Astragalus missouriensis var. humistratus
Author: Isely

Missouri milkvetch

Fabaceae (Pea Family)

Close up of Astragalus missouriensis var. humistratus flower by Peggy Lyon
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Close up of Astragalus missouriensis var. humistratus fruit by Peggy Lyon
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Taxonomic Comments

There are three varieties of A. missouriensis in Colorado. It is probable that a hybrid between A. humistratus and A. missouriensis is present in southwestern Colorado. Plants in this area have characteristics of both species - the lower stipules are connate and the plant is strongly caulescent but prostrate as in A. humistratus, however the straight, glabrous or sparsely pubescent pod is more similar to A.missouriensis. Astragalus humistratus has a lunately incurved pod or the pod is straight but incurved at the beak, and the surface is strigulose. The potential hybrid is here referred to as A. missouriensis var. humistratus (Ackerfield 2012).

Ranks and Status

Global rank: G5T1
State rank: S1
Federal protection status: USFS Sensitive
State protection status: None

[+] Description and Phenology

Astragalus missouriensis var. humistratus by Teresa Burkert.
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General description: Astragalus missouriensis var. humistratus is a low-growing perennial with greenish-gray foliage. In contrast to other A. missouriensis varieties, var. humistratus has noticeable aboveground prostrate stems 7 to 20 cm in length. The pinnately compound leaves are 4 to 14 cm in length, and typically have 11 to 17 narrowly elliptic leaflets. The racemes typically hold 8 to 12 ascending or spreading flowers with pinkish-purple, white-tipped petals. Flowers are large, with total length of calyx and banner typically in the range of 23 to 35 mm. The legumes (pods) are oblong-ellipsoid, straight, about 1.7 to 2 cm in length, without hairs (or very sparsely pubescent along the ventral suture and on the beak), and they remain on the plant after seeds have dispersed. Pods contain 33 to 40 ovules, and seeds are small (about 2 mm long).  Mature pods are necessary to distinguish this variety from related species A. amphioxys and A. chameleuce, as well as other varieties of A. missouriensis (Barneby 1964, Isely 1983, 1986, and 1998).

Look Alikes: Isely separates var. humistratus from var. amphibolus by the stipules, which are free in var. amphibolus but connate or amplexicaul in var. humistratus; the beak of the fruit, which is curved or incurved at the tip in var. amphibolus, but straight in var. humistratus; and the usual number of flowers on the stem-4-8 in var. amphibolus and 8-12 in var. humistratus.

Phenology: Plants flower in May, and set fruit in May and June (Colorado Natural Heritage Program 2012).

[+] Habitat

Habitat of Astragalus missouriensis var. humistratus by Teresa Burkert
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In Colorado this species is found in dry soils derived from Mancos Shale, on barrens, and in grasslands and Ponderosa pine woodlands. Associated species also include Picea engelmannii, Quercus gambellii, Ceanothus fendleri, Astragalus flavus, A. bisulcatus var. haydenianus, A. oocalycis, A. lonchocarpus, Ipomopsis polyantha, Lesquerella pruinosa, Townsendia glabella, Penstemon caespitosus, Penstemon teucroides, Erigeron flagellaris, Artemisia ludoviciana, Eriogonum racemosum, Tetraneruis torreyana, and others.

Elevation Range: 6,955 - 8,320 feet (2,120 - 2,536 meters)

[+] Distribution

Distribution of Astragalus missouriensis var. humistratus in Colorado according to mapped land ownership/management boundaries (CNHP 2012, COMaP v9 ).
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Distribution of Astragalus missouriensis var. humistratus in Colorado
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Colorado endemic: No
Global range: Known only from New Mexico (Rio Arriba) and Colorado (Archuleta and Hinsdale counties).
State range: Known from Archuleta, Hinsdale, and La Plata counties in Colorado.

[+] Threats and Management Issues

Summary results of an analysis of the status of Astragalus missouriensis var. humistratus based on several ranking factors. This species was concluded to be “Weakly Conserved”. From Rondeau et al. 2011.
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The primary threat is considered to be roads and housing and urban development in the vicinity of Pagosa Springs. One occurrence is on US Forest Service land. Most occurrences are along roadsides as the plants tend to grow in disturbed soil. Little is known about the effects of grazing or other disturbance on this species.

[+] References

    • Ackerfield, J. 2012. The Flora of Colorado. Colorado State University Herbarium. 433 pp.
    • Colorado Natural Heritage Program. 2012. Biodiversity Tracking and Conservation System. Colorado State University, Fort Collins, CO.
    • Decker, K. 2006. Astragalus missouriensis Nutt. var. humistratus Isely (Missouri milkvetch): a technical conservation assessment. USDA Forest Service, Rocky Mountain Region. Online. Available: (accessed 8 August 2011).
    • Heil, K.D. 2007. New Mexico Rare Plants: Astragalus missouriensis var. humistratus (Pagosa milkvetch). New Mexico Rare Plant Technical Council, Albuquerque, New Mexico. Online. Available: (Accessed 2011).
    • Heil, K.D., S.L. O'Kane Jr., L.M. Reeves, and A. Clifford, 2013. Flora of the Four Corners Region, Vascular Plants of the San Juan River Drainage; Arizona, Colorado, New Mexico, and Utah. Missouri Botanical Garden Press, St. Louis, Missouri. 1098 pp.
    • Isely, D. 1983. New combinations and two new varieties in Astragalus, Orophaca and Oxytropis. Syst. Bot. 8:422-423.
    • Isely, D. 1998. Native and naturalized Leguminosae (Fabaceae) of the United States (exclusive of Alaska and Hawaii). Monte L. Bean Life Science Museum, Brigham Young University; MLBM Press, Provo, Utah. 1007 pp.
    • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
    • Neely, B., S. Panjabi, E. Lane, P. Lewis, C. Dawson, A. Kratz, B. Kurzel, T. Hogan, J. Handwerk, S. Krishnan, J. Neale, and N. Ripley. 2009. Colorado Rare Plant Conservation Strategy, Developed by the Colorado Rare Plant conservation Initiative. The Nature Conservancy, Boulder, Colorado, 117 pp.
    • USDA, NRCS. 2013. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.
    • Weber, W. A. and R. C. Wittmann. 2012. Colorado Flora, Western Slope, A Field Guide to the Vascular Plants, Fourth Edition. Boulder, Colorado. 532 pp.
    • Welsh, S.L. 2007. North American Species of Astragalus Linnaeus (Leguminosae) A Taxonomic Revision. Brigham Young University, Provo, Utah. 932 pp.

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