Jepson eFlora: Taxon page
Vascular Plants of California
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Parthenium hysterophorus

Higher Taxonomy
Family: Asteraceae (Compositae)View DescriptionDichotomous Key
Habit: Annual to tree. Leaf: basal and/or cauline, alternate, opposite, rarely whorled, simple to 2+ × compound. Inflorescence: 1° inflorescence a head, resembling a flower, of several types (see below), 1--many in generally +- cyme-like cluster; each head generally with +- calyx-like involucre of 1--many series of phyllaries (involucre bracts); receptacle of head flat to conic or columnar, paleate (bearing paleae = receptacle bracts) or epaleate; flowers 1--many per head. Flower: bisexual, unisexual, or sterile, +- small, of several types (see below); calyx 0 or modified into +- persistent pappus of bristles, scales, and/or awns; corolla radial or bilateral (0), lobes generally (0)3--5; stamens 4--5, filaments generally free, generally fused to corolla at tube/throat junction, anthers generally fused into cylinder around style, anther base generally rounded or cordate (deeply sagittate or with tail-like appendages), tip (= flattened appendage) generally projecting beyond pollen sac; pistil 1, 2-carpeled, ovary inferior, 1-chambered, 1-seeded, placenta basal, style 1, tip generally +- 2-branched (except in some staminate disk flowers), branch tips truncate or generally bearing +- brush-like appendages; stigmas 2, generally on adaxial faces of style branches. Fruit: achene (also called a cypsela) (drupe in Chrysanthemoides), cylindric to ovoid, sometimes compressed, generally deciduous with pappus attached.
Genera In Family: +- 1500 genera, 23000 species: worldwide, many habitats. Note: Flower and head types differ in form and sexual condition. A disk flower has a generally radial corolla, with a cylindric tube, expanded throat, and generally 5 lobes. Disk flowers are generally bisexual and fertile but occasionally staminate with reduced ovaries. Discoid heads comprise only disk flowers. A radiant head is a variant of a discoid head, with peripheral disk flower corollas expanded, often bilateral. A ray flower corolla is bilateral, generally with a slender tube and flattened petal-like ray (single lip composed of generally 3 lobes). Ray flowers are generally pistillate or sterile (occasionally lacking styles). Radiate heads have peripheral ray flowers and central disk flowers. Disciform heads superficially resemble discoid heads, with pistillate or sterile flowers that lack rays, together with or separate from disk flowers. A ligulate flower is bisexual, with a bilateral, generally ephemeral corolla and 5-lobed ligule. Liguliflorous heads comprise only ligulate flowers. See glossary p. 31 for illustrations of family characteristics. Echinops sphaerocephalus L., Gaillardia aristata Pursh, Gaillardia pulchella Foug., Hymenothrix loomisii S.F. Blake, Tagetes erecta L., Thelesperma megapotamicum (Spreng.) Kuntze are waifs. Melampodium perfoliatum Kunth, historic urban waif. Ageratum conyzoides L., Guizotia abyssinica (L. f.) Cass., Santolina chamaecyparisus L., orth. var. are rare or uncommon escapes from cultivation. Dyssodia papposa, Ismelia carinata (Schousb.) Sch. Bip. [Chrysanthemum carinatum Schousb.], Mantisalca salmantica (L.) Briq. & Cavill. are historical or extirpated waifs in California. Inula helenium L. not documented in California. Taxa of Aster in TJM (1993) treated here in Almutaster, Doellingeria, Eurybia, Ionactis, Oreostemma, Sericocarpus, Symphyotrichum; Chamomilla in Matricaria; Bahia in Hymenothrix; Cnicus in Centaurea; Conyza in Erigeron and Laennecia; Dugaldia in Hymenoxys; Erechtites in Senecio; Hymenoclea in Ambrosia; Lembertia in Monolopia; Osteospermum ecklonis in Dimorphotheca; Picris echioides in Helminthotheca; Prionopsis in Grindelia; Raillardiopsis in Anisocarpus and Carlquistia; Schkuhria multiflora in Picradeniopsis; Trimorpha in Erigeron; Venidium in Arctotis; Viguiera in Aldama and Bahiopsis; Whitneya in Arnica. Amauriopsis in TJM2 (2012) treated here in Hymenothrix; Arida in Leucosyris; Bahia in Picradeniopsis; Eucephalus in Doellingeria.
Unabridged Note: Largest family of vascular plants in California and of eudicots globally.
eFlora Treatment Author: David J. Keil, except as noted
Scientific Editor: David J. Keil, Bruce G. Baldwin.
Genus: PartheniumView Description 

