CPC Plant Profile: Greene's Milkweed
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Plant Profile

Greene's Milkweed (Asclepias uncialis ssp. uncialis)

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  • Global Rank: N/A
  • Legal Status: N/A
  • Family: Apocynaceae
  • State: AZ, CO, NM, OK, UT
  • Nature Serve ID: 147486
  • Date Inducted in National Collection:

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, appearing in clusters at the tips of the stems, and are reported to have a strong fragrance (Zimmerman 1993).

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  • 08/19/2020
  • Orthodox Seed Banking

Seed collection and storage.

  • 08/19/2020
  • Seed Collection

Seed collection and storage.

Nature Serve Biotics
  • 05/02/2017

Asclepias uncialis ssp. uncialis ( = A. uncialis sensu stricto) is apparently very rare with tiny population sizes. This species and its habitat are in decline.

Greene, Mary VB Goshorn
  • 01/01/2010

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 t

Greene, Mary VB Goshorn
  • 01/01/2010

6 - 20 occurrences. 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.

Greene, Mary VB Goshorn
  • 01/01/2010


Greene, Mary VB Goshorn
  • 01/01/2010

On both BLM and USFS Sensitive Species lists

Greene, Mary VB Goshorn
  • 01/01/2010

Occurrences of Asclepias uncialis are small and generally isolated from each other. The species also has extremely low rates of sexual reproduction. These factors, and the fact that A. uncialis is apparently absent from many locations where it was collected historically make it difficult to confirm that populations are stable. There is some indication that A. uncialis requires intact native habitat; however, before appropriate conservation elements can be identified, surveys and research to define the distribution, abundance, and population ecology of the species are needed. A more accurate picture of population numbers, occurrence extent, and variability will allow the identification of conservation targets. Additional investigation of the biology and ecology of A. uncialis will eventually allow land managers to formulate management strategies for the conservation of the species (Decker 2006).

Greene, Mary VB Goshorn
  • 01/01/2010

Seed collection and storage.


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Taxon Asclepias uncialis ssp. uncialis
Authority Greene
Family Apocynaceae
CPC Number 5018
ITIS 524890
Common Names Greene's Milkweed | wheel milkweed
Associated Scientific Names Asclepias uncialis ssp. uncialis | Asclepias uncialis
Distribution Historically, this species appears to have been known from two or three disjunct geographical areas: the western Great Plains of eastern Colorado, northeastern New Mexico, and the adjacent Oklahoma.
State Rank
State State Rank
Arizona SNR
Colorado S2
New Mexico S2
Oklahoma SNR
Utah SNR

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.

Ecological Relationships

Asclepias uncialis is likely to be pollinated by generalist species since the sparse occurrences and their scattered distribution do not constitute a predictable floral resource (Locklear 1996a). Ants, crab spiders, and hesperiid butterflies are the only insects that have been observed in association with A. uncialis, and only the last are likely pollinators. Observers have reported that A. uncialis has a strong fragrance (Zimmerman, 1993, Locklear 1996a), described as an aroma suggesting rose fragrance or that of citrus blossoms (Zimmerman 1993). The scent may serve as an attractant for pollinators.

Common Name Name in Text Association Type Source InteractionID
Honey bees Apis mellifera Confirmed Pollinator Link
Bumble bees Bombus pennsylvanicus Confirmed Pollinator Link
Cellophane bees Colletes armatus Confirmed Pollinator Link
Leaf-cutting bees Mechachile brevis Confirmed Pollinator Link
Butterflies & Moths
Brush-footed butterflies Danaus plexipus Confirmed Pollinator Link
Hairstreaks Strymon titus Confirmed Pollinator Link
Thread-waisted wasps Sphex ichneumoneus Confirmed Pollinator Link
Seed bugs Lygaeus kalmii Confirmed Pollinator Link
Lead Institution State Reintroduction Type Year of First Outplanting

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