© Bruce McNitt/Panoramic Images (Virginia)

Shale-Barren Rockcress

(Arabis serotina)

Shale barren rock cress (Arabis serotina) is a biennial herb in the mustard family (Brassicaceae). In its characteristic nonreproductive stage, A. serotina has a subtle basal rosette of lobed leaves 1.6 to 3.5 cm in size. In its reproductive stage the leaves shrivel and the stem “bolts,” forming a paniculate inflorescence that produces tiny cream-white flowers with calyxes from mid-July to September. The mature plant is 16 to 40 inches tall and produces slender siliques that contain slightly elliptic yellowish-brown seeds.

Similar Species
Arabis serotina most closely resembles smooth rock cress (Arabis laevigata) which also shares shale barren habitat. A. serotina is taller, has narrower leaves, the flowers and seeds are smaller, and the inflorescence is wider and more branched. A. laevigata flowers from April to May and grows in a wider range of xeric habitats.

Habitat and Ecology
Shale barren rock cress is an endemic species; its growth is restricted to shale barrens found in western Virginia and eastern West Virginia. There are about 60 known populations, most of which have low numbers of less than 50 individuals. Shale barrens are eroding, generally south-facing shale slopes of 20 to 70 degrees that occur in mid-Appalachian Mountain ranges at an elevation of 1300 to 1500 feet. Shale barrens can cover 0.2 to 20 hectares and are typically undercut by a stream.  

Shale barren habitat consists of open herbaceous cover and scrubby oaks, pines, and junipers that form no more than 10% canopy cover. Arabis serotina can also grow in shale woodlands next to shale barrens. Associated species include Kates Mountain clover (Trifolium virginicum), lillydale onion (Allium oxyphilum), and whitehair leather flower (Clematis albicoma). A. serotina does not tolerate encroachment of grasses or exotic species such as Centauria maculata.

Reproduction and Life History
Arabis serotina is biennial (flowering and fruiting in the second year) and reproduces sexually, although self-fertilization is possible. It is a facultative biennial in which the basal rosettes can persist longer than a year, putting off flowering and fruit bearing. Pollinators include several lepidopteran species as well as bees and small insects such as flower flies of the family Syrphidae.  

Limiting factors to A. serotina reproduction include high temperatures, drought, herbivory, and over-collection by botanists. Widespread unoccupied areas of viable shale barren habitat indicate that there are dispersal problems or reproductive bottlenecks. The harsh surface conditions of shale barren habitat control germination and seedling propagation and should be considered in restoration measures.

Because of its extreme habitat, Arabis serotina is very intolerant to stressors and is the most vulnerable of all endemic shale barren species. Threats include herbivory, reproductive failure, invasive species, fungal blight, insect infestations, and habitat degradation. Habitat degradation occurs via construction of roads, dams, railroads, and hiking trails. Because of their open environments, shale barren endemics are especially vulnerable to pesticides and insecticides, the application of which results in pollinator death.  

Arabis serotina predominantly occurs on public land, mostly in National Forests (specifically George Washington National Forest and Monongahela National Forest). Conservation practices include protecting and monitoring existing populations, surveying for additional populations, more thorough study of life history and ecological attributes, and storing seeds to mitigate population loss.  

Arabis serotina was listed as a federally endangered species in 1989.  It is imperiled globally and statewide in Virginia and West Virginia.

More information
NatureServe Explorer: Shale-Barren Rockcress

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