Dyer’s woad is among a rogues' gallery of problematic invasive weeds in the Intermountain West, including far northern California. Although it can be controlled by properly timed herbicide applications prior to seed set, it continues to spread at an alarming rate along roadsides, fencerows, and ditchbanks, as well as on cropland, rangeland, and natural areas.
Dyer’s woad outcompetes native grasses in natural areas, and infests grain and alfalfa fields and pastures. It is an aggressive competitor and often forms dense monocultures. While not toxic to mammals, most livestock and wildlife do not readily graze on dyer’s woad. Like other perennial mustards, dyer’s woad can be difficult to control because of its early rosette growth habit and deep root system.
The epicenter of dyer’s woad trouble in California is the Scott Valley of Siskiyou County, where it is locally referred to as Marlahan mustard. Until a decade or two ago, it was primarily confined to Scott Valley, but it has subsequently spilled over into Shasta Valley and continues to spread throughout Siskiyou County and into Modoc, Shasta, and other northern California counties.
A weed is often defined as “a plant out of place,” and dyer’s woad certainly fits that bill. Dyer’s woad has successfully invaded and colonized extensive areas of California, Oregon, Utah, Wyoming and Montana. Dyer’s woad is a B-listed noxious weed in California.
During medieval times, dyer’s woad was perhaps the most valuable plant commodity in Europe. Dyer’s woad was cultivated in southeastern Russia as a source of blue dye as early as the 13th century and was first introduced to the eastern United States by colonists late in the 17th century for the same purpose. However, its dye is now only of very minor importance in the U.S.
Research during 2012 to 2014 has shown that some viable seeds are formed earlier than expected. Preliminary herbicide trials with glyphosate or 2,4-D applied at late bloom or during early- to mid-seedpod maturation showed that such applications may be too late.
IPM Plant Pathologist and Advisor Jim Stapleton, UCCE Farm Advisor Steve Orloff and Laboratory Assistant Nicole Stevens conducted a field trial to determine whether treating dyer’s woad with glyphosate or 2,4-D at bloom, early green seed pod or late green seed pod stages had an effect on seed production or seed viability.
A weed is often defined as "a plant out of place," and dyer's woad certainly fits that bill.
They found that applying an herbicide at bloom or the early green seed pod stage reduced seed production and, to a lesser extent, seed viability. However, herbicide treatments at these plant stages did not eliminate viable seed production. Since this is the ultimate goal, these late application timings are not recommended.
Additional studies were initiated to examine the feasibility of using solar tents as part of an IPM program to eliminate viable seeds on senescent skeleton plants in small stands of dyer’s woad.
Preliminary data showed moistened seed germination to be completely inhibited when exposed to 70°C for 20 minutes; 60°C for 75 minutes; and 50°C for 28 hours.
Field experiments using solar tents were conducted during summer months in Scott Valley, Siskiyou County. The trials indicated that germination of seed lots completely immersed in water could be greatly reduced in solar tents.
Solar tents can reduce germination of dyer’s woad seed. As part of an overall IPM program, use of solar tents could slow the rapid spread of dyer’s woad to new areas, potentially leading to reduced herbicide use. Solar tents have the most potential in remote or rural residential areas with low weed numbers where it is impractical to use herbicides or to hand pull weeds and take them to a site where they can be destroyed. Reducing herbicide use saves money and lessens the risk of harm to the environment. Stapleton’s research is also increasing our knowledge of IPM for rangeland and natural area weeds.