The Stinkwort Report: Persistence Pays Off

It’s been gratifying to hear from Bay Leaf readers with questions about the invasive weed stinkwort (Dittrichia graveolens) and their interest in helping to remove it in their neighborhoods and towns. Keep it up! And let me know if you have any observations or questions (stinkwort@ebcnps.org).

It might go without saying, but here it is: if you want to get rid of a weed, remove all of it. Every last one. And because a small proportion of stinkwort seed remains viable for up to three years, you may have to remove all of it several years in a row.

It can be done. I found a number of sites this year with no stinkwort—zero—places I’ve been clearing for up to five or six years. And many more had only a handful of isolated plants where there had been hundreds or even thousands in recent years. The seed bank has finally been exhausted!

In all humility, nature plays a part. Stinkwort is a pioneering plant that thrives in open ground and with disturbance. Take away the disturbance, and other plants may outcompete, depending on conditions. Also, in even more humility, eradication might have come sooner if I had actually succeeded in removing all the plants. Even for an experienced eye they can be hard to see, and the best way to clear an area is to walk over it slowly and carefully several times.

In February 2024, the California Invasive Plant Council (Cal-IPC), State Parks, USDA, UC Santa Cruz, and Santa Clara Valley Water District hosted a workshop on stinkwort, with 175 participants. The presentations were recorded and you can watch them on the Cal-IPC website. One pro tip I learned was that land managers often return two or three times during the growing season to clear stinkwort from a site. Stinkwort seems to germinate over a long period, so a population can contain large plants and very tiny ones. Each time we clear all the plants we see—maybe 90 percent of what’s there—there still are a few left. Clearing a site several times brings the number of remaining plants down to near zero. I’ve started doing that and it really works.

Here are more pro tips, many of which can also apply to other invasive plants.

• If you see a large stinkwort all alone (or with just a few friends), it’s time to act! The fact that the plant is robust means it’s in good site conditions and it can produce a lot of seed. “All alone” could mean a new infestation. Pull it now and you get rid of one (or a few plants) with no seed production. Leave it and you will have hundreds of offspring to deal with and a developing seed bank, potentially extending the removal process for several years.
  
  
• Even when you think you have removed all the stinkwort, keep checking the site for a couple more years. Seed remains viable for up to three years. We aren’t perfect (see above); sometimes we miss a few. Also, stinkwort can become harder to see as other vegetation fills in. If you keep checking on your sites, the amount of pulling is reduced, and your patrolling will begin to resemble a sharp-eyed ramble.
  
  
• Cover your wrists to avoid that nasty stinkwort skin rash. Even with long sleeves and gloves, there still is a little exposed, tender skin between. Take a pair of old socks, cut off the toe portion and cut a hole in the heel. Pull each sock over a wrist so the top covers the end of your sleeve, then pull on the gloves. Your wrist will remain protected.

• A sharp, narrow shovel is great for loosening large, deep taproots, especially in hard soil. The weight is worth it.

• Even with small infestations, it’s worthwhile to gather your pulled stinkwort into a pile (rather than scattering it) as you weed your way through the infestation. The small extra effort makes it easier to see the plants you missed.
  
  
• If the stinkwort is flowering, it’s important to bag and remove the uprooted plants from the site because flowers on an uprooted plant can still form viable seed.
  
  
• Always ask permission to work on someone’s land. Explain that you are engaged in a community service project to remove a highly invasive weed (everyone understands that, even if they don’t share your passion), hand them a brochure (you can get an example from me), and share a few facts about why you are working on stinkwort (toxic to wildlife, avoided by wildlife, highly invasive, and gives people a rash). Explain that you noticed some on their land and ask if you can remove it. I’ve had one person say no, and dozens of people say yes. Their reactions range from bemused to deeply appreciative; I’ll take them all. If you can, get their contact information so you can reach out again next year.
  
  
• Thank everyone you work with. (Sandy, Donita, I’m looking at you!) And thank your cooperators, who make access possible. Big props for some of my cooperators: EBMUD, CalTrans, cities of Orinda and Lafayette, town of Moraga, Acalanes Union High School District, ENGEO Inc. (manager of Bellavista Geologic Hazard Abatement District), and many private property owners.
  
  

Stinkwort Look-Alikes

First, a review. Stinkwort is an annual in the sunflower family. It can grow up to three feet tall and nearly as wide, or it can assume an erect, cylindrical shape. It is many-branched, from the base and throughout the stems. The leaves are narrow, pointed, hairy, and glandular, giving off a pungent, camphor-like odor. The veins on the leaves are slightly depressed. The yellow flower heads are extremely numerous, borne at the tips of the branches. Each flower head contains roughly 10 ray flowers and 10 disk flowers. Each achene (seed) is topped with a dandelion-like puff of bristles.

Sometimes other plants look a lot like stinkwort, and sometimes they grow in similar habitats. Bay Leaf readers are probably a lot better than the general public in distinguishing stinkwort from look-alikes, but here are a few that make me take a closer look.

Horseweed (Erigeron spp.)  These non-natives are more columnar in shape with white or cream-colored flower heads concentrated near the top of the plant. Some plants are bristly throughout and some are glabrous (hairless) but they are not glandular and do not have an odor. They may grow closely intermixed with stinkwort. From a distance, these are the easiest ones to mix up with stinkwort, although they tend to be a deeper, clearer green.

Coast tarplant (Madia sativa)  This native annual also sometimes grows intermixed with stinkwort. It is similar in having abundant hairs throughout, glands and a strong odor, and yellow flower heads. Coast tarplant differs by branching more in the upper half of the plant rather than the lower half, having larger flower heads, and bearing fewer, broader leaves.

Willowherb (Epilobium brachycarpum) This native annual grows in open areas and has a diffuse branching pattern reminiscent of stinkwort. However, the leaves are glabrous (no hairs) and not glandular, and the four-petaled flowers are pink.

— Barbara M. Leitner, Corresponding Secretary, CNPS East Bay Chapter, and enemy of stinkwort
September 2024