Asclepias uncialis ssp. uncialis

Dwarf milkweed

Asclepiadaceae (Milkweed Family)

Close up of Asclepias uncialis ssp. uncialis by Steve Olson.
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Close up of Asclepias uncialis ssp. uncialis by Renee Rondeau.
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Close up of Asclepias uncialis ssp. uncialis by Susan Panjabi.
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Taxonomic Comments

Weber and Wittmann (2012) list this as Asclepias uncialis, without reference to the subspecies.

Ranks and Status

Global rank: G3G4T2T3
State rank: S2
Federal protection status: USFS Sensitive, BLM Sensitive
State protection status: None

[+] Description and Phenology

Asclepias uncialis ssp. uncialis by Patricia Whalen.
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Asclepias uncialis ssp. uncialis by Carolyn Crawford.
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General description: A small, herbaceous perennial with several to many stems 1 to 2.5 inches high. Stems have milky sap. Leaves are primarily opposite, and are of two different forms - lower leaves are oval to lanced shaped, while upper leaves are much narrower. Flowers have five reflexed petals with attendant hoods and horns. Flowers of A. uncialis ssp. uncialis are rose-purple, 0.25 inches wide, appear in clusters at the tips of the stems, and are reported to have a strong fragrance (Zimmerman 1993). Plants are without hairs except occasionally along the leaf margins. Fruits (follicles) are spindle-shaped (thick but tapering toward the ends) and about 2 inches long. Seeds are about 0.25 inches long with a tuft of silky hairs about 1 inch long (FNA 1993+, Locklear 1991).

Look Alikes: The small stature, early blooming period, and heterophyllous leaves distinguish Asclepias uncialis ssp. uncialis from the sympatric and similarly small-sized A. pumila, which has white flowers, blooms from July to September, and has only filiform leaves (Locklear 1991). The low-growing A. involucrata may also be found in the southern portion of the range of A. uncialis ssp. uncialis. It has greenish-white flowers, blooms later than A. uncialis ssp. uncialis, and has longer leaves that are uniformly lanceolate (Locklear 1996). Asclepias uncialis ssp. ruthiae (Maguire) Kartesz & Gandhi is known from the four corners region, and has been reported from Utah, Arizona, and New Mexico. It could also occur in Montezuma Co., and should be looked for in desert scrub and pinyon-juniper communities. Subspecies ruthiae differs by having larger flowers (corolla lobes 4-6 mm long vs. 3-4 mm long in ssp. uncialis), and mostly ovate leaves as opposed to lanceolate or linear leaves (Ackerfield 2015).

Phenology: Asclepias uncialis ssp. uncialis is the earliest blooming milkweed in the Great Plains (Great Plains Flora Association 1986) although its flowering period can potentially overlap those of a few other species in its range (e.g., A. asperula, A. speciosa, and A. involucrata). In Colorado, flowering begins in late April and extends to the end of May. The small population in Weld County, Colorado, that did not flower in the dry spring of 2006 was observed flowering in early August of the same year after some substantial summer rains (Colorado Natural Heritage Program 2012).

[+] Habitat

Habitat of Asclepias uncialis ssp. uncialis courtesy of the Colorado Natural Heritage Program.
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Habitat of Asclepias uncialis ssp. uncialis by Steve Olson.
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Typical habitat for Asclepias uncialis ssp. uncialis is level to gently sloping terrain without notable micro-topographic features. Although plants are often found at the base of escarpments or mesas, the species does not occur on rock ledges or outcroppings, and is absent from highly disturbed habitats such as sand dunes, erosion channels, wash slopes, and badlands. Soils in the range of A. uncialis ssp. uncialis belong to orders characterized by dry, warm soils (Mollisols, Entisols, Aridisols, and Alfisols). Asclepias uncialis ssp. uncialis does not appear to have highly specific microsite requirements, and there is no evidence that A. uncialis ssp. uncialis is restricted to a particular soil type. Occurrences are known from soils derived from a variety of substrates, including sandstone, limestone, and shale, but are most often found in sandy loam soils. It does not occur in pure sand. Asclepias uncialis ssp. uncialis is primarily associated with species typical of shortgrass prairie. Associated vegetation is comprised mostly of grasses, with forbs, shrubs, and trees typically comprising less than 15% of the total vegetation cover. Plants are typically found growing in open spaces between bunch grasses. Associated forbs are variable throughout the range, since many species found with A. uncialis ssp. uncialis in southeastern Colorado (e.g., Melampodium leucanthum) are near the northern edge of their distribution in that area (Locklear 1996). Although A. uncialis ssp. uncialis is often associated with Juniper Woodland and Savanna ecological systems, it is always found in the prairie or grassland components of these systems.

