The Enemy - Dyers woad (Isatis tinctoria L.) is a biennial plant introduced into the U.S. in colonial times. It was brought here as a blue dye. This mustard plant is one that is probably showing its beautiful bright yellow bloom right now. It has inhabited thousands of acres near Pocatello and south (Utah especially) as it is one of their worst weeds. The plant has thick green leaves with a noticeably white midvein.
The Strategy - Dyers woad germinates late in the fall to early spring and gets established before many other plants. Its rosettes can spread out close to 12 inches and the plant will grow to a height of 4 feet. It produces dark purple to black looking seeds than ‘hang’ from the top of the plant which allows the seeds to spread long distances as well as keeps the seed viable in the soil for many years. The plant produces a tough taproot.
The Defense - As this is a biennial plant, mechanical control can be effective, in fact many counties have a “BAG-O-WOAD” program in which they will pay for garbage bags full of this weed. Herbicides such as Telar XP or Escort XP are most effective if applied during the active growth of the weed. There are currently no biocontrols available for this weed and most animals will not eat it. This canola looking plant is found in most counties so if you suspect you have seen it let your county weed superintendent know so that it does not become a serious problem.
U of I Idaho's Noxious Weeds Control Guidelines (183 KB PDF download)