CPC National Collection Plant Profile

Astragalus sinuatus


Heading for profile page
CPC Home Join now
About CPC
CPC National Collection
Conservation Directory Resources
Invasive Plant Species Plant News
Plant Links Participating Institutions
Search CPC
Search    Alphabetical List    Reference Finder    CPC Home

CPC National Collection Plant Profile

Astragalus sinuatus

Common Name: 
Whited's milkvetch
Growth Habit: 
CPC Number: 


Profile Links
 Fish & WildLife

Astragalus sinuatusenlarge
Image Owner: Washington NHP

Astragalus sinuatusenlarge
Image Owner: Washington NHP

Astragalus sinuatus is Not Sponsored
Primary custodian for this plant in the CPC National Collection of Endangered Plants is: 
Edward Guerrant, Ph.D. contributed to this Plant Profile.

Astragalus sinuatus

Astragalus sinuatus is found only within a ten square mile (26 sq. km) area in Central Washington. Although much of the historic range of this species has been converted or destroyed by agriculture and grazing, there is quite a bit of suitable habitat remaining. Scientists do not know why A. sinuatus has not spread to these areas of seemingly suitable habitat.

Whited's milkvetch presents quite a challenge to researchers and seed conservationists. The seed pods of this species are small and incredibly tough. It may take a pair of pliers to crack the fruit open and extract the seed. For all the work involved, the pods have only one or two (rarely more) seeds inside (and sometimes they are completely empty). In the wild, pods open at one end, releasing seeds as they shake in the wind or roll down the hill. Often, the pods are so tough that they do not open until weathered and rolled along the ground. Despite the tough pod, the seeds often fall victim to seed predation or larval damage. Certain insects can bore a tiny hole through the pod and the seed coat and deposit an egg within the seed. As the larvae develops it consumes the contents of the seed.

Distribution & Occurrence

State Range
State Range of  Astragalus sinuatus
  Astragalus sinuatus grows on rocky in the sagebrush/bunchgrass shrub steppe community. Associated species commonly include Lupinus sulphureus, Erigeron linearis, Phlox longifolia, Woodsia oregana, Balsamorhiza sagittata, Lomatium dissectum, Lithophragma bulberifera and Astragalus purshii. Astragalus sinuatus grows on southwest to southeast facing slopes which makes them warmer and drier than the reported temperatures. The region is very harsh and dry with reported winter lows around 20F (-6.6C) and summer highs around 87F (30.5C). Precipitation averages only 9 inches (22.9 cm) per year, most of which falls from October to June (fall to spring).

  WA: EC (Eastern Cascades)

Number Left
  As of 1995: 8 populations (one with 2 sub-populations) all in an area less than 10 square miles. Populations range in size from 5 to 2,900 individuals and from acre to 80 acres. The total population in 1994 was approximately 5,000 (Gamon 1995).


Global Rank:  
Guide to Global Ranks
Federal Status:  
Guide to Federal Status
Recovery Plan:  

State/Area Protection
  State/Area Rank Status Date  
  Washington S1 E 10/1/2001  

Conservation, Ecology & Research

Ecological Relationships
  The region that Astragalus sinuatus occupies is subject to occasional fire, localized erosion, slope movement, and frost heaving. Astragalus sinuatus does not seem to take full advantage of, or require these localized disturbances (Gamon 1995). Although plants may show a short-term positive response from fire and other mild disturbances, it seems unlikely that they are required for long term survival of the population. It is found in early, mid and late successional stages. Population numbers become stabilized as the climax ecosystem is reached (Gamon 1995).

Occasional fire (30-90 year interval) probably played a role historically in maintaining open habitat for this species. Now, fire may lead to increased success of weedy annuals (WNHP 1999). Other disturbances can also have a negative impact as they may increase the number of weedy species which may eventually out-compete A. sinuatus (Gamon 1995).

The plant actively grows for only a short period every year. Leaf emergence is most likely initiated in late March or early April. The plant is fully leafed-out by mid April, just when floral buds appear. Most flowers bloom between late April and early May, and the majority of fruits develop by mid-May. Seed is fully mature by the middle to end of July. Most leaves begin to senesce by June, and in summer, the only identifying features remaining may be the empty seedpods (Gamon 1995). Astragalus sinuatus is sensitive to fluctuating precipitation levels. During drought years, many plants remain dormant, never emerging from the ground. Those that do emerge may not flower (Gamon 1995).

Knowledge of the reproductive biology of Astragalus sinuatus is scant. It is believed that they are a predominantly out-crossing species, although some members of the genus Astragalus can self-pollinate as well. Bumblebees have been observed in and around the plant (Gamon 1995). The incredibly tough pods do not naturally split along the "seam" as a pea or a bean would. They open only at the end, and seeds may fall out by shaking in the wind or as the pod rolls downhill. Pods may open more as they are weathered and rolled along the ground. Seeds must be scarified (have their surface scratched slightly), before they will germinate. Most seeds are dispersed close to the parent plant or slightly downhill from it. A. sinuatus is more common lower on slopes (Gamon 1995).

