Arabis pycnocarpa (Hairy Rock Cress)
|Also known as:||Cream-flower Rock-cress|
|Life cycle:||biennial, short-lived perennial|
|Habitat:||part shade, sun; open woods, prairies, streambanks, rock outcrops, cliffs, gravel bars|
|Bloom season:||May - July|
|Plant height:||8 to 30 inches|
|Wetland Indicator Status:||GP: FACU MW: FACU NCNE: FACU|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Elongating cluster of stalked flowers, a few to several flowers in a cluster open at a time. Flowers are 1/8 to ¼ inch across with 4 rounded white petals and 6 creamy colored stamens in the center. The blooming cluster is erect or sometimes tilted to one side. A plant often has a single cluster at the top of the plant, sometimes multiples.
Leaves and stems:
Basal leaves are 1 to 3 inches long, 1/3 to 1 inch wide, spatula shaped to oblong, toothless or with a few small sharp teeth, usually with a blunt tip, tapering at the base to a short stalk, and covered in long stiff hairs. The leaf surface has a pimply appearance.
Stem leaves are 2/3 to 2¼ inches long, 1/8 to ¾ inch wide, egg-shaped to oblong, toothless or with a few small sharp teeth around the edges, blunt or pointed at the tip, stalkless or clasping the stem and often held erect, covered in long stiff hairs though the uppermost leaves may be hairless. The base of the leaf may be rounded or have a pair of small lobes (auricles). Stems are also covered in long stiff hairs at the base, becoming hairless to sparsely hairy in the upper plant. Stems are often single, though multiple stems may arise from the base of the plant.
Hairy Rock Cress, formerly Arabis hirsuta, seems to grow just about anywhere and is rather variable depending on the habitat. In a part-shade open woods, we found it pretty robust, the stems rather leafy and the stem leaves relatively large. In a sunny open prairie, it took on a much more spindly appearance, the leaves narrower and more widely spaced on the stem. Regardless of habitat, it can be distinguished from other members of the mustard family with small white flowers by the long stiff hairs on leaves and stems. There are 2 varieties in North America, with var. pycnocarpa found in Minnesota.
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- Hairy Rock Cress plant, shaded woods habitat
- Hairy Rock Cress plant, sunny prairie habitat
- more flowers
- basal leaves
- toothless stem leaves
Photos courtesy Peter M. Dziuk taken in Swift and Dakota counties.
Have you seen this plant in Minnesota, or have any other comments about it?