The host plants of western white butterfly larvae (Pontia occidentalis), the subject of my most recent post, are Brassica species. Adult western white females lay their eggs on various mustard, cabbage and related plants. Growing along the levee of Big Lake (Shasta County CA) where I photographed the western whites were common mustard (Brassica rapa) plants, surely not a coincidence.
A European native, this annual (occasionally biennial) has invaded most of North America and infests cultivated areas, roadsides and other waste areas and is often considered a noxious weed. Linneas originally classified the common mustard as B. campestris. However because it easily cross-fertilizes with other Brassica species, including domesticated turnips and oilseed rape, among others, these plants are often considered members of B. rapa and given different subspecies designations. Confusing! Both species designations are found in the literature.
The yellow-flowered mustards are often difficult to distinguish. Common mustard has a root that resembles a small turnip, an important identifying characteristic. (I could not get close enough to dig up a root without getting wet. For some unknown reason my better judgement did uncharacteristically take over that morning.) The erect stem grows 1 to 4 feet in height and can be either simple or branching. The lower leaves of common mustard are deeply lobed, bluish green and lyre-shaped. The upper leaves are not lobed and lance-shaped. The fact that the upper leaves broaden at the base and clasp the stem with two auricles (earlike projections) distinguishes common mustard from the other yellow mustards. Four-petaled yellow flowers with 1/2 to i inch stalks form small clusters terminally or at the leaf axes. The seeds are contained in a pod-like fruit. The entire plant has a whitish bloom (powdery look).
The leaves of common mustard were eaten as a potherb by native peoples and even today foragers collect and eat mustard greens. The bitterness increases with age so leaves are usually eaten young. I have eaten common mustard greens. They have an acceptable taste but I would not go out of my way to collect them. Common mustard seeds can be ground and used as a replacement for pepper. However care must be taken because ingesting large quantities of common mustard seeds can cause irritation of the stomach lining.
A pastelike poultice of common mustard seeds spread between layers of cloth and applied to the chest, the mustard plaster of lore, is reputed to relieve bronchial congestion. Since I never tried this remedy, I cannot vouch for its effectiveness.
Common mustard has many colloquial names including: wild turnip, birdrape mustard, wild mustard, yellow mustard, rape and field mustard.