Slender-petaled Mustard

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Slender-petaled Mustard

Thelypodium stenopetalum

Status Endangered
Listed August 31, 1984
Family Cruciferae (Brassicaceae)
Description Perennial herb with decumbent stems, lance-shaped leaves, and lavender flowers.
Habitat Big Bear Basin; wet alkaline meadows.
Threats Loss of wetlands; residential development.
Range California


Slender-petaled mustard (Thelypodium stenopetalum ) is a short-lived, hairless perennial herb with straggling, decumbent stems 12-21 in (30.5-53.3 cm) in length. Leaves at the top of the plant are lance shaped and about 2 in (5.1 cm) long, while leaves closer to the base are larger and more spatula shaped. Lavender to whitish flowers occur in an open, many-flowered inflorescence. The linear petals are 0.4-0.6 in (1-1.3 cm) in length and are crinkled at the base. The calyx has a purplish tinge that gives the inflorescence a purple hue. The fruit is a straight to slightly curved pod that is cylindrical and slightly narrowed between seeds. Plants of this species flower from June to July.


Slender-petaled mustard and a similar species called the pedate checkermallow occur in vernally moist meadows and sparsely vegetated drier meadows at elevations from 5,250-8,200 ft (1,600.2-2,499.4 m) in the Big Bear Valley, San Bernardino Mountains. The habitat for the slender-petaled mustard might be more accurately described as moist meadows, although the species does tend to occupy the drier portions of wet meadows or sparsely vegetated "dry" meadows. The habitat where the species is documented is generally dominated by open sagebrush scrub vegetation. Individual slender-petaled mustard plants often seen growing up through sagebrush (Artemisia tridentata ) shrubs may be up to 3.3 ft (1 m) tall. Sagebrush scrub habitat is closely associated with wet areas along the margins of drainages or low areas where water collects in the spring.

Although the limiting factors that influence the distribution of slender-petaled mustard are not well understood, soil moisture appears to be important. A mosaic of moisture gradients can be observed at various slender-petaled mustard locales from Eagle Point to Erwin Lake in California. At the north end of Baldwin Lake, this species occurs along the margins of a relatively moist swale. At Erwin Lake, the species has been observed in what appears to be relatively dry, alkaline flats around the margins of the dry lake bed. Although no quantitative data are available regarding the specific soil characteristics associated with this species, soil alkalinity and clay content may be important.

Slender-petaled mustard also occurs in moist meadow-pebble plain associations. Within these wet meadow areas, this species often occurs in association with other wet to moist meadow species that are considered rare.

One unique aspect of the ecological relationship of slender-petaled mustard to faunal species is the use of its flowering stalks by the caterpillar of the rare Andrew's marble butterfly. Andrew's marble is endemic to the San Bernardino Mountains, whee the species feeds on native mustard species, including slender-petaled mustard.. The adult butterfly lays one to several eggs on the tip of the flower stalk. Larvae emerge after several days to feed on the blossoms and fruits. They typically feed on the upper fruit stalks, leaving the lower fruits untouched. The caterpillars pupate and may remain in their cocoons for up to five years. Insect damage to individual slender-petaled mustard plants was recorded on as many as 20% of the plants monitored at the locale at the north end of Baldwin Lake.


Slender-petaled mustard was once found throughout the Big Bear Lake Basin in San Bernardino County, California.

Dam construction, diversion of water for irrigation or human use, and drainage have reduced the extent of meadow wetlands from an original 7,000 acres (2,832.8 hectares) to only about 1,000 acres (404.7 hectares), not all of which is suitable for this mustard. Slender-petaled mustard is known from six, possibly eight, populations in the Big Bear Basin: Big Bear Lake, Baldwin Lake, Erwin Lake, and Holcomb Valley. The size of the habitat in the six to eight sites ranges from 1-8 acres (0.4-3.2 hectares). The only population survey, conducted in 1989 during a drought year, located 439 flowering and 106 nonflowering plants. More recent estimates in 1996, during wetter years, placed the total population at 25,000-27,000 flowering plants.


Habitat conditions, particularly the abundance and quality of groundwater, have deteriorated since this plant was listed in 1984. The most immediate threat is the ongoing loss of habitat to residential development. Off-road vehicular use and alteration of natural surface runoff have desiccated the habitat in most areas. Intensive grazing has resulted in the destruction of habitat in Pan Hot Springs, South Baldwin Lake, and Erwin Lake. Grazing has resulted in the compaction and erosion of soil, which has extirpated the species from pastures. Grazing animals also eat the plants.

Three of the known populations are on private property and face impending development, over which federal agencies can exert little control. The U. S. Fish and Wildlife Service declined to designate critical habitat for this species because it was feared that the required maps might cause vandalism of population sites.

Conservation and Recovery

The U. S. Forest Service retains ownership of one locale of slender-petaled mustard at Upper Holcomb Valley/Belleville. This site has also been monitored on an annual basis since 1989. The Forest Service has 1) attempted to negotiate an exchange of lands to acquire rare plant habitat adjacent to existing Forest Service lands surrounding the Bluff Lake meadow, 2) participated in the development of a cooperative management plan for the Baldwin Lake Ecological Reserve, 3) contributed significantly to the planning and implementation process for protection and interpretation activities at the Baldwin Lake site, 4) restricted off-road vehicle access to the habitat area, and 5) rerouted a hiking trail to lessen human disturbance. Habitat rehabilitation is necessary where overgrazing has denuded the land of vegetation and compacted the soil, and the wetlands need to be managed to ensure adequate water table levels.


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


Krantz, T. P. 1980. " Thelypodium stenopetalum, the>Slender-Petaled Mustard: A Botanical Survey of the Species throughout Its Range." San Bernardino National Forest, San Bernardino, California.

U. S. Fish and Wildlife Service. 3 July 1998. "Recovery Plan for the Pedate Checkermallow and Slender-Petaled Mustard." U. S. Fish and Wildlife Service, Portland, 68 pp.

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Slender-petaled Mustard

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