Mustard family (Brassicaceae)
Description: This native perennial plant is 1-3' tall, branching occasionally. The foliage consists of basal leaves and bolting stems with alternate leaves; most vegetative growth occurs during the spring. The stems are glabrous and either ribbed or furrowed. The leaves are up to 5" long and 2" across, becoming smaller as they ascend the stems. They are more or less ovate, dentate along the margins, and hairless. The basal leaves and lower alternate leaves are often pinnately lobed; these lobes occur in 1-4 pairs near the base of each leaf.
The upper stems terminate in panicles of flowers about ½1½' in length. Each panicle is little-branched and erect, consisting of several narrow racemes of flowers. Shorter panicles of flowers also develop from the axils of the upper leaves. Each flower is about 1/3" across, consisting of 4 petals that are pale violet to nearly white, 4 pale violet sepals, 6 stamens with violet anthers, and a pistil with a stout style. The petals are broader toward their tips than at the base of the flower; they are longer than the sepals. The pedicel of each flower is about 1/3" long; it is often greenish violet or violet. Both the pedicels and flowering stalks (peduncles) are hairless. The tips of the sepals on young flowers are often hairy. The blooming period occurs during the early summer and lasts about 3 weeks. The flowers are often fragrant and many of them are in bloom at the same time. Each flower develops into a silique (an elongated seedpod) that is about 11¼" long, hairless, and linear in shape. Each silique contains a single row of oblongoid seeds; it is often violet-colored on the outer surface, except for a short beak at its tip that is green. Relative to the erect stalks of the inflorescence, the siliques are spreading and semi-erect. The root system consists of a taproot. This plant spreads by reseeding itself.
Cultivation: The preference is partial sun to medium shade, wet to moist conditions, and a rich soil with abundant organic matter. This plant withstands temporary flooding.
Range & Habitat: Purple Rocket occurs occasionally in east central and NE Illinois, while in other areas of the state it is uncommon or absent (see Distribution Map). Habitats include wet to mesic deciduous woodlands, particularly in floodplain areas along rivers and low-lying areas that are prone to occasional flooding. This plant also occurs in soggy meadows near woodland areas, including sunny areas that are adjacent to small streams and drainage ditches. Purple Rocket is a non-weedy member of the Mustard family that usually occurs in high quality habitats, less often at degraded sites.
Faunal Associations: There is very little information about floral-faunal relationships for this species. In sunny areas, the nectar of the flowers attracts bees and butterflies; in particular, the butterfly Pieris rapae (Cabbage White) is strongly attracted to the flowers. In shady areas, the flowers appear to attract about the same number of bees, but fewer butterflies.
Photographic Location: Along a drainage ditch in a moist meadow at Judge Webber Park in Urbana, Illinois. This meadow is rather weedy and located near a wooded area. Purple Rocket also occurs in damp shady areas at Busey Woods in the same city.
Comments: This is one of the more unusual members of the Mustard family because it is native, non-weedy, and has pale violet flowers. As these flowers become older, their color fades to white. In sunny areas, the elongated panicles of flowers are exceptionally long often comprising the upper half of the plant, if not more. They tend to be a little shorter and less showy in shady areas. Purple Rocket is fairly easy to identify in the field because it doesn't closely resemble any other member of the Mustard family. Perhaps the most similar species is Chorispora tenella (Blue Mustard). This introduced species is rare in Illinois; it differs from Purple Rocket in having pink or purplish pink flowers and glandular hairs on its stems and pedicels. The siliques of Blue Mustard curve upward and have longer beaks. Another member of the Mustard family, Hesperis matronalis (Dame's Rocket), has much larger flowers that are rosy pink. The petals of its flowers are well-rounded and overlapping, while the petals of Purple Rocket are more narrow and don't overlap.