Rape Mustard
Brassica rapa
Mustard family (Brassicaceae)

Description: This plant is an annual or biennial about 1-3' tall. Small plants are branched sparingly, while large plants branch abundantly in the upper half. The stems are gray-green or gray-blue, terete, glabrous, and glaucous. Plants that begin growth during the fall will overwinter as low rosettes with basal leaves, while plants that begin growth during the spring bolt upward almost immediately. Both the basal and lower leaves are up to 10" long and 2" across, but they are usually smaller than this. They are oblanceolate in overall shape and strongly pinnatifid with undulate or bluntly dentate margins; their terminal lobes are the largest in size. Both types of leaves have stout petioles. In contrast, the middle to upper leaves are smaller in size, lanceolate-oblong in shape, with margins that are smooth or bluntly dentate. These latter leaves have bases that usually clasp their stems, although some of them may be sessile. Like the stems, these various leaves are grey-green or blue-green, glaucous, and usually glabrous – occasionally the basal leaves have short bristly hairs. The lower, middle, and upper leaves are alternate.

The upper stems terminate in racemes of bright yellow flowers. The flowers bloom toward the apex of each raceme, while the seedpods develop below. Each flower is 1/3–1/2" (8-12 mm.) across, consisting of 4 yellow petals, 4 green to yellow sepals, several stamens, and a pistil with a single style. The sepals are narrowly lanceolate and hairless. The blooming period can occur anytime between late spring to early fall; it usually lasts about 1 month for a small colony of plants. Each flower is replaced by an ascending cylindrical seedpod (silique) that is 1–2" long at maturity and hairless. Each seedpod terminates in a seedless beak that is about one-fourth its entire length. At the base of each seedpod, there is a stout hairless pedicel about " long that is widely spreading to ascending. Each seedpod divides into 2 valves to release its small globoid seeds. The root system consists of a taproot. This plant reproduces by reseeding itself.

Cultivation: Rape Mustard prefers full sunlight, moist to dry conditions, and a neutral to alkaline soil containing loam, clay-loam, or gravelly material. The size of individual plants varies greatly according to moisture conditions and soil fertility.

Range & Habitat: Rape Mustard is occasional in most areas of Illinois (see Distribution Map). It is native to Eurasia. Typical habitats include cropland, weedy fields, roadsides, gravelly areas along railroads, and waste areas. This plant is usually found in areas with a history of disturbance where there is scant ground vegetation.

Faunal Associations: The nectar of the flowers attracts small bees and White butterflies (Pieridae); some bees may collect pollen from the flowers as well. The caterpillars of the butterflies Pieris napi (Mustard White), Pieris rapae (Cabbage White), and Pontia protodice (Checkered White) feed on Brassica spp. (Mustards), as do the caterpillars of the moths Evergestis pallidata (Purple-Backed Cabbageworm) and Plutella xylostella (Diamondback Moth). Several species of flea beetles (primarily Phyllotreta spp.), Murgantia histrionica (Harlequin Bug), and Adelphocoris superbus (Meadow Plant Bug) also feed on the foliage of these plants. The oily seeds of Rape Mustard and similar species are eaten by the Mourning Dove and Ring-Necked Pheasant, and the mild-tasting foliage can be eaten in limited amounts by livestock and other mammalian herbivores.

Photographic Location: A gravelly area along a railroad in Savoy, Illinois.

Comments: This is another weedy mustard species from Eurasia. It should not be confused with the agricultural crop, Oilseed Rape (or Canola), which is Brassica napus oleifera, or one of the cultivated vegetables. Rape Mustard has several common names, including Field Mustard and Birdseed Rape. In general, Rape Mustard can be distinguished from other Brassica spp. (Mustards) by its glaucous gray-blue or gray-green foliage and its clasping alternate leaves. A similar species, Brassica oleracea (Wild Cabbage), shares these characteristics, but this latter species has larger flowers (exceeding " across) and it is quite rare in Illinois. Oilseed Rape has foliage that is more green than either Rape Mustard or Wild Cabbage, and its foliage isn't glaucous.