Taxon Report

Castilleja ambigua (Hook. & Arn.) T.I. Chuang & Heckard var. humboldtiensis (D.D. Keck) J.M. Egger

Humboldt Bay owl's-clover

Print Report

Taxon Summary

Castilleja ambigua var. humboldtiensis, commonly known as Humboldt Bay owl's-clover, is a annual herb (hemiparasitic) in the Orobanchaceae that is found only in California. It occurs within Marshes and swamps, growing at elevations from 0 to 3 meters. Castilleja ambigua var. humboldtiensis is ranked 1B.2, Plants Rare, Threatened or Endangered in California and Elsewhere; Moderately threatened in California.

Classification


Scientific Name:	Castilleja ambigua (Hook. & Arn.) T.I. Chuang & Heckard var. humboldtiensis (D.D. Keck) J.M. Egger
Common Name:	Humboldt Bay owl's-clover
Family:	Orobanchaceae
Element Code:	PDSCR0D402
USDA Plants Symbol:
Synonyms/Other Names:	Castilleja ambigua ssp. humboldtiensis

Ecology and Life History


Lifeform:	annual herb (hemiparasitic)
Blooming Period: Apr-Aug	Apr-Aug
Elevation:	0-3 (0-10)
General Habitats:	Marshes and swamps
Microhabitat:
Microhabitat Details:

Conservation Status


CA Rare Plant Rank:	1B.2
Global Rank:	G5T2
State Rank:	S2
State List:	None
Fed List:	None
Other Status:	BLM_S; SB_UCBG
CRPR Changes:

Occurrence Data from the CNDDB


Total Occurrences:	31
Element Occurrence Ranks:
Excellent (A)	5
Good (B)	11
Fair (C)	4
Poor (D)	0
None (X)	0
Unknown (U)	11

Location


California Endemic: True
California Counties and Islands: Name (Code)
Humboldt (HUM), Marin (MRN), Mendocino (MEN)
Quads: Name (Quad Code)
Arcata North (4012481), Arcata South (4012471), Cannibal Island (4012463), Eureka (4012472), Ferndale (4012453), Fields Landing (4012462), Inverness (3812217), Mendocino (3912337), Point Arena (3812386), Rodgers Peak (4112421), Tomales (3812228), Trinidad (4112412), Tyee City (4012482)

Threat List Data from the CNDDB


Threat List Total:	12
EOs with Threat Listed:	Total EOs	% of EOs
	17	55 %
Development	7	22%
Foot traffic/trampling	6	19%
Non-native plant impacts	4	12%
Other	4	12%
Grazing	3	9%
Altered flood/tidal/hydrologic regime	3	9%
Agriculture	2	6%
Road/trail construction/maint.	2	6%
Waterway bank protection/maintenance	2	6%
Surface water diversion	1	3%
Recreational use (non-ORV)	1	3%
ORV activity	1	3%

Notes



Threatened by coastal development and non-native plants. See C. ambigua ssp. humboldtiensis in TJM 2. See Proceedings of the American Academy of Arts and Sciences IV 16:536 (1927) for original description, Phytologia 90(1):63-82 (2008) for revised nomenclature, and Madroño 45(4):326 for distribution information.
Threats:

Taxonomy:

Selected References

Citation California Native Plant Society, Rare Plant Program. 2025. Rare Plant Inventory (online edition, v9.5.1). Website https://www.rareplants.cnps.org [accessed 10 May 2025].