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English
: Santa Maria, bitterweed, carrot grass, false ragweed, feverfew, parthenium weed, ragweed parthenium, whitetop
French
: fausse camomille
Spanish
: escoba amarga
Habit:
herb
Description:
"Taprooted annual
herbs
3-10 dm tall;
stems
unbranched in lower part, branched in upper part, longitudinally striate, hirsute. Lower
leaves
in a basal rosette, these deciduous at maturity and stem leaves remaining, 3-20 cm long, 2-10 cm wide, bipinnatifid or pinnatifid.
Heads
small, numerous in open panicles; rays white, 0.6 mm long; disk corollas cream-colored, ca. 1 mm long; pappus of 2 petaloid scales inserted close to the corolla on either side, entire or notched.
Achenes
black, obovate, 2-2.5 mm long" (Wagner
et al
., 1999; p. 347).
Habitat/ecology:
"A pioneer species it invades grazing land, disturbed areas, roadsides and cultivated areas" (Smith, 2002; p. 64). "A weed of croplands, pastures, and natural and recreation areas. Can cause 90% reduction in carrying capacity of pastures. Contains sesquiterpines and phonolics, which makes parthenium weed allelopathic (suppressive) and causes dermatitis and other allergic reactions in humans and livestock, especially horses. High water requirement. Does best in neutral to high pH soils, less prolific on other soils" (Motooka
et al
., 2003). "In Hawaii, "naturalized in low elevation, dry pastures or along roadsides." (Wagner
et al
., 1999; p. 347). In New Caledonia, "espèce largement répandue et parfois envahissante" (MacKee, 1994; p. 37). In Taiwan, "widely naturalized on open seashores and in lowland waste places throughout the island" (
Digital Flora of Taiwan
).
Propagation:
"Seed is dispersed by wind, water, as a contaminant of harvested material and by vehicles, machinery or animals. Seeds are light and produced in large quantities, often within three weeks of germination" (Smith, 2002; p. 64).
Native range:
Tropical America, but widely naturalized.
Presence:
Pacific
Country/Terr./St.
&
Island group
Location
Cited status
&
Cited as invasive
&
Cited as cultivated
&
Cited as aboriginal introduction?
Comments:
Moderately invasive in New Caledonia (Meyer, 2000; p. 100). A "weed of national significance" in Australia. "A declared noxious weed in Queensland, Northern Territory and Western Australia" (Smith, 2002; p. 64).
Control:
Physical:
"Australian authorities have imposed strict quarantines on contaminated equipment and stock to avoid spread into new areas. Since the weed only invades bare soil, good pasture management is effective for prevention" (Motooka
et al
., 2003).
Chemical:
"Effective herbicides include 2,4-D, atrazine, hexazinone, and metsulfuron. Triclopyr is effective on most Asteraceae" (Motooka
et al
., 2003).
Biological:
"Australian scientists report the moth
Epiblema strenuana
promising. They are also looking at the rust
Puccina abrupta
var.
parthenenicicola
and fungi that affect the weed in Mexico" (Motooka
et al
., 2003).
7.
Parthenium hysterophorus
Linnaeus, Sp. Pl. 2: 988. 1753.
Annuals,
(10—)30—120+ cm.
Leaf blades
ovate to elliptic, 30—180+ × 10—50(—90+) mm, (1—)2-pinnately lobed (ultimate lobes lanceolate to linear, 3—50 × 2—15 mm), faces sparsely to densely scabrellous and gland-dotted (seldom with additional erect hairs 1—2 mm).
Heads
obscurely radiate, borne in open, paniculiform arrays.
Peduncles
1—8(—15+) mm.
Phyllaries:
outer 5(—6), lance-elliptic, 2—4 mm, inner 5(—6) ovate to ± orbiculate, 2.5—4 mm.
Pistillate florets
5(—6); corolla laminae reniform or orbiculate to oblong, 0.3—1 mm.
Disc florets
12—30[—60].
Cypselae
obovoid, 1.5—2(—3.5) mm; pappus-like enations erect, deltate to ovate, 0.5—1 mm (sometimes a third, subulate spur near apex adaxially).
2
n
= 34.
Flowering Mar—Nov. Disturbed sites; 0—300 m; Ala., Ark., Conn., Del., D.C., Fla., Ill., Kans., La., Md., Mass., Mich., Miss., Mo., N.J., N.Mex., N.Y., Ohio, Okla., Pa., Tex., Va.; Mexico; introduced in subtropical, tropical, and warm-temperate New World and Old World.
In some places, especially in Old World,
Parthenium hysterophorus
causes serious contact dermatitis in humans. The plants should be handled with caution.