Isatis tinctoria • Class A

This plant grows from 1 to 4 feet tall and has a 3 to 5 foot long taproot. The basal rosette produces stalked, bluish green leaves covered with fine hairs. The leaves are 1 ••• to 7 inches long and are succulent. When the rosette bolts, up to 20 stems can be produced. Its small yellow flowers bloom from April to June, they are cross shaped with 4 sepals, 4 petals, 2 short outer stamens, and 4 longer inner stamens. 

Rotting seed pods contain a water soluble chemical with allelopathic properties that work as a germination inhibitor and limit the root elongation of other plant seedlings. This inhibitor impacts other plants growing nearby, until the chemical is thoroughly flushed away by rains, which gives dyers woad seedlings the ability to outcompete other plants, and contributes to seed bank formation. 

Dyers Woad will establish in rocky soils, along road sides, gravel pits, levees and railroad rights-of-ways. From there it has the ability to spread by seed to well vegetated pastures, forests, waterways, hayfields and crop lands. 

Dyers woad is known as a source for blue dye (indigo) since the 13th century and it was cultivated till the 1930’s in England. Indigotine, the blue dye chemical, is located in the leaves. 

Dyers Woad will establish in rocky soils, along road sides, gravel pits, levees and railroad rights-of-ways. From there it has the ability to spread by seed to well vegetated pastures, forests, waterways, hayfields and crop lands. 

Mowing is not considered an effective treatment due to re-sprouting from the crown. However, hand pulling may be very effective in reducing infestations. It is critical to remove the crown to prevent re-sprouting. Hand pulling is easiest after the plants have bolted, but should be done before seed set. Most hand pulling programs have indicated it is necessary to follow up for several years to prevent re-infestation 

• The selective, translocated active ingredient 2, 4-D (used in Weedmaster) is the most effective treatment. Plants should be treated in the seedling to rosette stages. Late season control of flowering plants is difficult and may not eliminate seed production. 

• When using herbicides, read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product names and formulations may vary from year to year.) 

• A native rust pathogen is effective in preventing seed or fruit production and seems to be spreading naturally to new populations. 

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels