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“Weed of the Week”

Posted: May 7, 2013

Weed of the week - Birdsrape mustard (Brassica rapa)
Image 1. Birdsrape mustard.  Left: clasping leaves, Steve Dewey, Utah State University; Center: yellow flowers, Joseph M. DiTomaso, University of California - Davis, and; Right: silique fruit structures, John D. Byrd, Mississippi State University, Bugwood

Image 1. Birdsrape mustard. Left: clasping leaves, Steve Dewey, Utah State University; Center: yellow flowers, Joseph M. DiTomaso, University of California - Davis, and; Right: silique fruit structures, John D. Byrd, Mississippi State University, Bugwood

Over the last few years we have noticed a mustard species becoming much more common along roadsides, in hay fields and in other areas. Brassica rapa known as birdsrape mustard, wild turnip, or field mustard is beginning to bloom in the southern half of Pennsylvania. This plant is an introduced annual/biennial from Europe that grows up to 3 feet in height, has smooth pale green leaves that clasp the stem, is highly branched, and has showy yellow flowers clustered at the top of each shoot (image 1). After the petals fall, long slender siliques (pods) form that are 1 to 3 inches in length. It can sometimes be confused with yellow rocket which tends to be shorter in height and has glabrous (shiny) leaves and also produces silique-type fruit. In our area, birdsrape mustard is most similar to rapeseed mustard or canola (Brassica napus), which is an oil-seed crop sometimes grown in our area.

Last year, a few of us had discussions about some Brassica species that did not die after being treated with glyphosate.  Was it volunteer Roundup Ready canola or birdsrape mustard or something else? We left that discussion hoping that it was “volunteer” canola and not a new resistant Brassica species. According the literature, B. napus can be differentiated from B. rapa by its longer flower buds that overtop the open flowers, flowers that are cream -colored to pale yellow as opposed to yellow, broader petals that are 10 to 16 mm long and 6 to 9 mm wide, and basal leaves that are usually glabrous. 

Seems less than clear to me. If you treat an infested area with glyphosate and they don’t show any injury symptoms, my bet is on volunteer RR canola. If the plants are injured, yet don’t die, then it’s likely it’s not canola and another Brassica species. Brassica species in general are quite tolerant of glyphosate and especially if they are flowing and nearing maturity. Brassica species can also have some hard seed allowing seed to remain dormant and persist for more than one season. Good luck identifying those yellow, white, and sometimes purple flowing plants that are out there now or in the near future.

Contact Information

William S. Curran
  • Professor of Weed Science
Phone: 814-863-1014
Dwight Lingenfelter
  • Program Development Specialist
Email:
Phone: 814-865-2242