Elevation Range: 3,881 - 7,730 feet (1,183 - 2,356 meters)

[+] Distribution

Distribution of Asclepias uncialis ssp. uncialis in Colorado according to mapped land ownership/management boundaries (CNHP 2015, COMaP v9).
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Distribution of Asclepias uncialis ssp. uncialis in Colorado.
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Colorado endemic: No
Global range: Historically, this species appears to have been known from two or three disjunct geographical areas: 1) the western Great Plains of eastern Colorado, northeastern New Mexico, and the adjacent Oklahoma panhandle; 2) central to southwestern New Mexico and scattered locations in Arizona; and 3) Sweetwater County in southwestern Wyoming. Some botanists consider the location of the Wyoming collection (C.C. Parry #246) to be an error in labeling and speculate that it may have come from northeastern Colorado (Fertig 2000, Fishbein personal communication 2004). Recent observations (i.e., those less than 20 years old) are confined to the first two areas mentioned plus a few observations in central New Mexico. Based on collection location and frequency, the range of the species appears to have contracted in northeastern Colorado since the mid to late 1800's.
State range: Estimated range in Colorado is 71,964 square kilometers (27,785 square miles), calculated by the Colorado Natural Heritage Program in GIS by drawing a minimum convex polygon around the known occurrences. Currently known from nine Colorado counties (Las Animas, Weld, Kit Carson, Huerfano, Pueblo, Otero, Prowers, Fremont, and El Paso), and historically known from at least eight additional counties (Arapaho, Adams, Baca, Bent, Cheyenne, Larimer, Denver and Washington). Occurrences are primarily in southeastern Colorado.

[+] Threats and Management Issues

The primary threat at this time is considered to be agricultural development. In general, A. uncialis ssp. uncialis habitat, shortgrass prairie, is threatened by extensive human alterations for agricultural, residential, and recreational uses. Specific threats to extant occurrences include: recreational use, agricultural use, and military tank traffic. Other threats are population limitation by unknown biological requirements, altered disturbance regime, habitat loss, spread of exotic species, and global climate change. A lack of understanding of population trends and habitat conditions for A. uncialis ssp. uncialis, and the lack of knowledge about its life cycle, population extent, and demographics also contribute to the possibility that one or more of these factors will threaten the long-term persistence of the species (Decker 2006). Locklear (1996) identified several patterns exhibited by Asclepias uncialis ssp. uncialis that are of concern: 1) A. uncialis ssp. uncialis is often not found at historical sites that retain native vegetation. In these cases, absence of A.uncialis ssp. uncialis may be due to causes peculiar to the biology of A.uncialis ssp. uncialis, instead of habitat degradation, 2) Most of the known populations are small, discrete, and isolated from each other. Large areas of intervening, apparently suitable habitat are not occupied. Gene flow between these isolated populations is unlikely, and may lead to a decline in species viability over time, and 3) A.uncialis ssp. uncialis exhibits extremely low rates of sexual reproduction, perhaps even lower than is characteristic of the genus. Although known populations are exposed to grazing, potential recreational use and development, and military training maneuvers, the degree of threat from these disturbances is not known.

[+] References

    • Ackerfield, J. 2015. Flora of Colorado. Brit Press, Botanical Research Institute of Texas, Fort Worth, TX. 818 pp.
    • Decker, K. (2006, April 24). Asclepias uncialis Greene (wheel milkweed): a technical conservation assessment. [Online]. USDA Forest Service, Rocky Mountain Region. Available: [March 2006].
    • Fertig, W. 2000. Asclepias uncialis State Species Abstract. Wyoming Natural Diversity Database, Laramie, WY. Available online:
    • Flora of North America Editorial Committee, ed. (FNA). 1993+. Flora of North America North of Mexico. Oxford Univ. Press, New York, Oxford.
    • Great Plains Flora Association. 1986. Flora of the Great Plains. University of Kansas Press, Lawrence. 1392 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.
    • Locklear, J. H. 1991. Status of Asclepias uncialis in eastern Colorado and northwestern New Mexico. Unpublished report prepared for the Nature Conservancy Colorado Field Office, Boulder, CO.
    • Locklear, J.H. 1996. The biology, ecology, and conservation needs of Asclepias uncialis in Colorado. Unpublished report prepared for the Colorado Natural Areas Program, Denver, CO.
    • 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. 2015. The PLANTS Database ( National Plant Data Team, Greensboro, NC 27401-4901 USA.
    • Weber, W. A. and R. C. Wittmann. 2012. Colorado Flora, Eastern Slope, A Field Guide to the Vascular Plants, Fourth Edition. Boulder, Colorado. 555 pp.
    • Wyatt, R., and S.B. Broyles. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25:423-441
    • Zimmerman, D. 1993. More on Asclepias uncialis. Native Plant Society of New Mexico Newsletter 18(3):11.

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