Species Factsheets

Oenothera argillicola

Shale-barren Evening-primrose

View as PDF

State Status: Pennsylvania Threatened (PT)
PBS Status: Pennsylvania Threatened (PT)
Federal Status:

Global Rank: G3G4 rank interpretation
State Rank: S2

Did You Know?

The phenotype of species hybrids in the genus Oenothera aren't compatible with the concept of "pure species" when compared with recognized species, so it is believed most Oenothera are constant hybrids.

Oenothera argillicola


Shale-barren evening-primrose (Oenothera argillicola) is a showy biennial herb that can reach 1.5m in hieght. Its erect or ascending stems are smooth and grow from strong, fleshy roots. The leaves grow both basally and alternately along the stem. Stem leaves are narrowly lance-shaped, and up to 1cm wide by 18cm long. Leaves are glossy, dark green and sometimes finely hairy. The flowers are yellow and usually last only one day. Each flower has a round ovary and four overlapping petals that are 2-4cm long. Flowers are displayed from July through September in a spike-like cluster that droops at the top.

Rank Justification

Imperiled in the nation or state because of rarity due to very restricted range, very few populations (often 20 or fewer), steep declines, or other factors making it very vulnerable to extirpation from the nation or state.


The PA Biological Survey considers the shale-barren evening-primrose to be a species of special concern, based on the relatively few occurrences that have been confirmed and the very specialized habitat. It has been assigned a rarity status of Threatened.


The species tends to grow on “shale barrens”, which occur on dry, open, usually steep slopes, banks, and cliffs, with shale substrate, typically on southerly or westerly aspects overlooking streams.

Survey Dates

Flowers July - September


In Pennsylvania, the occurrences are restricted to a few southcentral counties.



Conservation of shale-barren evening-primrose will depend on the protection of existing populations and shale barrens habitat. Many shale barrens may require no active management, although some sites that are more susceptible to woody encroachment may benefit from prescribed fire. Ending gypsy moth pesticide spraying in shale barrens, as well as creating unsprayed buffers around shale barrens, may be necessary to protect this species' insect pollinators.

Conservation Status Map


Map Legend

NatureServe. 2017. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org.


  • NatureServe. 2018. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Available at https://www.natureserve.org/explorer
  • Pennsylvania Natural Heritage Program. 2018.
  • Rhoads, A.F. and W.M. Klein, Jr. 1993. The Vascular Flora of Pennsylvania. American Philosophical Society, Philadelphia, Pennsylvania. Rhoads, A.F. and T.A. Block.
  • 2007. The Plants of Pennsylvania: An Illustrated Manual. 2nd edition. University of Pennsylvania Press, Philadelphia, Pennsylvania.
PNHP is a partnership between The Department of Conservation and Natural Resources,
the Pennsylvania Game Commission, the Pennsylvania Fish and Boat Commission,
and the Western Pennsylvania Conservancy.
DCNR Home Page
PA Game Commission Home Page
PA Fish and Boat Commission Home Page
Western PA Conservancy Home Page
DCNR Home Page PNHP | Forestry Home | Contact Us | Search This Site
© 2019 PA Department of Conservation and Natural Resources
DCNR Home Page