Hill mustard is an erect biennial or perennial (observation suggest that it acts as a perennial in Wisconsin). A key characteristic of hill mustard is the “warty bumps” (tubercules) on the stems which are easily felt by running your finger over the stem surface. Leaves may also have tubercules. The plants are extremely well anchored and are impossible to pull out and difficult to dig by hand. The highly lobed leaves are lanceolate and sharply pointed somewhat resembling dandelion leaves in shape. On mature plants can be 12 or more inches long in lower regions of the stem and become progressively smaller up the stem. Both leaves and stems are somewhat hairy. Flowers have bright yellow petals, and are very fragrant and are borne on dense racemes. Fruits are ovate, irregularly warty, 0.25 to 0.4 inches long, contain 2 to 4 seeds, and are borne on stalks about 0.5 inch long.
Hill mustard resembles yellow rocket but is easily distinguished by its leaf shape and stem texture. Leaves of yellow rocket do not have pointed lobes and are hairless unlike hill mustard which has toothed and hairy leaves. Yellow rocket stems never have the warty bumps found on hill mustard. Additionally, yellow rocket tends to be shorter than hill mustard and seldom becomes a monoculture. Yellow rocket flowering is waning as hill mustard begins to flower and the flowering period of hill mustard is considerably longer than that of yellow rocket. The fruits of the two species are also quite distinct with yellow rocket forming a narrow pod with many very small seeds while hill mustard has tear-shaped pods with few seeds.
Hill mustard inhabits non-disturbed sites and once established, forms a monoculture of only hill mustard plants. Hill mustard is native to southern Europe and has invaded most European countries. It is known to exist in the northeastern states, Virginia, Michigan, and Wisconsin. The University of Wisconsin-Madison Herbarium documented the original infestation in Green County west of the intersection of County Highway N and Buhler Road (north of Monroe approximately 3 miles). An inspection of all roads in the vicinity of this site in 2005 found than most hill mustard infestations are within 5 miles of the site of its original appearance. However, we have now seen hill mustard in several locations in Lafayette County.
As soon as yellow flowers are seen, plants must be cut as low as practical for the terrain. Additional mowing should be done if plants continue to flower. We do not know if repeated mowing will kill established plants but mowing each time flowers appear will at least greatly reduce seed production. Another approach to mechanical control is to till the soil or otherwise dislodge the roots of hill mustard from the soil. We need field trials to determine which tillage tools and how frequently tillage is needed to kill existing plants. Establishment of desired vegetation after tillage is essential and will reduce the likelihood of hill mustard once again becoming a monoculture.