Like other members of the Pea Family (Fabaceae), Astragalus sinuatus can develop a symbiotic relationship with nitrogen fixing bacteria that live in their roots. This gives them a competitive advantage over other species in nitrogen poor soils and increases the available nitrogen in the soil (Gamon 1995).

Cattle do not seem to eat Astragalus sinuatus plants. They do, however, compact the soil, accelerate soil erosion, and contribute to an increase in weedy, annual, non-native species, such as cheatgrass (Bromus tectorum) (Gamon 1995). Areas most heavily used by cattle have lower population densities and lower seedling recruitment than areas less heavily used by livestock.

  Years of fire suppression has lead to increased vegetative cover. Now, in the event that a fire does start, this increased fuel load will lead to hotter burning fires, which can kill native vegetation. (WNHP 1999).
Invasion by weedy annual species (WNHP 1999).
Damage from roadside herbicide application (WNHP 1999).
Habitat loss to agriculture, grazing and gravel pits (WNHP 1999)
Seed predation by insect larvae leading to lower reproductive fitness. One year it was estimated that roughly 20% of fruits contained larvae. This threat may be even greater during stressful conditions, such as drought or high levels of competition (Gamon 1995).

Current Research Summary
  Germination trials at the Berry Botanic Garden revealed that 8 weeks of cold stratification, followed by either a constant 68F (20C) or alternating 50/68F (10/20C) temperature yielded slightly higher germination rates (67%) than the same temperature treatments with no cold stratification (40% at 68F and 50% at 50/68F) (BBG file).

Current Management Summary
  All populations have been subject to grazing by domestic livestock, especially cattle and horses, at some point since at least 1900, possibly as early at the 1880's (Gamon 1995).
One population is within a "Natural Area Preserve" managed by the Washington Department of Natural Resources (Gamon 1995). The area is fenced to exclude livestock.
A portion of another population on BLM land is designated as an ACEC (Area of Critical Environmental Concern). There is a grazing lease on the land, although the lessee has not grazed livestock there since around 1985 (Gamon 1995).
All other sites are used periodically as range land for livestock (Gamon 1995)
Seed from one population is stored at The Berry Botanic Garden

Research Management Needs
  Research ways to reduce big sagebrush cover other than fire that benefit Astragalus sinuatus (WNHP 1999).
Determine why individuals appear to reproduce successfully, yet seemingly suitable habitat remains unoccupied (WNHP 1999).
Determine factors influencing limited distribution.

Ex Situ Needs
  Collect and store seeds from all known populations.
Determine optimum germination procedures.
Determine propagation and reintroduction protocols.


Books (Single Authors)

Abrams, L.; Ferris, R.S. 1944. Illustrated Flora of the Pacific States: Washington, Oregon, and California. Stanford, CA: Stanford University Press.

WNHP. 1981. An illustrated guide to the endangered, threatened and sensitive vascular plants of Washington. Olympia: Washington Natural Heritage Program. 328p.

Books (Sections)

Kartesz, J.T. 1999. A synonymized checklist of the vascular flora of the U.S., Canada, and Greenland. In: Kartesz, J.T.; Meacham, C.A., editors. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden. Chapel Hill, NC.

Electronic Sources

WNHP. (2000). Washington Natural Heritage Program Database. Olympia, Washington.

Journal Articles

1901. (Original Publication). Bulletin of the Torrey Botanical Club. 28, 1: 40.


Ciacco, S.L. 1993. Natural Features Report: Upper Dry Gulch Natural Area Preserve. Olympia, Washington: Presented to the Division of Land and Water Conservation, Department of Natural Resources. p.7.

Franklin, J.F.; Dyrness, C.T. 1973. Natural vegetation of Oregon and Washington, General Technical Report. Portland, OR: U.S. Forest Service. PNW-8.

Gamon, J.G. 1995. Report on the status of Astragalus sinuatus Piper. Olympia, WA: Washington Natural Heritage Program, Washington Department of Natural Resources.

WNHP. 1994. Endangered, threatened and sensitive vascular plants of Washington. Olympia, Washington: Washington Department of Natural Resources. p.52.

WNHP. 1999. Field Guide to Selected Rare Vascular Plants of Washington. Produced as part of a cooperative project between the Washington Department of Natural Resources, Washington Natural Heritage Program, and the U.S.D.I. Bureau of Land Management, Spokane District.

  This profile was updated on 4/29/2011
New Mexico
North Dakota
South Dakota
South Carolina
North Carolina
West Virginia
New Jersey
Rhode Island
New Hampshire
New York
New York