Asclepias uncialis ssp. uncialis
Greene's Milkweed
Other English Common Names: Wheel Milkweed
Other Common Names: wheel milkweed
Synonym(s): Asclepias uncialis Greene
Taxonomic Status: Accepted
Related ITIS Name(s): Asclepias uncialis Greene (TSN 30318) ;Asclepias uncialis ssp. uncialis Greene (TSN 524890)
Unique Identifier: ELEMENT_GLOBAL.2.147486
Element Code: PDASC02222
Informal Taxonomy: Plants, Vascular - Flowering Plants - Milkweed Family
 
Kingdom Phylum Class Order Family Genus
Plantae Anthophyta Dicotyledoneae Gentianales Asclepiadaceae Asclepias
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Concept Reference
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Concept Reference: 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.
Concept Reference Code: B94KAR01HQUS
Name Used in Concept Reference: Asclepias uncialis ssp. uncialis
Taxonomic Comments: A. uncialis ssp. uncialis in the Kartesz (1994) sense is the same entity as A. uncialis sensu stricto (excluding A. uncialis ssp. ruthiae).
Conservation Status
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NatureServe Status

Global Status: G3G4T2T3
Global Status Last Reviewed: 14Dec2007
Global Status Last Changed: 26Jan1998
Rounded Global Status: T2 - Imperiled
Reasons: Asclepias uncialis ssp. uncialis ( = A. uncialis sensu stricto) is apparently very rare with tiny population sizes. This species and its habitat are in decline.
Nation: United States
National Status: NNR

U.S. & Canada State/Province Status
Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
United States Arizona (SNR), Colorado (S2), New Mexico (S2S3), Oklahoma (SNR), Utah (SNR)

Other Statuses

NatureServe Global Conservation Status Factors

Range Extent Comments: 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.

Number of Occurrences: 6 - 20
Number of Occurrences Comments: Most of the occurrences documented from Colorado, New Mexico, Arizona, and Wyoming are historical and general records. All of the known extant occurrences are restricted to 6 sites: Garden Park (CO), Sheep Pen Canyon Road (CO), Pueblo Reservoir (CO), Van Bremer Arroyo (CO), Sonoita (AZ) and Shiprock (NM). It is likely that further survey will locate additional occurrences. Jim Locklear (Kansas Arboretum) recognizes a total of 18 extant occurrences range wide as of May 1997.

Population Size Comments: Total number of individuals documented at all extant sites is estimated to be less than 200. Eleven individuals are documented in New Mexico; approximately 175 individuals are known to occur in Colorado. The Arizona record does not indicate population size. Wyoming occurrences are all historical. Typically there are 10 individuals per occurrence. There seems to be a low rate of reproduction.

Overall Threat Impact Comments: In general, A. 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.

Short-term Trend: Decline of >30%
Short-term Trend Comments: Data gathered by James Locklear from 23 herbaria indicate that this species was probably more common 100 years ago. Several herbarium sheets contain more plants than can be found at many of the extant occurrences. Many botanists made collections of this species in the late 1800's, presumably without a specific search for this species. Few botanists in our region today have seen A. uncialis, and many have been unable to locate the species, even with considerable effort. It is also likely that this species has declined over the past 100 years because its habitat, shortgrass prairie, has been extensively modified for human uses such as agriculture, livestock operations, and residential development.

Intrinsic Vulnerability Comments: Asclepias uncialis has been found primarily in areas with modest human disturbances, therefore, it may be relatively fragile.

Other NatureServe Conservation Status Information

Distribution
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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.

U.S. States and Canadian Provinces

Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
Color legend for Distribution Map

U.S. & Canada State/Province Distribution
United States AZ, CO, NM, OK, UT

Range Map
No map available.


