Parthenium confertum

Parthenium confertum, or Gray's feverfew, is a biennial plant belonging to the family Asteraceae.^[2]

As can be confirmed on a page displaying images of different Parthenium species—the "feverfews"—documented by citizen scientists throughout the world,^[3] Gray's feverfew is typical of the 16 or so feverfew species by producing a certain kind of flowering arrangement with these traits:^[4]

• Flowering heads are quite small and white.
• They are composed of five or so unusually short, widely spaced, petal-like "ray florets" surrounding numerous small, closely packed "disk florets".
• The ray florets produce tiny, one-seeded, cypsela-type fruits, while the disk florets usually provide only pollen
• The heads are arrayed in more or less panicle-type inflorescences.

Among features distinguishing Gray's feverfew from similar species of Parthenium are these:^[2]

• It is a biennial.
• It stands up to 60+ cm tall (~2 feet).
• Its leaves are oval to rounded-triangular or elliptic.
• Leaf blades usually have irregularly pinnately lobed margins.
• Leaves can develop to 120+ mm in length and 40+ mm in width (~4+3⁄4 x 1+1⁄2 inches).
• Both leaf surfaces are gland-dotted.
• Leaves bear stiff, slender, pressed-flat hairs, or trichomes interspersed with erect trichomes up to 2mm tall (+1⁄16 inch).
• Disk florets usually number 20-30.

Unlike some other Parthenium species whose injured parts produce white latex historically used as a natural source for rubber, Parthenium confertum produces no white latex.^[5]

In the Southwestern United States Parthenium confertum occurs in the states of Arizona, New Mexico and Texas.^[2] In eastern Mexico it reaches as far south as the state of Querétaro in the country's central highlands.^[6] In western Mexico it occurs in the states of Baja California, Sonora and Sinaloa.^[1]

In the US state of New Mexico Gray's feverfew inhabits dry, sandy, rocky soils of mesas, canyons, hillsides, prairies, pinyon-juniper woodlands, scrub, grasslands and disturbed areas such as roadsides, at elevations of ~1200 – 2100 meters (3580–7000 feet).^[7] Pictures on this page show an individual occupying a patch of prairie in an open forest on a slope of the Edwards Plateau at an elevation of ~1750m (~5750 feet).^[8]

Four varieties of Parthenium confertum are accepted:^[1]

• Parthenium confertum var. confertum
• Parthenium confertum var. divaricatum Rollins
• Parthenium confertum var. lyratum (A.Gray) Rollins
• Parthenium confertum var. microcephalum Rollins

Through natural hybridization in northeastern Mexico, Parthenium confertum along with Parthenium bipinnatifidum are reported to have produced the famously invasive and toxic weed Parthenium hysterophorus.^[9]

Parthenium confertum, with a chromosome count of 35, is a polyploid.^[10]

The genus name Parthenium probably either derives from the Greek parthenos, which means "virgin", or parthenion, the ancient name of some kind of plant. The allusion is unclear.^[11]

The species name confertum apparently is derived from the New Latin confertus, meaning "crowded/pressed together/thronging,"^[12] which is a good description of how the numerous disk florets are crowded into such a small space in the flowering heads.