How to Appreciate, and Then Eradicate, California’s Least Favorite Wildflower

If you've ever gone for a hike or driven along the 101 freeway in LA, or just scrolled through social media during springtime, you've likely noticed the stunning array of wildflower blossoms that engulf the hillsides, transforming the land into a vivid but short-lived spectacle. 

But there’s one flower that stands out in particular in early Spring, and it’s become even more of a flashpoint than the California Poppy—black mustard. From February to May it’s impossible to avoid these tall stalks of bright yellow flowers taking over the grasslands, scattered right next to a busy highway, blanketing hills, or carefully placed in between rows of grapes in vineyards.

This is Brassica nigra, a quick-growing, rapidly spreading wild plant that often shoots up into the air as high as six feet tall and thrives up and down California. With their dense thickets of dazzling yellow, they make beautiful photo backdrops, but their intrusive nature isn’t always good for California’s ecosystems. They’re an invasive species, deeply rooted in California’s history for better and for worse.

For some, they signify the dawn of spring, and chemical-free protection for their precious vineyards. For others, they’re a symbol of oppression and the colonial history of California. Here’s what you should know about this fascinating flora next time you spy it in the wild, and everything to consider about this unique plant, before, after, or during your next flower selfie.

The prevailing theory is that black mustard's arrival in California is tied to Spanish colonization in the 1700s. Legends claim these plants were intentionally scattered by Spanish missionaries to mark the way from mission to mission with a trail of flowers on El Camino Real.

This story hasn’t been officially proven, but it makes sense in context. Moreover, before the Spanish incursion there was no evidence of this flower growing in California, nor any signs of invasive plant species like this at all. The theory then suggests that the invasive plant spread further as people moved and cattle migrated. The seed traveled with the colonists and their livestock, and slowly but surely, plant life on the West Coast changed forever.

In today’s California ecosystem, many botanists and plant experts consider these blooms problematic, especially because they’re difficult to root out and aggressively expansionist—black mustard often kills off native plants in its habitat.

Senior Rare Plant Conservation Scientist Heather Schneider, Ph.D., of the Santa Barbara Botanic Garden explains how black mustard has such a pernicious influence: “[It] has two ways that it competes with native plants. First, it germinates and grows very quickly. Black mustard plants are very large (up to six feet or taller when they are thriving) and take up space and soil resources, creating conditions that make it hard for natives to germinate and reproduce,” she says. But that’s not all, according to Schneider, “black mustard's secret weapon is ‘allelopathy,’ which is basically chemical warfare. The roots of black mustard exude chemicals in the soil that inhibit the germination of seeds from other kinds of plants, preventing them from standing a chance against black mustard.”

As if this wasn’t enough, after their expansive spring bloom, the black mustard stalks die and dry out, making excellent fuel for wildfires during crispy California summers. These fires also change the ecosystem, creating conditions in which other nonnative plants thrive, further degrading biodiversity. And that degradation has a harmful knock-on effect on the many animals (and people) that depend on native plants.

But not everyone shares those ecological concerns, and in fact, some people have been able to put black mustard to good use. Brian Talley, Owner & Vintner of Talley Vineyards in Arroyo Grande, sees the problems the plants can cause—but his team does its best to work with them instead of fighting against them. They remove the plants before they can take over since it’s difficult to work with, but they also use it for its practical fumigation properties.

“I’m pulled both ways on the topic of mustard,” he says. “Currently, we mow it or use mechanical tillage to remove it... On the other hand, Talley Farms is part of the family and we plant it due to its fumigation properties. Some of the land around here harbors a soil pathogen called verticillium and mustard, being in the brassica family, has soil fumigation properties that are helpful."

Kevin Jussila, Winemaker and Owner of Kukkula Vineyards in Paso Robles, also recognizes the blooms are a multidimensional challenge. He agrees there are benefits to working with the mustard plants as natural agents against pests, and as a ground stabilizer instead of unnatural chemicals—but acknowledges the plant’s aggressive nature.

"As an organic dry farmer, it and all the other native plants make it less likely to necessitate additions to our soil during the growing season,” says Jussila.

Dry farming means using little irrigation during a dry season, a sort of location-based approach to farming—in this case, working holistically with California’s dry weather to find water naturally. Jussila explains his team tries to harness Mustard plants for good, by using it as mulch since it provides phosphorus and other nutrients for the soil, and to fight erosion on the farm’s steep slopes. He says it also helps ward off pests like the vineyard’s nematode population.

Gelert Hart, Vineyard Manager and Winemaker at Ambyth Estate in Templeton, acknowledges he’s not a mustard plant specialist but says his team relies on propagating grasses that include mustard by way of intentional animal grazing and calculated mowing and weed control.

“Though we don’t have the plant everywhere, it seems to really thrive and attracts loads of beneficial insects,” says Hart. “So, we semi-love it and tend to let [it] flourish rather than control it.”

Winemakers have put the invasive plant to work, and it’s also been a boon for tourism-focused communities in California.

Birgitt Vaughan, Director of Public Relations at Sonoma County Tourism, sees the annual spectacle's positive impact: “The blooming mustard fields create a breathtaking scene that enchants many visitors, providing a picturesque setting for capturing memorable photographs and portraits.” And, she notes, “the vibrant yellow hues of mustard flowers not only allure human admirers but also attract bees and other insects and serve as a vital source of nectar and pollen.”

Vaughan also reminds visitors that this is a gorgeous natural phenomenon, and to be respectful of the blossoms and not step on, sit on, pick, or otherwise disturb them. And if you do want to see the remarkable waves of yellow blossoms, make sure to time your visit to the mustard flower season—ideally soon, before these early-bloomers dry out.

The flowers are undeniably beautiful, a spectacle worth seeing, and they’re also a problematic invasive species. Perhaps the best bet for California’s future, then, is to appreciate black mustard for its natural beauty and for what it can give to our farmers in soil improvement and natural pest prevention, while also working to slow its spread where we can.

According to Dr. Schneider, the plant is so widespread and also so survivable and difficult to remove that eradication is already impossible anyway. The bloom might seem fleeting, but those seeds can live for decades underneath the soil, meaning it takes years of consistent time and effort to effectively treat invaded spaces.

That’s not to say that it’s hopeless, of course—ecological organizations like Friends of Elysian Park have hosted weeding parties, and they have seen some good results. The more people who get involved, the better chance we have to preserve as much space as possible for native plants to thrive.

“The displays can be quite striking, blanketing hillsides as far as the eye can see,” says Schneider. “But if the mustard wasn't there, those yellow swaths should be a patchwork of yellow, orange and purple native wildflowers, showcasing our famous floral diversity.”

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