Western tansy mustard (Descurainia pinnata) is a native annual found throughout most of North America up to about 7,000 feet elevation. It is particularly common in the Pacific States. Western tansy mustard occupies various habitats, particularly dry, desert areas and disturbed sites, and can easily become weedy.
A variable plant morphologically, western tansy mustard arises from a taproot and appears early in the spring as a basal rosette of leaves. The basal leaves are usually dry by the bloom period. The lacy-looking cauline (stem) leaves are alternate and pinnately dissected. Although the plant can grow to about 3 feet in height, it is usually smaller.
The inflorescence of bright yellow flowers is a raceme that elongates as the plant matures, with older flowers and seedpods lower on the stem while flowers continue to blossom at the tip of the stem. Western tansy mustard flowers have four petals in a Maltese Cross arrangement and four sepals. The plant has scattered stinging hairs.
The short-tipped linear seedpods (siliques) hang on threadlike pedicels (stalks) and have two rows of seeds. Western tansy mustard reproduces only from seeds. Once the seedpods open, the seeds are spread by wind or rain wash. When wet, western tansy mustard seeds are mucilaginous and stick to fur, feathers and beaks resulting in animals and birds also aiding in their dispersal.
Western tansy mustard contains nitrates and thiocyanides. When eaten in quantity by grazing livestock, the animals can become poisoned resulting in blindness, paralysis of the tongue and aimless wandering. Continued ingestion over a long period of time may even cause death. However, butterflies and other insects feast on the flowers.
Some people eat western tansy mustard, especially in the spring while the plant remains young, tender and less bitter. Personally, I avoid eating plants containing poisons.
Native Americans ground the tiny seeds and utilized them to treat stomach ailments.
Other common names for western tansy mustard include pinnate tansy mustard and yellow tansy mustard. Often the two words, “tansy mustard”, are seen in the literature as a single word, “tansymustard”.
These western tansy mustard specimens were photographed along Hat Creek just downstream of Hydro Plant #1 (Shasta County CA).