Ambrosia confertiflora
Weak leaved burweed
Photo by Chez Brungraber
Family: Asteraceae · Type: Herb · Native
Ambrosia confertiflora (Weak leaved burweed) is a native California herb in the Asteraceae family, recognized by rhizomatous habit and tiny vestigial spines.
Weak leaved burweed is a California native perennial found in the San Joaquin Valley, central coast, southern coast, and eastern Mojave Desert in disturbed areas at elevations below 1,250 meters. Flowering from July to October, this plant produces small green and white flower heads in dense raceme-like clusters. Growing with strigose stems 30 to 180 centimeters tall that emerge from rhizome-like roots, it has a spreading, somewhat bristly habit. Its leaves are sessile or short-petioled, with divided blades less than 16 centimeters long that are roughly ovate and pinnately lobed. The distinctive fruit is a small bur with 5 to 12 spiraled spines, which are cylindric and hooked, measuring 2 to 5 millimeters across.
Identification and appearance
Key characters: 15 sympatric species; rhizomatous habit and tiny vestigial spines
Habitat and range
Habitat: Disturbed areas
Elevation: < 1250 m
Outside California: to Kansas, Oklahoma, Texas, northern Mexico
Bloom period
Ambrosia confertiflora blooms Jul-Oct.
How to tell it apart
vs Artemisia: Ambrosia produces spiny burs as fruits; Artemisia has tiny dry fruits without spines and strongly aromatic foliage.; vs Atriplex: Ambrosia has flower heads in spikes producing spiny burs; Atriplex has inconspicuous flowers with papery bracts and scurfy gray foliage.; vs Baccharis: Ambrosia has both male and female flowers on the same plant; Baccharis has male and female flowers on separate plants.; vs Eriogonum: Ambrosia has greenish flower heads producing spiny burs; Eriogonum has showy clusters of white, pink, or yellow flowers with papery bracts.; vs Salvia: Ambrosia has inconspicuous greenish flowers in the Asteraceae; Salvia has showy tubular lipped flowers typical of mints.
Frequently asked questions
Is Ambrosia confertiflora native to California?
Yes. Ambrosia confertiflora is native to California (recorded as native).
When does Ambrosia confertiflora bloom?
Ambrosia confertiflora blooms Jul-Oct.
What plant family does Ambrosia confertiflora belong to?
Ambrosia confertiflora is in the Asteraceae family, genus Ambrosia.
What habitat does Ambrosia confertiflora grow in?
Ambrosia confertiflora grows in Disturbed areas.
Related species
Other California species in the same genus, family, or bioregion.
Sources: Jepson eFlora · Index of California Plant Names · CalPhotos
Last updated 2026-10-07
Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, Cal-IPC, and more.