A beautiful but unwelcome tide of golden blossom is sweeping over the Santa Monica Mountains this year. The late—and light—rainy season has resulted in few native wildflowers but an abundance of mustard, an opportunistic invasive that thrives even in drought conditions and has become synonymous with spring in coastal California.
It’s hard to imagine the local mountains without the sea of bright gold and the dusty gray stalks it leaves behind, but this plant only arrived 200 years ago. The Indigenous people who lived on the California Coast for millennia before the European invaders arrived had a land management technique that involved periodic burns to control the grassland environment, promoting the growth of useful plants. The Spanish colonists and their Mexican and American successors replaced that tradition with grass-based crops like wheat and barley and forage for grazers, creating perfect conditions for the weeds they also brought with them.
The story that mustard was deliberately seeded by the missionaries along the Camino Real to mark the road between the missions is romantic fiction, but there is proof that mustard arrived in California with the first Spanish colonialists. A 1925 study of plant material in the adobe bricks used to build the missions and mission-era ranchos identified a number of non native species that traveled to California with the Spanish and Mexican invaders. The oldest sample of mustard in this study dated to 1793-1797 and was obtained from bricks used to build Mission la Soledad in Monterey. Other invasives that arrived during the early years of Spanish occupation include sow thistle and filaree.
It is unlikely that the missionaries would have carried hundreds of pounds of mustard seed to spread along the route from southern to Northern California, not when they needed to carry essential supplies for survival, but odds are high that they brought mustard seed to grown in the mission’s gardens for food and medicine, and that mustard and other weed seeds were present among the grain seeds they brought to sow. It wouldn’t have taken long for the non-natives to naturalize in their new environment.
Mustard is native to the Mediterranean climate in Southern Europe, North Africa and the Middle East, where it has served not only as the source of mustard seed for a condiment but as a spring vegetable, and a source of medicine. It is perfectly adapted to life in California. Mustard can grow all the way up to around 7000 feet of elevation, although it is happiest at the coast. It has become the dominant plant in many areas here, and it had plenty of help. The Spanish colonists unleashed mustard on the California landscape but it wasn’t until American took California from Mexico that mustard began to spread like wildfire.
Mustard needs disturbed soil to thrive. The sudden demand for agricultural products that began during the gold rush in the 1850s converted large swaths of coastal land into agricultural use, creating ideal mustard habitat.
Mustard was widely used as a cover crop in orchards and vineyards and it was also cultivated for a modestly profitable mustard industry. Mustard rated highly enough to be included in California agricultural commodity reports in the late nineteenth century. Some spice merchants specifically advertised “California mustard” as a selling point.
By the start of the twentieth century, mustard was already ubiquitous. Then the government thought it would be a good idea to use mustard to stabilize the soil in wildfire burn areas, further spreading the invasive species into wild land areas where it did not already have a foothold.
Mustard has evolved to germinate with the first rain of the season and grow quickly even when conditions are dry, pushing out any potential competition. That’s why it was chosen to reseed burned slopes. That, and it was cheap and abundant.
A 1935 Herald Examiner news story describes how 12,500 pounds of mustard was spread over a 2,500-acre area in the Altadena in 1935 following a wildfire. “Because of the recent rains, the seed will sprout rapidly and will provide temporary protection against erosion,” the article states.
The goal was to prevent mudslides, but mustard also increases fire risk, because it dries out as fast as it grows, leaving a forest of tinder-like stalks once its flowers and goes to seed, and those seeds are highly fire resistant, enabling rapid regeneration after the next fire, and the next one. And once mustard is established, it is extremely difficult to remove. The practice of seeding mustard after wildfires wasn’t discontinued until the late 1950s in Southern California. A 1956 article, also in the Examiner, boasts that a helicopter was being used to sow “mustard and rye seed” in the Verdugo Hills following a wildfire. The aerial approach enabled more seed to be broadcast over a wider area. A 2004 US Forestry Service paper on the practice of postfire seeding concludes that mustard was discontinued not because of its environmental impact but because it was interfering with downslope agriculture.
The practice of discing, or plowing, undeveloped land and the verge of millions of miles of roadway every year for fire clearance has also contributed to the spread of mustard. Few native plants will grow on disturbed ground, but mustard thrives there and it has a powerful weapon to ensure it doesn’t have competition. Many members of this family produce allelopathic chemicals that poison the soil, preventing the competition from growing or even germinating. A 2020 study found that black mustard produced chemicals that inhibited the growth of a wide range of non mustard species, not just potential competition from plants like grasses. Mustard has another defence. Its leaves and flowers, while edible and a good source of vitamins C and K, contain oxalic acid which prevents many animals—including humans—from being able to digest it in quantity.
Numerous non native members of the brassica family have colonized the local mountains, ranging from the wild radish, Raphanus sativus, with its pastel pink and white flowers, to the tiny, delicate-appearing shepherd’s purse, Capsella bursa-pastoris, with its heart-shaped seed pods, but there are two main mustard species that dominate the local landscape. Brassica nigra, or black mustard—named for the color of its seeds—is the bright golden flower that covers entire hillsides in a dazzling sea of yellow. It can grow to be more than eight feet tall in just a couple of months when conditions are right.
Hirschfeldia incana, hoary mustard, is shorter and sparser than black mustard, but it is still incredibly invasive and abundant. Its flowers are less golden—a faded yellow instead of a brilliant glow—and it has grayish, hairy leaves.
No one has anything poetic to say about hoary mustard: it’s tough, bitter, deep rooted, prickly when dry, and impossible to eradicate once it is established, but there are plenty of romantic stories about black mustard. People have written poems about it and glowing descriptions used to lure East Coasters to the West. Fields of mustard flowers were one of the things used to sell the myth of the perfect “Golden State”. Mustard flowers were a popular decoration at balls, teas, and other society events in the early twentieth century, and the spring mustard bloom remains a popular attraction in Napa Valley, where the plant is still used as a cover crop for the wine industry, and its vivid golden flowers are celebrated at the Napa Valley Mustard Celebration each spring.
While rolling hills of yellow delight the eye, here in Southern California mustard is a bane not a blessing. It’s also a reminder that things change—sometimes with appalling speed, and not necessarily for the best. We can’t change the past, but we can learn from it.
As beautiful as the flowers of mustard are, this is one family of weeds that deserves no mercy in the garden. Uprooting mustard before it goes to seed is the only way to stem the tide. Hand-pulling is more labor-intensive than using a string trimmer, but it prevents seeds from spreading. Seeds should be disposed of in the garbage, not the compost pile where they can survive high heat and still germinate.
Black and hoary mustard leaves and flowers are edible in moderation. The seeds can be gathered, winnowed, and ground up to make the spice mustard (try using an old pillowcase that won’t be missed to gather, dry and crush the seed pods, and wear gloves—those pods are sharp!). It’s a lot of work but there is a certain satisfaction in it. These seeds, at least, will not contribute to the mustard problem in the Santa Monica Mountains.
