Wormseed mustard (Erysimum cheiranthoides) is an annual herb that grows to about 1 m in height. It has multi-branched, reddish stems covered with small, sharply toothed leaves. It bears numerous small yellow flowers in clusters along its stems. Following the flowers, it produces thousands of tiny black seeds in small fruit clusters. E. cheiranthoides is easily spread and regrown from the numerous seeds it produces which is why some consider it an invasive weed. The whole plant gives off a strong and distinctive odor.
During certain environmental conditions, E. cheiranthoides is able to accumulate high quantities of nitrates. High levels of nitrates can be toxic to horses, and some horses are more susceptible than others.