/index.htm

    CABI, Undated. CABI Compendium: Status inferred from regional distribution. Wallingford, UK: CABI

    Corlett RT, 1992. The naturalized flora of Hong Kong: a comparison with Singapore. In: Journal of Biogeography, 19 (4) 421-430.

    Cronk Q C B, 1989. The past and present vegetation of St. Helena. Journal of Biogeography. 16 (1), 47-64. DOI:10.2307/2845310

    DAISIE, 2016. Delivering Alien Invasive Species Inventories for Europe. http://www.europe-aliens.org/

    Funk V, Hollowell T, Berry P, Kelloff C, Alexander S N, 2007. Contributions from the United States National Herbarium, Washington, USA: Department of Systematic Biology - Botany, National Museum of Natural History, Smithsonian Institution. 55, 584 pp.

    Grier CR, Grier NM, 1929. A list of plants growing under cultivation in the vicinity of Cold Spring Harbor, N.Y. In: The American Midland Naturalist, 11 (8) 389-434.

    Johnson DS, 1921. Invasion of virgin soil in the Tropics. In: Botanical Gazette, 72 (5) 305-312.

    JOHNSON M F, 1971. A monograph of the genus Ageratum L. Compositae-Eupatorieae. Annals of the Missouri Botanical Garden. 58 (1), 6-88. DOI:10.2307/2394925

    Komarov VL, 1968. Flora of the USSR, Volume 25, Compositae., 25 Leningrad, USSR: Academiae Scientiarum.

    Maroyi A, 2006. Preliminary checklist of introduced and naturalized plants in Zimbabwe. In: Kirkia, 18 (2) 177-247.

    Miller P, 1768. Gardeners dictionary.,

    Missouri Botanical Garden, 2016. Tropicos database., St. Louis, Missouri, USA: Missouri Botanical Garden. http://www.tropicos.org/

    Moldenke H N, 1946. A contribution to our knowledge of the wild and cultivated flora of Pennsylvania. American Midland Naturalist. 35 (2), 289-399. DOI:10.2307/2421669

    Nakahara S, Hilburn D J, 1989. Annotated checklist of the whiteflies (Homoptera: Aleyrodidae) of Bermuda. Journal of the New York Entomological Society. 97 (3), 261-264.

    NOBANIS, 2016. North European and Baltic Network on Invasive Alien Species., http://www.NOBANIS.org

    Oliver RW, 1973. Annual flowers for Canadian gardens., Ottawa, Canada: Plant Research Institute.

    Oviedo Prieto R, Herrera Oliver P, Caluff M G, et al, 2012. National list of invasive and potentially invasive plants in the Republic of Cuba - 2011. (Lista nacional de especies de plantas invasoras y potencialmente invasoras en la República de Cuba - 2011). Bissea: Boletín sobre Conservación de Plantas del Jardín Botánico Nacional de Cuba. 6 (Special Issue No. 1), 22-96.

    PFAF, 2016. Plants for a Future., http://www.pfaf.org/user/default.aspx

    PIER, 2016. Pacific Island Ecosystems at Risk., Honolulu, USA: HEAR, University of Hawaii. http://www.hear.org/pier/index.html

    Robinson BL, 1913. Revisions of Alomia, Ageratum, and Oxylobus. In: Contributions from the Gray Herbarium of Harvard University, 42 438-491.

    SANBI, 2016. South African National Biodiversity Institute (SANBI). Declared weeds & alien invader plants., http://www.plantzafrica.com/miscell/aliens1.htm

    Shaltout KH, 2004. An updated flora of Egypt. In: Diversity and Distributions, 10 75-79.

    Sharma VS, 1987. Comments on the identity of Ageratum conyzoides L., and A. houstonianum Mill. -two naturalized weeds in India. In: Feddes Repertorium, 98 (11-12) 557-560.

    Sorrie B A, 2005. Alien vascular plants in Massachusetts. Rhodora. 107 (931), 284-329. DOI:10.3119/0035-4902(2005)107[0284:AVPIM]2.0.CO;2

    Thoden T C, Hallmann J, Boppré M, 2009. Effects of plants containing pyrrolizidine alkaloids on the northern root-knot nematode Meloidogyne hapla. European Journal of Plant Pathology. 123 (1), 27-36. http://springerlink.metapress.com/link.asp?id=100265 DOI:10.1007/s10658-008-9335-9

    Torres AM, Liogier AH, 1970. Chromosome numbers of Dominican Compositae. In: Brittonia, 22 (3) 240-245.

    USDA-ARS, 2016. Germplasm Resources Information Network (GRIN). Online Database. Beltsville, Maryland, USA: National Germplasm Resources Laboratory. https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomysimple.aspx

    Wester L, Juvik J O, 1983. Roadside Plant-Communities on Mauna-Loa, Hawaii. Journal of Biogeography. 10 (4), 307-316.

    Witt A, Luke Q, 2017. Guide to the naturalized and invasive plants of Eastern Africa. [ed. by Witt A, Luke Q]. Wallingford, UK: CABI. vi + 601 pp. http://www.cabi.org/cabebooks/ebook/20173158959 DOI:10.1079/9781786392145.0000

    Contributors

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    26/05/2016 Original text by:

    Jeanine Vélez-Gavilán, University of Puerto Rico at Mayagüez

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