Highway patrolling: hand removal of stinkwort (Dittrichia graveolens) on Highway 24, Contra Costa County

Stinkwort (Dittrichia graveolens) was first reported in 1984 in Santa Clara County and has now spread to 41 of California’s 58 counties, with most records in the Bay Area (CalFlora 2016). The current California Invasive Plant Council (Cal-IPC) assessment of its invasiveness is “Moderate” statewide, with the additional designation of “Red Alert” for rapidity of spread. Stinkwort is still spreading, and impacts may not yet be fully realized (Cal-IPC 2010). 

This Mediterranean annual is generally avoided by grazing livestock. It is toxic to sheep and horses, and can taint the meat and milk of cattle. In humans, it causes contact dermatitis (rash), producing blistering and itchiness equal to that of poison oak.

Stinkwort produces copious seed with three dispersal mechanisms—the dandelion-like pappus; barbed hairs on the achene (seed); and sticky resin all over the plant which may allow seeds to stick to dispersal agents. Stinkwort spreads rapidly along highway corridors and trails. Vehicles, especially construction vehicles, are important vectors in stinkwort spread. 

I first noticed stinkwort along Highway 24 in late 2007 and decided to try to limit its spread into nearby wildlands. I can’t rid the whole world of undesirable weeds, I thought, but maybe I can do something around where I live.

The only technique I use is pulling the plant up by its roots. Plants even beginning to flower can set mature seed, so if it’s past about Labor Day, plants need to be bagged and disposed of. The seed bank doesn’t last long, so if we pull up all stinkwort for a couple of years the infestation drops to near zero or even disappears. 

CalTrans provides safety equipment and a limitless supply of trash bags and promptly picks up bags filled with stinkwort debris. I get similar cooperation from the Town of Moraga, Cities of Orinda and Lafayette, and the County Public Works. Most large infestations we’ve found on public land require only minor cleanups after a year or two, but we have to keep after new infestations as soon as we find them. Many private landowners are also very cooperative.

Here’s a shout-out to some people who are also helping to clean up stinkwort: Janet Gawthrop, Sandy Greenwald, Bob Sorenson, Sally de Becker, and my family, plus many others.

You can help, too! Stinkwort season has started. You can join me, or I will work with you to get things started in your part of the world.

Each year, We pulled up stinkwort by the roots or used loppers to cut at or slightly below ground level. Plants with any open flowers or maturing seeds were placed in trash bags, the bags tied shut, and stacked for CalTrans pickup. 

During 2007-2009, most plants were flowering and all plant material was bagged. In 2010, most work was earlier, August 11-September 15. Virtually all stinkwort was vegetative and could be left in place after pulling. Plants with even a few flowers open were bagged, as uprooted stinkwort appears capable of setting seeds from open flowers and possibly buds. 

Results

1.  Areas where infestations were removed effectively had very much reduced populations of stinkwort in subsequent years. Although seed viability is reported for two or three years, the great majority of seed appears to germinate the first year.

2.  The appearance of significant new infestations on the visible right-of-way led us to “source” populations not visible from the highway itself. Most removal work is now focused there. 

3.  Most large “source” infestations were associated with parking or staging areas where vehicle activity has the potential to facilitate the spread of stinkwort. 

4.  Roughly 60 to 80 person-hours were spent each year. 

5.  Work will continue, hopefully at a somewhat reduced level. New introductions will continue to occur, and some local infestations cannot be accessed, such as along the BART tracks and on private property, or are yet to be discovered.

Incidental Observations

1.  One infestation was not entirely removed in 2009 and the site was not mowed or grazed in 2010. Where grass was dense, the resulting stinkwort infestation was much reduced.

2.  Many new infestations on Highway 24 were associated with off-pavement driving.

3.  Late spring mowing for fire hazard reduction does not reduce late-developing stinkwort.

4.  We saw no vertebrate herbivory on stinkwort whatsoever. Argentine ants and western fence lizards used it for cover. Yellow jacket wasps, usually dead, were occasionally observed at the bases of plants. No pathology was observed, except at one site. 

5.  Stinkwort decomposes very slowly. Skeletons are visible for at least a year, and stacked piles of pulled plants are also visible for a year or two.

Words to the Wise for Weed Warrior Wannabes

1.  Hand pulling is best before flowering, i.e., before mid-September, depending on site.

2.  Train your crew. Stinkwort is variable in form and can be mistaken for Salsola, Conyza, Hemizonia, Madia and a variety of other, less-noxious plants.

3.  Wear long sleeves, long pants, gloves. Protect wrists. Ivy Block or similar preventive may help.

4.  If any flowers are open, bag the plant and tie the bag shut! Stinkwort apparently can mature seeds after uprooting from open flowers and possibly buds as well.

5.  Clean up right away with Technu or thoroughly wash with soap and water. 

6.  Rubbing alcohol removes black, tarry residue from gloves, boots, and tools.

 

Conclusions

1.  Ecological Value: In the world of photosynthetic organisms, stinkwort comes about as close to zero as possible for habitat value.

2. Short term: Hand pulling stinkwort can significantly reduce the spread of this noxious weed, especially in light infestations.

3.  Prevention: Vehicles appear to be the main vector for dispersal along the highway. Construction vehicles are especially effective long-distance vectors. Sanitation protocols could include:

a.  Avoid off-pavement driving.

b.  Clean vehicles before and after driving in infested areas. 

c.  Do not move soil into or out of infested areas, and do not accept contaminated fill. 

d.  Remove stinkwort from corporation yards, and monitor and promptly control re-introductions.

4.  Range Management: Limited observations suggest that stinkwort is a poor competitor against Mediterranean annual grasses such as Avena and Bromus. Resting a pasture or greatly reducing grazing pressure may be an effective way to reduce stinkwort abundance. 

5. Policy: Because of its strong rash-producing effect, this weed should be prevented from spreading to parkland and open space.  It tends to grow on open bare soil at trail edges where it can contact unsuspecting recreationists.

Barbara Malloch Leitner

Stinkwort (Dittrichia gravoleons). Photo by Joseph do Thomaso, USDA

Stinkwort (Dittrichia gravoleons). Photo by Joseph do Thomaso, USDA