Marsh Sandwort

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Marsh Sandwort

Arenaria paludicola

ListedAugust 3, 1993
FamilyCaryophyllaceae (Pink)
DescriptionHerbaceous perennial with singular flowers, and angled and smooth stem.
HabitatSmall freshwater marsh with active to partially stabilized beach dunes.
ThreatsOverpopulation of eucalyptus trees, alterations in hydrologic regime, erosion and sedimentation.
RangeCalifornia, Oregon, Washington


The Arenaria paludicola (marsh sandwort) is an herbaceous perennial member of the pink family. This slender plant roots at the nodes of procumbent stems. The small, inconspicuous, singular flowers are located in the axils on the narrow, opposite leaves. The stem is angled and smooth. Flowering occurs from May through August.


The marsh sandwort is classified as an obligate wetland plant. Obligate means that a plant almost always (greater than 99% of the time), under natural conditions, occurs in wetlands. Historically, it was found in freshwater marshes and swamps along the California and Washington coasts. Marsh sandwort is now restricted to a small persistent emergent marsh complex near the Nipomo Dunes Mesa, San Luis Obispo County, California. Marsh sandwort is found in association with Gambel's watercress, another endangered plant, stream orchis, bur-reed, sedges, and rushes.

This marsh complex is characterized as a series of small freshwater marshes associated with active to partially stabilized beach dunes which extend a distance of 5 mi (8 km) from Oceano south to the Oso Flaco Lakes area. Just inland from this "dune lakes" area lies the Nipomo Mesa, a broad mesa comprised of old Oceano sands deposited 40,000 years ago, and bisected by Black Lake Canyon. Pockets of freshwater marsh habitat in Black Lake Canyon and the dune lakes area harbor a unique flora that includes remnant populations of marsh sandwort and Gambel's watercress.


The marsh sandwort has been reduced to one remnant population located in a wetland complex near the Nipomo Mesa in San Luis Obispo County, California. Marsh sandwort is restricted to one site, and recent surveys found only three individual plants.

Because of this species' limited population size and limited distribution, it is vulnerable to extinction. The small population size also means there is a limited gene pool. The lack of genetic diversity means the species may not have the capabilities to evolve to adjust to changes in its environment. The Fish and Wildlife Service also considers this species to be endangered in Oregon and Washington.


The wetland complex supporting the marsh sandwort population is threatened by alterations in hydrologic regime, natural succession, runoff from upstream developments, erosion and sedimentation, and filling for development.

The area's hydrologic regime may have been altered over an extended period of time by the planting of eucalyptus trees. This species was introduced in the late 1800s and has spread throughout the lower half of the canyon. The eucalyptus has an extensive root system that can draw out soil moisture; that, coupled with the large evaporative surface of its foliage, has probably resulted in a significant drawdown of the water table compared to that which the surrounding native vegetation would have accomplished. Agricultural and urban use of groundwater may also impact these wetlands but studies concerning groundwater hydrology have not focused on which portions of the canyon have been affected. Additional urban development and the associated increase in drilling of wells could potentially impact these wetlands.

Natural succession in the area from a wetland to a more mesic grass or shrub-dominated ecosystem has apparently been accelerated, possibly due, in part, to the above mentioned changes in hydrologic regime. Drops in water table levels will decrease the soil moisture levels in certain areas which could allow the invasion of upland plant species. Aerial photographs from 1949 show the lower portion of the canyon as one wetland with open water and freshwater marsh or bog vegetation along its margins. By 1956, however, aerial photographs showed that willows had encroached into the wetlands and 30% of the area was covered with trees.

Plans to develop a golf course with 515 residential units upstream of the wetland complex were approved; there are additional plans to subdivide other parcels in the area for further residential and commercial development.

Erosion in the canyon has been exacerbated by off-road vehicles, horses, hikers, and pipeline easement construction and maintenance. Eucalyptu trees may also contribute to increased sedimentation in bog and pond areas by inhibiting the decay of debris because of acid tannins contained in the tree's leaves. Large or old trees that fall tend to destabilize the sandy slopes of the canyon, exposing unconsolidated patches of loose soil.

The U.S. Army Corps of Engineers, under Section 404 of the Clean Water Act, regulates the discharge of fill into waters of the United States, including wetlands. However, no permit is required to fill less than 1 acre (0.4 hectares), and if the fill is between 1-10 acres (0.4-4 hectares) in size, a Nationwide Permit Number 26 is issued by default within 20 days unless it is determined that an individual permit is required. Ongoing activities related to urban and agricultural use of the area that may result in the filling of wetlands in the Black Lake Canyon may, therefore, have little to no regulation by the Corps, since these areas are typically less than 10 acres (4 hectares) in size.

Conservation and Recovery

The Fish and Wildlife Service published a Recovery Plan for the marsh sandwort in 1998. Its critical habitats are on private land and are potentially threatened. These habitats should be protected by acquiring the land and designating ecological reserves, or by negotiating conservation easements with the owners. The populations of the marsh sandwort should be monitored, and research undertaken into its biology, habitat needs, and beneficial management practices. The rare plant should be propagated in captivity to provide stock for out-planting to increase the size of existing populations and to establish new ones in suitable wetland habitat. Surveys should be made in the Mexican range of the rare plant to determine its status there.


U. S. Fish and Wildlife Service
Sacramento Fish and Wildlife Office
Federal Building
2800 Cottage Way, Room W-2605
Sacramento, California 95825-1846
Telephone: (916) 414-6600
Fax: (916) 460-4619

U.S. Fish and Wildlife Service
Ventura Fish and Wildlife Office
2493 Portola Road, Suite B
Ventura, California 93003-7726
Telephone: (805) 644-1766

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


U.S. Fish and Wildlife Service. 3 Aug. 1993. "Endangered and threatened wildlife and plants; determination of Endangered status for Arenaria paludicola (Marsh Sandwort)." Federal Register 58(147).

U.S. Fish and Wildlife Service. 1998. "Recovery Plan for Marsh Sandwort (Arenaria paludicola ) and Gambel's Watercress (Rorippa gambelii )." U.S. Fish and Wildlife Service, Portland, Oregon.