Dyer's Woad
Isatis tinctoria
Washington State Classification
Class A — Eradication Required Statewide
Dyers woad is a non-palatable, aggressive weed with allelopathic properties.
Quick Reference
Identification
Growth Habit
Dyers woad grows as a winter annual, biennial or short-lived perennial, ranging from 1 to 4 feet tall, from a 3 to 5 foot long taproot. The basal rosette produces stalked, bluish green leaves covered with a fine hair.
Leaves
It has basal leaves, leaves in a radiating cluster at the stem base, and stem leaves. Basal leaves are stalked, bluish green and are up to 7 inches long. Stem leaves are lance shaped, alternate and not stalked. All leaves have a cream-colored mid vein.
Flowers
The flowers are small and yellow, cross-shaped with 4 sepals, 4 petals and 6 stamens. The yellow petals are about 1/8 inch wide and just over 1/8 inch long. Flowers are found in terminal clusters on the branch tips.
Fruit & Seeds
The fruit is a flattened seed pod (silicle), 3/8” long and 1/4” wide, winged and slightly pear shaped and it hangs from a small stalk. Initially the pod is hairless and green, but it matures to a black or dark purplish brown. Each pod produces 1 seed.
Impact
Dyers woad is a non-palatable, aggressive weed with allelopathic properties. Originally cultivated as a source of Indigo dye, this species escaped cultivation and has a history of establishing across thousands of acres of western rangeland.
Ecology & Spread
Habitat
Dyers woad will establish in rocky soils with minimum water holding capacity, and the highest threat of establishment is in rangelands, pastures and forest lands. Please click hereto see a county level distribution map of dyer's woad in Washington.
Spread Mechanisms
Reproduction
Dyers woad reproduces by seed, each plant can produce from 350 to 500 seeds, and some plants produce as many as 10,000 seeds.
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Control Methods
Mechanical
- Hand pulling is the recommended control option - after the plant bolts, and before seed production. This is considered the only practical control method of hard to reach, or difficult terrain. An active mowing program will control orchard populations of dyers woad.
Biological Control
- The native rust pathogen Puccinia thlaspeos is effective in preventing seed or fruit production. The rust seems to be spreading naturally to new dyers woad populations, and it is controlling further spread (James et al. 1991). Sheep will not voluntarily graze the plant.