Ambrosia trifida
Giant ragweed
(c) Roz Hunter, some rights reserved (CC BY) · licence
Family: Asteraceae · Type: Subshrub · Not Native
Conservation status: Cal-IPC Yes
Ambrosia trifida (Giant ragweed) is a non-native California subshrub in the Asteraceae family, recognized by giant size and palmately lobed leaves diagnostic.
Giant ragweed is a naturalized annual found in northern San Joaquin Valley, central and western California, and southwestern and Mojave Plateau regions in dry, disturbed places at elevations below 800 meters. Flowering from June to September, this plant produces small greenish flower heads in dense clusters. Growing up to 2 meters tall with thick, fleshy stems that are soft-hairy and black-lined, it develops a slightly woody base. Its opposite leaves are large, often 6 to 35 centimeters long, with distinctive palmately lobed blades that are lanceolate to widely ovate and deeply serrated. The plant produces unusual bur-like fruits 6 to 12 millimeters long with 4 to 8 short spines arranged in a ring around the fruit's tip.
Identification and appearance
Key characters: 12 sympatric species; giant size and palmately lobed leaves diagnostic
Habitat and range
Habitat: Dry, disturbed places
Elevation: < 800 m
Outside California: native to central and eastern United States
Bloom period
Ambrosia trifida blooms Jun-Sep.
Conservation status
Cal-IPC Yes
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 trifida native to California?
No. Ambrosia trifida is not native to California (recorded as not native).
When does Ambrosia trifida bloom?
Ambrosia trifida blooms Jun-Sep.
What plant family does Ambrosia trifida belong to?
Ambrosia trifida is in the Asteraceae family, genus Ambrosia.
Is Ambrosia trifida invasive in California?
Ambrosia trifida is listed by Cal-IPC with an invasiveness rating of Yes.
What habitat does Ambrosia trifida grow in?
Ambrosia trifida grows in Dry, disturbed places.
Related species
Other California species in the same genus, family, or bioregion.
- Ambrosia acanthicarpa — Annual bur-sage
- Ambrosia ambrosioides — Big bur-sage
- Ambrosia artemisiifolia — Common ragweed
- Ambrosia chamissonis — Beach bur-sage
- Ambrosia chenopodiifolia — San diego bur-sage
- Ambrosia confertiflora — Weak leaved burweed
- Amphipappus fremontii — Chaffbush
- Amphipappus fremontii var. fremontii — Fremont's chaff bush
- Amphipappus fremontii var. spinosus — Spiny fremont's chaffbush
- Amphipappus sp. — Chaff-bush
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.