Asclepias stenophylla A. Gray 



NARROWLEAF MILKWEED 



Milkweed Family (Asclepiadaceae) 



CONSERVATION STATUS 



U.S. Fish and Wildlife Service: None. 



Bureau of Land Management: Watch. 



Montana Natural Heritage Program: G4G5 SI; The species may be demonstrably secure 

 throughout its range, but may be critically imperiled in Montana where it is extremely rare. 



DESRIPTION: Narrow-leaved milkweed is a perennial herb that grows from a stout underground 

 rootstock. The stems, usually a single or pair per plant, are decumbent or upright, usually 

 unbranched, 2-8 dm (8-30 in) long, and are filled with milky sap. The leaves are borne alternately to 

 sub-oppositely along the stem and are linear shaped and 8-18 cm (3-7 in) long, by 1-5 mm (0.2 in) 

 wide. The foliage is moderately to sparsely hairy. Umbrella-shaped clusters of 10-25 flowers are 

 nearly stemless in the leaf axils of the upper half of the stem. Each greenish- white to yellow flower 

 is 7-9 mm long and is borne on a hairy pedicel up to 1 cm long. Flowers have 5 reflexed corolla 

 lobes and 5 erect "hoods" which are tri-lobed at the tip and surround a central column. The erect, 

 spindle-shaped fhiits are 9-12 cm (4-5 in) long and less than 1 cm wide. Each fruit contains 

 numerous seeds which have white tails about 3 cm (1 in) long. Flowering in June-early July. 



Both whorled milkweed {Asclepias verticillata) and plains milkweed (A. pumila) also have narrow 

 leaves but they both have more crowded often filiform leaves usually less than 1 .5 mm wide and 

 uniformly short in length. The flower clusters are borne on stalks that are greater than 5 mm long 

 rather than being nearly stemless in the leaf axils. The leaves of green milkweed {A. vihdiflora) , a 

 highly variable species, are sometimes linear, but the hoods of the flowers have rounded apices. A 

 hand lens reveals the three-lobed hoods of A. stenophylla that are diagnostic of the species. The 

 middle lobe of the hood is considered to be analogous to the "horn" of other milkweeds (adapted 

 from Great Plains Flora Association 1985, Dom 1984). 



GEOGRAPHICAL DISTRIBUTION 



Global distribution: Great Plains and Midwest; 

 from western Illinois west to eastern Montana, 

 eastern Wyoming and central Colorado, south to 

 western Arkansas and Texas (Great Plains Flora 

 Association 1986). 



Montana distribution: Known from six locations 

 in Carter and Rosebud Counties. 



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Asclepias stenophylla 

 Narrowleaf I 



