Marin Dwarf-Flax
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Sacramento Fish & Wildlife Office

Species Account

MARIN DWARF-FLAX
(Hesperolinon congestum)

CLASSIFICATION: Federal Threatened Species (Federal Register 60:6671 (pdf); February 3, 1995).

CRITICAL HABITAT: None designated

RECOVERY PLAN: Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area, September 30, 1998 (pdf format)

DESCRIPTION:

Marin dwarf-flax, (Hesperolinon congestum), also known as Marin western flax, is a herbaceous annual of the flax family (Linaceae). It has slender, threadlike stems, 4-16 inches tall. The leaves are linear.

Flowers bloom from May to July. They are borne in congested clusters. Stems are 0.01-0.08 inch long. Sepals are hairy and the five petals are rose to whitish. The anthers are deep pink to purple. This helps distinguish Marin dwarf-flax from California dwarf-flax (Hesperolinon californicum), found in the same geographic area, which has white to rose anthers, as well as hairless sepals.

Two other species that are found in the same region are small-flower dwarf-flax (Hesperolinon micranthum) and slender dwarf-flax (Hesperolinon spergulinum). They differ from Marin dwarf-flax in having hairless sepals and a long, open inflorescence, with pedicels 0.08-1.0 inch long. See Hickman (1993) in General Information about California Plants, below, for a detailed description of these species.

DISTRIBUTION:

Marin dwarf-flax is restricted to serpentine soils from Marin County south to San Mateo County, California. Two populations are found in serpentine chaparral; the others occur in serpentine bunchgrass habitat. There are six populations known from Marin County, one from San Francisco County, and seven from San Mateo County. Populations fluctuate in size from hundreds to thousands of plants.

Serpentine soils are formed from weathered volcanic (ultramafic) rocks such as serpentinite. dunite, and peridotite. These soils provide a harsh environment for plant growth. Several factors contribute to the inhospitability of serpentine soils to plant growth including: 1) a low calcium-magnesium ratio; 2) lack of essential nutrients such as nitrogen, potassium, and phosphorous; and 3) high concentrations of heavy metals (mineral toxicity). However, species such as Marin dwarf-flax have adapted to serpentine soils and require them to survive.

SPECIAL CONSIDERATIONS:

Threats to Marin dwarf-flax include residential and recreational development, foot traffic and competition with nonnative species. Serpentine outcrops in the San Francisco Bay area are limited; 20 percent of those outcrops have already been eliminated as plant habitat due to development. The pressure to build more houses, roads, and other facilities for humans is great. Serpentine habitats also have been fragmented by the construction of roads. Habitat fragmentation increases the risks of extinction due to chance events such as fire, flood, landslide, pest or disease outbreaks, severe drought, or other natural or human-caused disaster.

This species was listed as endangered by the California Department of Fish and Game in June 1992 under the name Marin western flax. The California Native Plant Society has placed it on List 1B (rare or endangered throughout its range), also under the alternate name.

REFERENCES FOR ADDITIONAL INFORMATION:

Sharsmith, H.K. 1961. The genus Hesperolinon (Linaceae). University of California Publications in Botany. 32:235-314.

U.S. Fish and Wildlife Service. 1995. Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Ten Plants and Threatened Status for Two Plants From Serpentine Habitats in the San Francisco Bay Region of California. Portland, Oregon.

U.S. Fish and Wildlife Service. 1998.  Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area. Portland, Oregon.

General Information about California Plants

Photo credit: Marin Dwarf-Flax by Rick York © 1986 California Native Plant Society Calphoto ID: 0000 0000 0502 1138

Prepared by Endangered Species Div., Sacramento Fish & Wildlife Office, U.S. Fish & Wildlife Service

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