Blue mustard [Chorispora tenella (Pallas) D.C.] was introduced into the United States from Siberia in 1929 and was first reported in Lewiston, Idaho. It was identified in Keith County, Nebraska in 1953.
Blue mustard is a winter annual weed, with seed germinating in late summer and fall. The plant overwinters as a rosette. The flower stalk usually elongates, a process known as bolting, in March. Cold weather in February results in late March elongation. Typically, other common mustard species such as tansy [Descurainia pinnata (Walt.) Britt] or tumble (Sisymbrium altissimum L.) mustard begin bolting two or three weeks after blue mustard. Blue mustard flowers are bluish-purple to purple and typically appear in early April. Viable seed can be produced approximately 10 days after bloom. Blue mustard is a problem in witner annual crops such as winter wheat, perennial crops like alfalfa, and noncrop areas such as roadsides and railroad right-of-ways.
Blue mustard is more difficult to control with 2,4-D than other mustard species. Early April applications of 2,4-D give excellent control of tansy and other mustards but only fair control of blue mustard because it has already bolted. Blue mustard flowers very early, and 2,4-D is often applied too late to be effective.
Blue mustard competition early in the spring reduces wheat yields. In Washington state, competition also occurs during the winter. Once blue mustard plant per square foot growing with wheat in the winter reduced yields 13 percent, while allowing blue mustard to mature reduced wheat yields 28 percent.
Rotations. Changing the crop rotation is suggested for heavily infested fields. Including a spring seeded crop such as corn, sorghum, soybean, oat, proso millet or sunflower in the rotation breaks the life cycle of blue mustard. This will reduce the blue mustard population in future winter wheat crops as the seed bank gradually decreases. Blue mustard plants must be killed before they produce seed prior to planting late spring crops. If early spring crops such as oat are planted, blue mustard must be killed before planting with tillage or a contact herbicide. It also may be necessary to spray the oat with a herbicide to kill late germinating blue mustard.
Winter Wheat. Winter wheat should be planted as near the optimum planting date as possible. Tilled ground should be firmed by rodweeding just before wheat planting. It’s very important to scout fields after wheat emergence and in February in order to determine blue mustard populations. This is especially true if significant precipitation occurs shortly after winter wheat is seeded. Note the location of blue mustard patches when they flower so these areas can be scouted in subsequent years. Roadsides and field borders should be sprayed to keep blue mustard from spreading into winter wheat fields. Blue mustard often invades a field from the edges, and in many fields, only field edges may need to be treated.
The best time for spring herbicide application is when blue mustard is in the rosette stage. Typically, this occurs from late February to about March 15, depending on weather and location in the state. Control decreases after blue mustard bolts. It is a common mistake, especially with 2,4-D, to spray too late, and control is diminished because blue mustard plants become too large.