Common Name: FEVERFEW
Habit: Annual herb [perennial herb to shrub]. Stem: generally erect, distally branched. Leaf: basal and cauline, alternate, [simple to] 2--3-pinnately divided, +- reduced distally on stem, puberulent to +- hairy, often gland-dotted. Inflorescence: heads radiate [disciform], many, small, in +- flat-topped to panicle-like clusters; involucre bowl-shaped to hemispheric; phyllaries in 2 series, outer phyllaries each subtending inner, +- ovate, obtuse, margins membranous, inner phyllaries subtending ray flowers; receptacle flat to conic, paleate, paleae each enfolding a disk flower. Ray Flower: 5(8); corolla persistent on fruit, tube short, glandular, ray [0] short, +- round, white. Disk Flower: 12--many, staminate, outer deciduous in pairs with ray fruits, inner deciduous as a group; corolla short, white; pollen abundant, wind-dispersed; ovary vestigial, style club-like, unbranched. Fruit: falling together with subtending phyllary, 2 adjacent disk flowers, and subtending paleae; body oblanceoloid to obovoid, +- compressed or not; pappus 0 or of 2--3 scales.
Etymology: (ancient name for a plant, derived from Greek word for virgin)
eFlora Treatment Author: David J. Keil
Reference: Kaur et al. 2014 Effects and Management of Parthenium hysterophorus: A Weed of Global Significance (International Scholarly Research Notices. Volume 2014 Article ID 368647, 12 pages.
Parthenium hysterophorus L.
Habit: Plant 30--200 cm. Stem: much branched, ribbed, minutely strigose, proximally sparsely spreading-hairy. Leaf: proximal petioled, blade minutely scabrous or strigose, gland-dotted; distal +- sessile; distalmost reduced, less divided, with a linear terminal segment. Inflorescence: heads in panicle-like clusters; peduncles 5--8 mm, elongating in fruit; involucre 4--5 mm diameter; phyllaries in 2 series of 5(7) each, outer +- 2.5 mm, green with colorless margins, 3-veined, abaxially appressed-puberulent, inner 5 fan-shaped, cupped around ray flowers, scarious-margined, abaxially +- glabrous, ciliate; receptacle flat, paleae wedge-shaped, cupped around disk flowers, distally densely short-hairy. Ray Flower: 5(7); tube 0.4 ? 0.6 mm, ray 0.5--1 mm, <= 1.5 mm wide, distally notched or entire. Disk Flower: 12--30; corolla +- 1.5 mm, 4-lobed; anthers 4, 0.8 mm; pappus 0. Fruit: ray fruit 1.5--2.5+ mm, obovoid, 3-angled, black, adaxially papillate, pappus of 2 ovate white scales 0.5--1 mm, 1 on each side of ray on abaxial side of fruit, sometimes an awl-like third scale present on adaxial side of fruit; disk fruit 0. Chromosomes: 2n=34.
Ecology: Disturbed ground, dry creek channels; Elevation: < 50 m. Bioregional Distribution: SCo; Distribution Outside California: native from Mexico to northern South America; widely introduced in warm-temperate to tropical areas +- worldwide. Flowering Time: Mar--Nov. Note: An emergent invasive weed of great concern; causes crop losses, degrades rangeland, and can cause severe contact dermatitis in both humans and livestock.
Jepson eFlora Author: David J. Keil
Reference: Kaur et al. 2014 Effects and Management of Parthenium hysterophorus: A Weed of Global Significance (International Scholarly Research Notices. Volume 2014 Article ID 368647, 12 pages.
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Citation for this treatment: David J. Keil 2019, Parthenium hysterophorus, in Jepson Flora Project (eds.) Jepson eFlora, Revision 7,, accessed on February 28, 2024.

Citation for the whole project: Jepson Flora Project (eds.) 2024, Jepson eFlora,, accessed on February 28, 2024.

No expert verified images found for Parthenium hysterophorus.

Geographic subdivisions for Parthenium hysterophorus:
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map of distribution 1
(Note: any qualifiers in the taxon distribution description, such as 'northern', 'southern', 'adjacent' etc., are not reflected in the map above, and in some cases indication of a taxon in a subdivision is based on a single collection or author-verified occurence).


Data provided by the participants of the  Consortium of California Herbaria.
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Blue markers indicate specimens that map to one of the expected Jepson geographic subdivisions (see left map). Purple markers indicate specimens collected from a garden, greenhouse, or other non-wild location.
Yellow markers indicate records that may provide evidence for eFlora range revision or may have georeferencing or identification issues.

CCH collections by month

Duplicates counted once; synonyms included.
Species do not include records of infraspecific taxa, if there are more than 1 infraspecific taxon in CA.
Blue line denotes eFlora flowering time (fruiting time in some monocot genera).