"To stop or slow the spread of this newly invasive plant in California,
we must quickly develop effective management tools
and an informed management approach."

Stinkwort (A) is a late-season winter annual. The aromatic leaves (B) have sticky hairs covered in resin. Flowering (C) occurs late summer, in response to day length.

With several weather related events, fire, extreme heat, etc., there has been an increase of undesired species growing on rangelands. Invasive species out compete with the more desired species such as grass, forbs - forage for grazing animals, both livestock and wildlife.  Once of these species is Dittrichia graveolens (L.) Greuter, commonly known as Stinkwort, is an invasive species that is expending on lrnaglends in Sonoma and Marin Counties. 

Stinkwort is not considered as a palatable species to animals and can be poisonous to livestock and cause contact allergic dermatitis in humans. Minimizing the impact of this noxious weed requires a good weed management plan. It will be important to begin a control program for this and other invasive species given our recent fire. Noxious weed can thrive in the burned areas, and the recent fire exposed ground surfaces, reduced shade and increased light, creating a flush of nutrients. All of these conditions favor this weed.

In California, this weed is found primarily along roadsides. However, the biology of this annual plant suggests that it could also invade open riparian areas and overgrazed rangelands. Stinkwort has an unusual life cycle among annual plants: unlike most summer or late-season winter annuals, stinkwort flowers and produces seeds from September to December. Such basic biological information is critical to developing timely and effective control strategies for this rapidly expanding weed.

Stinkwort seeds are likely spread by wind, on the fur and feathers of mammals and birds and on motor vehicles and equipment, thus moving along transportation corridors. Stinkwort has very high seed viability, with an average of about 90% of the seeds capable of germination at the time they disperse from the plant. While the primary expansion has moved radially from the original infestation in Santa Clara County, it has been present in Sonoma County since early 2000.

What makes stinkwort's life cycle rather unusual is that it matures much later in the season than most annuals, even other late-season winter annuals. For example, yellow starthistle begins to send up a flowering shoot (bolt, referred to as moderate growth in April, begins flowering in late June, and — like most late-winter or summer annuals — has completed its life cycle by September or October. In contrast, stinkwort begins to bolt in mid-May, grows most of its branches and leaves between June and September and flowers and produces seeds from September to December. What makes stinkwort's life cycle rather unusual is that it matures much later in the season than most annuals, even other late-season winter annuals.

Aside from the tarweeds, there are few other late-season winter annual species with a similar life cycle in the native California flora. Some other weedy species, such as Russian-thistle, horseweed (Conyza canadensis (L.) Cronquist) and yellow starthistle (Centaurea solstitialis L.), have similar life history strategies, with only Russian-thistle and horseweed flowering within the same time frame as stinkwort. In contrast to stinkwort, Russian-thistle is a summer annual that germinates in spring.

Stinkwort can grow roots as deep as yellowstarthistle andvirgatetarweed, this occurs several weeks after these other grassland annuals grow their roots. Thus, it may be that stinkwort will not be a significant invasive plant of rangelands, except in years when there is significant late-season rain or when competitive winter annual species are removed by overgrazing. Nevertheless,stinkwort has also been observed in open riparian systems, where water is not a limiting factor and a slow-growing shallow root system will not limit its competitive ability. It is possible that this non-native species may eventually become a problematic weed in these more-open wetland areas.

Current control methods.

The challenge in controlling stinkwort is applying the appropriate management at the proper time. Although traditional methods of control, including mechanical and chemical techniques, can be effective, determining the most appropriate timing has been difficult. If management actions are not taken before plants begin to produce seeds in the fall, there is a risk that they will help disperse seeds rather than control stinkwort populations. For example, mowing may move seeds on equipment for long distances when conducted too late in the season. Unfortunately, a poor understanding of the biology of this plant and of how to control it effectively have led to unsuccessful management of growing infestations as well as much wasted time and money. However, management tools that prevent seed production for 1 to 2 years have the potential to greatly reduce the soil seed bank and, thus, the population size.

Biological or cultural practices.

There are currently two biological or cultural practices that can be employed to limit the ability of stinkwort to invade an area. One is to minimize disturbances such as overgrazing and soil manipulation in natural andrangeland sites. Second, pastures should be managed for dense, competitive stands of desirable perennial or annual grasses that maximize ground cover in spring, when stinkwort seedlings are beginning to establish.

