The Colonial Roots of the Black Mustard Plant

In 1968, California made its official state nickname the "Golden State," referencing both its gold rush history and the colorful native poppies that festoon its rolling hills each spring. But if you didn’t know better, you might think the designation was inspired by the proliferation of another bright yellow plant that dominates California’s landscape — Brassica nigra, aka black mustard.

Thick stands of this quick-growing plant, which can shoot up to six feet tall in a matter of months, thrive in the state’s low-lying and coastal areas, and can be found growing along hiking trails, freeway medians, and abandoned lots throughout the spring and early summer. But while black mustard’s vibrant flowers do make for a good selfie backdrop, it turns out this annual "super bloom" is not so super after all — at least not for the region’s native ecosystems.

Introduced by Spanish colonizers, mustard is an invasive species in the West, and its spread is intrinsically linked with the development, land use changes, and environmental shifts that have happened since California’s mission period. Like many of the legacies of colonialism, it continues to have harmful impacts on the plants, animals, and people that are indigenous to the region today.

A widely-cited story claims mustard was planted intentionally by Spanish missionaries, who scattered it along El Camino Real as they traveled, marking the path from mission to mission with a trail of bright yellow flowers. Citing a legend passed down from her grandfather, Napa Historical Society founding member Ivy Loeber claims it was Father Junipero Serra who scattered the seeds as the missionaries traveled north, creating a "ribbon of gold" they followed back south the following year.

"So wherever you see the Spanish mustard in California, you know the Spanish fathers visited there," wrote Loeber.

The use of mustard seeds for missionary purposes does seem apt — in Christianity, the plant’s voracious growing habits have long been a metaphor for religion’s spread. The "Parable of the Mustard Seed," which appears in several books of the gospel, compares God’s kingdom to mustard’s seeds, which start out small and humble but have the potential to grow into massive plants that are "greater than all the herbs." However, definitive proof that Serra himself is responsible for California’s first mustard crop is hard to come by.

What we do know is that there is no evidence of mustard growing in the region before the padres arrived. In fact, there were essentially no invasive plants in California at all prior to Spanish colonization. A study of plant matter found in adobe bricks from historic structures in California determined that only three invasive weeds had been introduced prior to colonization in 1769. Fifteen weeds — including black mustard — were found in bricks dating from the Mission era, and another sixteen were introduced after 1824, when California was a Mexican territory.

"We know that plants were mobile," says Tom Boellstorff, a professor of Anthropology at UC Irvine who has researched the link between ecology and settler colonialism. "Indigenous peoples of the Americas moved around, and birds pooped and carried [seeds] around. But they were much more ecologically linked, [whereas] colonization was a [more intensive] planting and agricultural project."

Shifts in land use to prioritize agricultural production also made it easier for mustard and other invasive plants to spread in a post-colonial California. During the gold rush of 1849, demand for beef in the state skyrocketed, and swaths of land were converted into pastures for cows. Thomas Gillespie, a geography professor at UCLA, created a map of black mustard’s range in Los Angeles County. He found a link between historic grazing sites and the spread of mustard, which thrives in soil that has been "disturbed."

"If you look at abandoned agricultural areas, [mustard is] almost an indicator of where cattle used to graze," says Gillespie. "You can see this out in the Channel Islands, as you're driving in the Santa Monica Mountains, or in the Central Valley. Because there were no native plants to compete with it, it's just kind of gone to that mono-dominant form."

Once mustard takes hold in a landscape, it can be extremely difficult for native plants to return. It germinates earlier than other plants, grows in extremely dense patches, and its roots leech out allelopathic chemicals into the soil, preventing anything else from growing nearby.

"They're not part of the same network as a lot of native plants and other plants are," says Andrea Williams, Director of Biodiversity Initiatives for the California Native Plant Society. "Part of their success is that they're able to live on the land in a different way — to kind of suppress other plants’ growth, giving them an advantage."

When mustard dries up in the summer months, it also creates fuel for quick-burning wildfires. Before the local plants have time to regrow following a burn, mustard seeds, which can remain active in the soil for up to 50 years, germinate once again, resulting in monocultures that have a ripple effect in the ecosystem.

Birds, local pollinators, and other animals who rely on the suppressed native plants lose important habitat and food sources. And Indigenous communities who use native plants for food, art, medicine, and other cultural practices are impacted, too.

"The colonizers came in and choked out the native people here," says Heidi Lucero, chairwoman of the Juaneño Band of Mission Indians tribal council. "And now the plants that they brought are choking out the native plants here. It’s kind of a good comparison."

Lucero recently helped organize a mustard mitigation effort at a site in the Cleveland National Forest, where basket-weaving materials like basket rush and laurel sumac are gathered. Following a cultural burn the previous year meant to clear fuel weeds and help native plants grow, the mustard began to crop up once again, and a team of basket weavers, archaeologists, and U.S. Forestry Service workers descended on the site to clear it before it went to seed.

"We need to make sure that our basket materials are growing really straight," says Lucero. "That's one of the reasons it's important for us to go and remove the mustard, because it gets so densely packed in there."

Abe Sanchez, a founding member of the Indigenous foodways group Chia Café Collective, also participated in the mustard cleanup. He said the plant has made it difficult for his group to harvest native chia seeds in many wilderness areas. In recent years, he has started recommending that people interested in foraging focus on eating mustard instead of over-harvested native plants. The leaves, flowers, and roots of Brassica nigra are all edible, with a sharp, wasabi-like flavor, and the seeds can be used to make — you guessed it — mustard.

"If you pull it out when it’s young and tender — which is better, because you're gonna get it before it seeds — then it’s edible," says Sanchez. "[You can have it] sautéed with onions and garlic and a little meat in there, or just steamed like green spinach."

Because mustard has spread so far and wide in California, state and national park officials in most areas have given up on trying to eliminate it — it’s not even listed as one of the "Evil 25" invasive plant species biologists are most concerned about in the Santa Monica Mountains. However, volunteer groups like the one Lucero and Sanchez participated in continue to engage in restoration efforts in areas throughout the city.

On a clear morning in May, Emily Cobar, Community Program Director at Nature Nexus Institute, looks down at the steep hillside of the Baldwin Hills Scenic Overlook, where she has been helping to restore the area’s native habitat for six years. Every year, teams of volunteers come to the park to help remove patches of mustard and other invasive weeds, replacing them with native plants like toyon, buckwheat, and white sage.

Fully removing these mustard stands is a tedious process that can take years — each individual plant’s thick tap root must be pulled up by hand, and seeds left in the disturbed soil often sprout up again the next spring. It can feel impossible to put a stop to the endless cycle of regrowth, even on this small 57-acre patch of parkland. But for Cobar, putting in the effort is worth it. Gesturing towards a patchwork of native plants with a clump of invasive grass she pulled from the ground on the walk over, she describes how she has slowly watched the native ecosystem come back to life.

"We have a site called area E, and there is a little bit of mustard there. But two years ago during the pandemic, my coworker and I worked on that area, and we took it all out. And since then, there has been no mustard. If you actually go to the site, there's a lot of sagebrush growing," says Cobar. "To see the animals thrive in those sites with the other plants growing in — it's really nice."