Flower:

Loose, elongating cluster at the top of the plant and arising from upper leaf axils, the longest extending 12 inches or more with many open flowers above and fruit forming below. Flowers are about ¼ inch across, have 4 pale violet to white petals with rounded tips, and 6 dark purple-gray stamens surrounding a stout style in the center. The 4 sepals behind the flower are oblong, shorter than the petals, and pale pinkish-purple with a spot of light green covered in a few white hairs at the tip. Flower stalks are hairless, green to purple, and up to about 1/3 inch long.

Leaves and stems:

Leaves are alternate, thin and hairless, the lower leaves up to 6 inches long and 1½ inches wide, lance-oblong to elliptic in outline, blunt to pointed at the tip, tapering to a winged stalk, often with a pair of small lobes (auricles) at the base of the stalk that clasp the stem. Larger leaves may have 1 to 4 pairs of small lobes along the stalk. Edges are sharply toothed to nearly toothless.

Leaves become smaller, shorter stalked, and more lance-shaped as they ascend the stem, minutely to coarsely toothed around the edges, pointed at the tip, the uppermost leaves stalkless. Stems are erect, hairless, ridged, unbranched or few branched in the upper plant.

Fruit:

Fruit is a thin pod up to 1½ inches long, ascending to spreading (rarely erect), straight to slightly curved, and contain a single row of seeds. There is very little constriction in between the seeds.

Seeds are about 1 mm long, oval-oblong, golden brown, and have a fine network pattern across the surface.

Notes:

Purple Rocket is a very rare species in Minnesota where it reaches the northern fringe of its range. It is distinguished by the long, loose racemes of ¼-inch pale violet to white flowers, thin, hairless and toothed leaves, the larger of which often have auricled or clasping bases. Only 4 populations have been recorded in the state, the first from Red Wing in 1886; it and 2 other historical populations along the Root River have never been relocated and are presumed destroyed. According to the DNR, its preferred habitat in MN is floodplain forest, which has been degraded or destroyed by several causes, including clearing for agriculture, grazing where clearing was impractical, water-level manipulation from the lock and dam system on the Mississippi River, and invasive species that travel along waterways or infest woodlands. Buckthorn, Garlic Mustard and non-native Honeysuckles are common invasives in these areas but Japanese Hops and Poison Hemlock are quickly taking hold, especially along the Root River. Purple Rocket was listed as a Special Concern species in 1984 but elevated to Endangered in 1996 after intensive biological surveys in the southeast counties located only a single population. It is currently a Special Concern species in Wisconsin. While a number of non-native mustard species in Minnesota are invasive, Purple Rocket is struggling to survive, as are many other native mustards.