Mechanical practices.

Mechanical control options can take advantage of the stinkwort root system, which is slow growing and initially relatively shallow. Plants may be controlled by hoeing or pulling. However, because stinkwort can cause dermatitis, it is important to wear appropriate protective clothing (long sleeves, long pants, gloves) to minimize exposure to the irritating oils. Once in flower, stinkwort plants should be bagged and removed from the site to prevent seeds from maturing and dispersing after the plants have been cut and left on the soil surface. Mowing can provide partial control when conducted late in the season. However, buds remaining on branches below the level of the mower may regrow. Mowing a second time can give improved control, especially when conducted after the soil has dried out in mid- to late summer. In contrast, mowing too early, as is done on highways to reduce the threat of grass fires, will favor stinkwort by removing competing annuals while this weed is still small and lower than the mowing blades.

Postemergence herbicides.

Postemergence herbicides are applied to small germinated seedlings or young plants. Thus, in contrast to preemergence herbicides that are generally applied to larger areas before seeds germinate, postemergence applications can directly target known infestations visible to the applicator. However, the sticky oils on the foliage, especially on mature plants, make it difficult to control stinkwort with postemergence herbicides. To overcome this, it may be necessary to use ester formulations of postemergence phenoxy-type herbicides (2,4-D, MCPA, triclopyr, etc.). However, these compounds are more volatile compared to salt formulations (commonly used in summer), and some should not be applied when ambient temperatures will reach or exceed 80°F.

In University of California experiments conducted for the postemergence control of stinkwort, it was found that the salt formulation of triclopyr at 24-ounce acid equivalent (a.e.) per acre (2 quarts Garlon 3A per acre) gave the most effective level of control following a postemergence application. Triclopyr is selective and relatively safe on grasses, but it must be used cautiously around vineyards, as grapevines are extremely sensitive to triclopyr drift. It is also important to note that control with postemergence herbicides is most effective when plants are young, actively growing and not exposed to stresses such as drought. For stinkwort, this is generally just before or at the time of bolting.

Glyphosate (Roundup Pro) at 1 quart product per acre also gave fairly good control, and anecdotal information from other land managers indicates that a rate of 2 quarts product per acre gives control similar totriclopyr at 2 quarts product per acre. Unfortunately, other herbicides, including aminopyralid (Milestone) and aminocyclopyrachlor (one of the active ingredients in herbicide Perspective), did not provide effective late-season post-emergence control of stinkwort. As previously discussed, plants also partially recovered from late-season mowing.

Pre- and early postemergence herbicides.

Because stinkwort germinates throughout the rainy season, the most effective control options are likely to be broadleaf selective herbicides with both pre- and early postemergence activity, which can control both new germinants and young emerged seedlings. A fairly new group of foliar- and soil-active growth regulator herbicides have proven very effective in winter and spring applications for control of yellow starthistle and other members of the sunflower family (Asteraceae). These herbicides have the ability to control both emerged young plants through foliar activity, as well as germinating seedlings through soil activity. These chemicals include clopyralid (Transline), aminopyralid (Milestone) and aminocyclopyrachlor, and they are generally safe on grasses. In preliminary research, UC found that winter applications of aminocyclopyrachlor and spring applications of Milestone VM+ (aminopyralid plus triclopyr) showed the greatest potential for controlling stinkwort. Early-season application of glyphosate, however, controlled competing vegetation and so allowed late-germinating stinkwort to thrive. Thus, glyphosate is best used later in the season as a post-emergence application.

It will be critical to assess the impact that stinkwort is making on your rangelands. A rancher should inventory and map the impacted areas of the ranch; then develop a management strategy to control this invasive plant. Having invaded sites mapped allows ranchers to assess future expansion and can help prioritize management activities. This work also lays a foundation for future implementation of specific management methods. To stop or slow the spread of this newly invasive plant in California, we must quickly develop effective management tools and an informed management approach.

For more information on control strategies: Stinkwort is rapidly expanding its range in California by Rachel Brownsey, UC Davis, Guy B. Kyser, UC Davis and Joseph DiTomaso, UC Davis for UCANR California Agriculture magazine.