Dyers Woad (Isatis tinctoria L. Brassicaceae)

All plant descriptions and control methods courtesy University of Nevada Cooperative Extension

Dyer’s woad, also known as Marlahan mustard, can be a winter annual, a biennial, or a short-lived perennial. It is native to Europe and invades rangeland, grain fields, pastures, ditch banks, waste areas, roadsides, and fence rows. It can also be found in orchards and in cultivated row crops. Unlike many other weeds in the mustard family, Dyer’s woad does well in undisturbed land. Its mustard yellow flower is seen as early as April, and seeds mature in June or July. A rust fungus has been found that is effective in keeping this weed in check on rangelands.

Distinguishing features:

• The plant grows 1 to 3 feet tall.
• Multiple stems can arise from the base.
• The outer flower stalks are longer than the inner ones.
• The alternate, lance-shaped leaves are bluish green with a white to cream-colored midvein.
• The upper leaves are smaller and clasp the stem with ear-like projections.
• The small, bright yellow flowers are borne in clusters in late April and May.
• Seeds are produced in abundance.
• The seed pod is 1/2 to 3/4 inch long and contains a single brownish yellow seed.
• Its thick taproot can exceed 5 feet in depth, and can produce a new stem if the original is clipped off.

Most Effective Strategy:
Dyer’s woad is a very difficult plant to control once it becomes established. Research and experience have shown that the most effective treatment is a combination of herbicides and cultural controls. Cleaning equipment and using weed-free hay limits the number of infestations. Finding Dyer’s woad in its first year of establishment and initiating control efforts then is the best opportunity for control. Metsulfuron or chlorsulfuron are effective at killing Dyer’s woad, but these chemicals remain active in alkaline soils for many years, making establishment and maintenance of competitive vegetation difficult after treatment and limiting the effectiveness of these herbicides over the long term. If the infestation is on highly alkaline soil, use repeated applications of 2,4-D coupled with efforts to stimulate growth of competitive vegetation. This may include seeding with an appropriate species and fertilization for maximum production of the competitive vegetation. A longterm effort over many years is required to have any effect on an infestation. Missing even 1 year of control severely reduces the effectiveness of past efforts.

Mechanical Control:
Repeated tilling or hand-pulling small infestations is effective. Cutting off the tops before seed is produced will reduce the spread of Dyer’s woad. Bag the pulled or cut plants, remove them from the area, and burn them.

Cultural Control:
Sheep will graze Dyer’s woad if there is no other forage available. This does more damage than good since they distribute seed and cause the plant to branch more and ultimately produce more seeds. Planting desirable vegetation after eradication of Dyer’s woad will reduce chances for reinvasion and invasion by other weeds.

Biological Control:
The rust, Puccinia thlaspeos, is used successfully to control Dyer’s woad. To use it, chop up Dyer’s woad that has been infested with the rust before it produces seed and spread the clippings over uninfested Dyer’s woad patches.

Read and follow the label. It is the law!

Chemical Control:

2,4-D amine or ester
Rate: Use 2 pounds a.e. per acre.
Time: Apply 2,4-D in the spring prior to flower bud formation. Repeated herbicide applications may be necessary during the growing season whenever green leaf material is present (every 4 to 6 weeks). When Dyer’s woad is present in cereal crops, apply 2,4-D before the cereal crop reaches the boot stage.
Remarks: When possible, use 2,4-D on Dyer’s woad populations before cultivating fields in the spring. Reapply 2,4-D in the fall if new weed growth occurs.
Caution: It is not lawful to apply more than 2 pounds a.i. per acre per year. Avoid drift to crops, especially with a volatile ester formulation. Follow the label directions. Do not contaminate water unless using a product specifically formulated for use in water.

Metsulfuron (Escort®)
Rate: Use 1/2 to 1 ounce per acre.
Time: Apply from emergence to the full bloom stages of growth.
Remarks: Use a nonionic surfactant to increase the herbicide’s effectiveness. In alkaline soils, above pH 7.5, reestablishment of competitive vegetation may be difficult due to residual herbicidal action on germinating seedlings.
Caution: Only apply to pasture, rangeland, and noncrop sites. Follow the label directions. Do not contaminate water.

Chlorsulfuron (Telar®)
Rate: Use 0.75 to 2 1/4 ounces per acre of chlorsulfuron (1 to 3 ounce of Telar® per acre).
Time: Apply during preemergence or early postemergence when plants are actively germinating or growing.
Remarks: Use a nonionic surfactant to increase the herbicide’s effectiveness. In alkaline soils, above pH 7.5, reestablishment of competitive vegetation may be difficult due to residual herbicidal action on germinating seedlings.
Caution: Only apply to noncrop sites. Chlorsulfuron is a selective broadleaf herbicide that also affects many grasses. Follow the label directions, especially as they apply to herbicide persistence in the soil and effect on any competitive vegetation. Do not contaminate water.

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