Tansy mustard is a circumboreal biennial or annual plant that is initially one half inch to two inches tall, which bolts in the spring and produces non-fragrant yellow flowers at the top of the plant. The plants eventually may grow as high as three feet tall, involving small orange seeds that manifest in long, narrow seed pods.
It branches sparingly and is more or less erect, with round stems covered with short glandular hairs. Young plants consist of a low rosette of basal leaves spanning four inches to eight inches across, from which there develops during the spring a flowering stalk with alternate cauline leaves. They are double or triple pinnately lobed and glabrous or slightly pubescent. The narrow lobes are oblong to oblanceolate, providing the cauline leaves with a fern-like appearance. The lobes become more narrow in the upper cauline leaves. The basal leaves are quite similar to the lower and middle cauline leaves in appearance.
The foliage of the tansy mustard is usually some shade of green. The petioles of the leaves are rather long, although they become progressively shorter as the cauline leaves ascend the stems. The upper stems terminate in racemes of flowers about two to twelve inches in length. The small yellow flowers bloom in whorls at the apex of each raceme, while the slender seedpods, known as siliques, develop below. Each flower spans about 1/8 inch across, consisting of four pale yellow petals, four green sepals, six stamens with yellow anthers, and a pistil with a stout style. The petals are about the same length as the sepals; they are both oblong-lanceolate.
The blooming period for the tansy mustard ccurs from mid- to late spring and lasts about 2 months. There is no noticeable floral scent. Each flower is replaced by a cylindrical silique that is about 1/3 inch long. This silique contains two rows of tiny seeds that are separated by a fine membrane. The spreading pedicels of the siliques (or flowers) are about ½” long when they are fully mature; the siliques angle upward from their pedicels and are more ascending. The tiny seeds are somewhat flattened, oblongoid or ovoid, and some shade orange-brown. These are ligght enough to be disseminated by the wind. The root system consists of a stout taproot. This plant spreads by reseeding itself and often forms loose colonies.
Tansy Mustard is typically found in full sun, mesic to dry conditions, and sterile soil that is sandy or gravelly. It also grows readily in fertile soil in open disturbed areas, in which case the plants will be larger in size.
Tansy Mustard is widely distributed in Along railroad lines, including the ballast, this plant is rather common. It is native to both the Old World and New World and botanists generally hold that the plant may be adventive from Eurasia. Habitats include gravelly prairies, sandstone and limestone glades, rocky bluffs and cliffs, areas along roads and railroads (including the ballast), fields, and sterile waste areas. The plant is most likely to occur in disturbed areas, but is also present in higher quality natural habitats.
The nectar and pollen of the flowers attract Syrphid flies and possibly other insects. The caterpillars of the butterflies Pieris rapae (Cabbage White), Pontia protodice (Checkered White), and Anthocharis midea (Falcate Orangetip) feed on the foliage. With regard to the plant’s relationships to birds and mammalian herbivores, in the drier Western states various species of quail eat the seedpods, while Bighorn Sheep, elk, and mule deer eat the foliage sparingly. The foliage is somewhat toxic to horses and cattle. It can cause blindness, staggering, and paralysis of the tongue if consumed in large quantities over a sufficiently long period of time. Sheep and goats are more immune to the toxic properties of the foliage. Because the tiny seeds become sticky when wet, it is possible that they are transported by the feathers of birds, the fur of animals, and the shoes of humans.
Tansy Mustard is one of the earliest plants to bloom along railroads, although it is usually rather small-sized and inconspicuous. It is somewhat similar in appearance to the adventive Descurainia sophia (Flixweed), differing from that plant by its greener foliage, earlier bloom, and the shape of its siliques, which are broader (about 1.5 millimeter. in diameter) and shorter (about 1/3 inch in length). Flixweed has greyish blue foliage and a tendency to bloom later. Its siliques are narrower (about 1 millimeter in diameter) and longer (about 1” in length). While the seeds of tansy mustard are arranged in two rows in each silique, the seeds of flixweed are arranged in a single row. Tansy mustard and flixweed are annual broadleaves and emerge in the fall. They grow as a rosette with finely lobed compound leaves.