The following Participating Institutions are custodians for this species in the CPC National Collection:
Missouri Botanical Garden
The conservation of Oenothera pilosella ssp. sessilis is fully sponsored.
Matthew Albrecht contributed to this Plant Profile.
Oenothera pilosella subsp. sessilis is beautiful native perennial herb with showy yellow flowers that bloom between May-June. Restricted primarily to remnant tallgrass prairies in the lower Mississippi River Valley, this species has declined dramatically in the wild due to conversion of prairie habitat to cropland and inappropriate management practices. Oenothera pilosella subsp. sessilis is one of two octoploid subspecies of Oenothera pilosella. According to Straley (1977), Oenothera pilosella subsp. sessilis can be distinguished from Oenothera pilosella subsp. pilosella in being nonrhizomatous, by narrower leaves, sessile capsules, and densely appressed hairs pubescence throughout the entire plant.
According to Tucker (1983), the following characteristics help distinguish Oenothera pilosella ssp. sessilis from Oenothera pilosella subsp. pilosella:
Pubescence of erect hairs 1-2 mm long throughout (rarely glabrous); mature ovary 9-12 mm long; sepal tips divergent. (O. pilosellas subsp. pilosella)
Pubescence of densely appressed hairs less than 1 mm long throughout; mature ovary 4.5-6.5 mm long; sepal tips connivent. (O. pilosella subsp. sessilis)
New research based on molecular, morphological, and breeding system analyses indicate that O. pilosella subsp. sessilis is clearly distinct from Oenothera pilosella subsp. pilosella and merits recognition as a species (Wagner, Hoch, and Krakos, personal communication, September 2010).
Distribution & Occurrence
Prairie evening primrose is restricted primarily to remnant moist to dry tallgrass prairies in the Grand Prairie of Arkansas, often occurring in swales within the habitat (NatureServe 2009). The Grand Prairie sites are typically flat and underlain by poorly drained sandy or silty Alfisols over claypan on ancient river terraces of the Mississippi Alluvial Plain. Interestingly, one occurrence was discovered in a willow oak (Quercos phellos) fragipan flatwoods site on the Gulf Coastal Plain of Arkansas (Lafayette County, Arkansas), but its not clear whether this occurrence is an anomaly or represents favorable habitat for the species (NatureServe 2009).
|Known from only about 20 occurrences, prairie evening primrose is rapidly declining in the wild due primarily to conversion of prairie habitat to cropland and possible improper management practices. In Arkansas, five occurrences in remnant prairies are protected by the Arkansas Natural Heritage Commission. Occurrences in the Grand Prairie of Arkansas are often located in rights-of-ways where maintenance disturbance potentially threatens the populations. Only about 600 acres of remnant prairie remains in east-central Arkansas, indicating little potential for discovery of new populations unless more occurrences are found in willow-oak flatwoods of the Gulf Coastal Plain. Populations in Arkansas are currently small and have declined in abundance over the past decade (Theo Witsell, Botanist, Arkansas Natural Heritage Program, May 2010).
Arkansas: 12 counties (Arkansas, Clay, Drew, Hot Spring, Lafayette, Little River, Lonoke, Monroe, Prairie, Pulaski, St. Francois, and Yell). Most collections are historical have not been recently assessed.
Louisiana: 5 counties, (Bossier, Jefferson Davis, Morehouse, Rapides, and Tensas). Status is unknown at most sites.
Texas: 1 site, Galveston County, Texas (historical)
Conservation, Ecology & Research
Unlike Oenothera pilosellas subsp. pilosella in which flowers are insect pollinated and visited frequently by bees, butterflies, and skippers (Straley 1977), Oenothera pilosellas subsp. sessilis is rarely visited by insects. Flowers of Oenothera pilosellas subsp. sessilis do not produce nectar and are most likely autogamous (Kyra Krakos, Washington University, personal communication).
The fruit is a capsule that ripens in early July in Arkansas populations (M.A. Albrecht, personal observation). The capsules gradually dehisce, releasing small seeds that are dispersed by wind and rain (Straley 1977).
Specific ecological conditions that promote seedling recruitment are unknown. Seed require a cold stratification treatment to break seed dormancy, indicating that seedlings emerge in spring in the wild (Kyra Krakos, personal communication). Understanding the light requirements for seedling recruitment would help determine whether the species can form viable populations in the deeply shaded willow oak flatwoods of the Gulf Coastal Plain.
Habitat loss due to conversion of remnant prairies to cropland.
Disturbance from maintenance activities in utility, railroad, and highway rights-of-ways.
One hypothesis is that prairie evening primrose requires unplowed prairies and recurrent management to thwart succession (NatureServe 2009). Prairie mounds, which are indicators of unplowed prairies, are found in at least two natural occurrences (Downs and Roth Prairies, Arkansas County, Arkansas) of prairie evening primrose (GHF 2010). Currently, there are no best management practice plans for the species.
Life-history studies are needed to understand the ecological factors that promote seedling recruitment and maintenance of reproductive individuals in the populations.
Distributional surveys are needed to determine: 1) whether the taxa persists at historical sites, and 2) whether it occurs more broadly in willow oak flatwoods along the Gulf Coastal Plain or is restricted primarily to remnant tallgrass prairie sites in the Grand Prairie.
Propagation protocols are needed to grow plants from seed to maturity for life-history study and to develop ex situ population seed sources.
Given the rarity of the species, experimental reintroductions into remnant tallgrass prairies may be warranted once best habitat management practices are determined.
GHF. (2010). The Grassland Heritage Foundation. [Web]. http://www.grasslandheritage.org/.. Accessed: 2010.
LNHP. (2008). Louisiana Natural Heritage Program. [Web]. http://www.wlf.louisiana.gov/wildlife/rare-plants-fact-sheets. Accessed: .
NatureServe. (2009). NatureServe Explorer: An online encyclopedia of life. [web application].NatureServe, Arlington, Virginia. Version 7.1. http://www.natureserve.org/explorer. Accessed:
TPWD. (2010). Texas Parks and Wildlife Department.. [Web]. http://gis.tpwd.state.tx.us/TpwEndangeredS