Suisun Thistle

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Suisun Thistle

Cirsium hydrophilum var. hydrophilum

ListedNovember 20, 1997
FamilyCompositae (Asteraceae)
DescriptionA perennial, herbaceous wildflower with pale lavender-rose flower heads.
HabitatUpper reaches of tidal marshes.
ThreatsHabitat conversion by urban development, and degradation by projects alter the tidal regime, mosquito abatement activities, and competition with invasive alien plants.


Cirsium hydrophilum var. hydrophilum (Suisun thistle) is a perennial herb in the aster family (Compositae or Asteraceae) that flowers between July and September. Slender, erect stems 3.0-4.5 ft (1-1.4 m) tall are well branched above, while the spiny leaves are deeply lobed. The lower leaves have ear-like basal lobes; the upper leaves are reduced to narrow strips with strongly spine-toothed margins. Pale lavender-rose flower heads occur singly or in loose groups, each head being 1 in (2.5 cm) long.

The bracts of the flower heads have a distinct green, glutinous ridge on the back that distinguishes C. hydrophilum var. hydrophilum from other Cirsium species in the area.


Suisun thistle grows in the upper reaches of tidal marshes associated with narrow-leaf cattail (Typha angustifolia ), Olney's bulrush (Scirpus americanus ), Baltic rush (Juncus balticus ), and saltgrass (Distichlis spicata ).


Suisun thistle is restricted to Suisun Marsh in Solano County. The plant was reported as possibly extinct in 1975 because it had not been collected for about 15 years. Extensive surveys later found the thistle at two locations within Suisun Marsh; however, unoccupied suitable habitat for this plant exists outside these sites in the upper reaches of tidal marshes in Solano County. The species numbers only a few thousand individuals and occupies a total area of less than 1 acre. One population is found on State land under the jurisdiction of the California Department of Fish and Game; the other population is on Solano County Farmland and Open Space Foundation lands. There is currently no active management of this plant at either location.

The California Department of Water Resources conducted surveys in the middle 1990s for Suisun thistle, even searching areas outside its former historical range. They surveyed potential habitat throughout Suisun Marsh, searched portions of the potential habitat along the Contra Costa shoreline, assisted with searches downstream of Suisun Bay in the Carquinez Strait and Napa marshes, and surveyed diked wetlands managed for waterfowl. These surveys failed to find any new populations of Suisun thistle.


Habitat conversion and fragmentation, indirect effects from urban development, increased salinity, projects that alter the natural tidal regime, mosquito abatement activities, and competition with non-native plants threaten the two populations of Suisun thistle. The highly restricted distribution of this species increases its susceptibility to catastrophic events such as pest outbreaks, severe drought, and oil spills.

Widening of Highway 12 near the Suisun Marsh will threaten Suisun thistle through increased fragmentation of habitat and the damaging effects of increased runoff.

Both populations of Suisun thistle are adversely affected by invasive alien plants. Perennial pepper-grass (Lepidium latifolium ), a rated noxious weed that has encroached very significantly on Suisun thistle habitat in the last 5-10 years, now threatens to out-compete the listed plant where they occur together. Bull thistle (Cirsium vulgare ), which hybridizes readily with other Cirsium, definitely appears to be a threat to this species. Hybridization with Cirsium vulgare was suggested as a possible explanation for the previously presumed extinction of Suisun thistle.

Predatory insects are now known to be a threat to this species. The California Department of Water Resources discovered the thistle weevil (Rhinocyllus conicus ) in one population of this species in June 1996; many of the Suisun thistle flower heads had no seeds, and researchers collected thistle weevils from flower heads that still had seeds. The larval stage of the thistle weevil feeds on flower head seeds. Phyciods mylitta caterpillars were collected on a population of Suisun thistle in September 1996. These caterpillars have caused significant damage to the rosettes of plants.

Conservation and Recovery

The Suisun thistle only survives in two locations within Suisun Marsh. One population is on land under the jurisdiction of the California Department of Fish and Game, and another is on land of the Solano County Farmland and Open Space Foundation. These publicly owned critical habitats must be protected against any threatening activities. The populations of the Suisun thistle should be monitored, and studies made of its habitat needs. It should be cultivated in captivity, to provide stock for the establishment of additional populations in suitable protected habitat elsewhere on the coast of San Francisco Bay.


U.S. Fish and Wildlife Service
Sacramento Fish and Wildlife Office
Federal Building
2800 Cottage Way, Room W-2605
Telephone: (916) 414-6600
Fax: (916) 460-4619

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
(503) 231-6121


U.S. Fish and Wildlife Service. 20 November 1997. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Two Tidal Marsh Plants Cirsium hydrophilum var. hydrophilum (Suisun Thistle) and Cordylanthus mollis ssp. mollis (Soft Bird's-Beak) From the San Francisco Bay Area of California." Federal Register 62(224): 61916-61925