U.S. Distribution by County Help
State County Name (FIPS Code)
CO Adams (08001)*, Arapahoe (08005)*, Baca (08009)*, Bent (08011)*, Cheyenne (08017)*, Denver (08031)*, El Paso (08041), Fremont (08043), Huerfano (08055), Kit Carson (08063), Larimer (08069)*, Las Animas (08071), Otero (08089), Prowers (08099), Pueblo (08101), Weld (08123)
NM Grant (35017), Quay (35037)*, San Miguel (35047)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
10 Middle South Platte-Cherry Creek (10190003)+*, Big Thompson (10190006)+*, Cache La Poudre (10190007)+*, Crow (10190009)+, South Fork Republican (10250003)+
11 Upper Arkansas (11020002)+, Fountain (11020003)+*, Chico (11020004)+, Upper Arkansas-Lake Meredith (11020005)+, Huerfano (11020006)+, Apishapa (11020007)+, Upper Arkansas-John Martin (11020009)+, Purgatoire (11020010)+, Big Sandy (11020011)+*, Two Butte (11020013)+, Cimarron headwaters (11040001)+*, Upper Cimarron (11040002)+*, Revuelto (11080008)+*, Tierra Blanca (11120101)+*
13 Pecos headwaters (13060001)+, Taiban (13060004)+*
15 San Francisco (15040004)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
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Basic Description: Dicot, herbaceous perennial, Asclepiadaceae, Milkweed.
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).
Technical Description: As described by Hartman (in Great Plains Flora Association 1986) and Locklear (1991), Asclepias uncialis is a diminutive, herbaceous perennial with several to many spreading or erect stems 1 to 2.5 inches high. The stems contain a milky sap, and they appear to elongate when the plant is in fruit. The leaves are primarily opposite, and the species is distinguished by the presence of two different forms of leaves: lower leaves are oval to lance-shaped, 0.5 to 0.75 inches long and 0.23 inches wide while upper leaves are much narrower (about 0.125 inches) and 0.75 to 1.5 inches long. Plants are without hairs except occasionally along the leaf margins. The rose-purple flowers are 0.25 inches wide and generally occur in clusters of seven to 12 at the tips of the stems. 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.
Diagnostic Characteristics: Small stature, early blooming period, and heterophyllous leaves are diagnostic field characteristics.
Reproduction Comments: The Asclepiadaceae share with the Orchidaceae the character of transmitting pollen grains in discrete packets (Wyatt and Broyles 1994). Flowers possess two separate, superior ovaries. Between each pair of stamens, the two adjacent anther sacs are joined by translator arms and a corpusculum to form the pollinarium. The pollinarium is typically removed from the flower when the leg of an insect visitor slips into the opening between the anthers. As the insect pulls its leg upward and out, the corpusculum is attached to the appendage and pulled out of the flower. A bend forms in each translator arm as it dries, and the attached pollinium rotates 90 degrees. This change in configuration of the pollinarium is essential for correct pollination. Pollination is completed when the reconfigured pollinarium is inserted in the correct orientation in the stigmatic chamber of another flower. As the insect visitor withdraws its leg from the chamber, the translator arm breaks, leaving the pollinarium to germinate in the stigmatic chamber (Decker 2006).
Habitat Comments: 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. Elevations of extant occurrences in Colorado range from 3,920-7,640 feet (1,190-2,330 m). 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.

Economic Attributes
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Economically Important Genus: Y
Management Summary Not yet assessed
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Population/Occurrence Delineation Not yet assessed
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Population/Occurrence Viability
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U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
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Authors/Contributors
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NatureServe Conservation Status Factors Edition Date: 15Nov1995
NatureServe Conservation Status Factors Author: Spackman, Susan (1995), rev. D. Gries (1998), rev. K. Decker (2007)
Element Ecology & Life History Edition Date: 20Dec2007
Element Ecology & Life History Author(s): Decker, K., rev. SSP (2015)

Botanical data developed by NatureServe and its network of natural heritage programs (see Local Programs), The North Carolina Botanical Garden, and other contributors and cooperators (see Sources).

References
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  • Flora of North America Editorial Committee, ed. (FNA). 1993+. Flora of North America North of Mexico. Oxford Univ. Press, New York, Oxford.

  • Ackerfield, J. 2015. Flora of Colorado. Brit Press, Botanical Research Institute of Texas, Fort Worth, TX. 818 pp.

  • Data for WCMC Threatened Plants of the World project. 1994. Unpublished notes on state ranks from heritage botanists, sent to L. Kutner at The Nature Conservancy Home Office.

  • Decker, K. (2006, April 24). Asclepias uncialis Greene (wheel milkweed): a technical conservation assessment. [Online]. USDA Forest Service, Rocky Mountain Region. Available: http://www.fs.fed.us/r2/projects/scp/assessments/asclepiasuncialis.pdf [March 2006].

  • Decker, K. (2006, April 24). Asclepias uncialis Greene (wheel milkweed): a technical conservation assessment. [Online]. USDA Forest Service, Rocky Mountain Region. Available: http://www.fs.fed.us/r2/projects/scp/assessments/asclepiasuncialis.pdf [March 2006].

  • Fertig, W. 2000. Asclepias uncialis State Species Abstract. Wyoming Natural Diversity Database, Laramie, WY. Available .

  • Fertig, W. 2000. Asclepias uncialis State Species Abstract. Wyoming Natural Diversity Database, Laramie, WY. Available online: http://uwadmnweb.uwyo.edu/WYNDD/Plants/state_spp_abstracts/A/Asclepias_uncialis_draft.pdf.

  • Great Plains Flora Association. 1986. Flora of the Great Plains. University Press of Kansas, Lawrence, KS.

  • 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. 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.

  • 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 (http://plants.usda.gov). 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

  • 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.

  • Zimmerman, D. 1993a. More on Asclepias uncialis. Native Plant Society of New Mexico Newsletter 18(3):11.

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