On the test plots in Tubb Canyon last Friday, McDonald pointed out clusters of mustard sprouts. They crowded around a native chuparosa shrub, sheltering under its branches. They also encircled the few, fragile wildflowers that had sprouted in the dry, cool spring.
Borrego Springs has received only about half of its normal rainfall in recent months, McDonald said, so the mustard hasn’t had a banner year. The native wildflowers haven’t, either.
“This year isn’t as awful, but if you walk around, there’s a lot more mustard than wildflowers,” McDonald said.
Going back years, Sahara mustard isn’t the only interloper to have messed with the state’s ecosystems. About 600 “naturalized” invasives have taken root in California soil, said Jon Rebman, curator of botany for the San Diego Natural History Museum.
The mustard also isn’t the worst invasive species, said Minnich at UC Riverside.
For example, he said, Spanish explorers extolled the vast prairies of wildflowers they spotted in their travels and then promptly introduced wild oats and black mustard, which decimated coastal flower blooms before the Gold Rush. Subsequent pioneers brought in bromes, spiky grasses that displace native flora in inland valleys and vex hikers — who find them stuck to their socks.
“We’ve seen waves of invasive plants ... threatening biodiversity,” said Travis Huxman, a biology professor at UC Irvine and director of the Steele/Burnand Anza-Borrego Desert Research Center.
Non-natives such as Sahara mustard run roughshod over their new environments because they are able to flourish outside the constraints of their original ecosystems.
“They all come from a different continent, and they are liberated from their natural pathogens and predators,” Minnich said. “They have free reign here to reproduce in massive numbers.”
Scientists are unsure whether the mustard will eventually reach a state of equilibrium with native plants or continue to march across the landscape.
“The big question is, is it driving extinction of some species?” Huxman said. “It’s certainly driving loss of local species from big plots of land. Whether that means we’ll lose some of those species altogether is uncertain.”
Researchers are seeking biological agents that could keep Sahara mustard in check. But they’re foiled by the plant’s relationship to valuable food crops such as broccoli, cauliflower and kale, which might also be targets of the mustard’s natural predators.
Insects brought in to counter a non-native invader sometimes work “spectacularly well,” McDonald said, and in other cases “don’t do a darned thing.”
Scientists are also testing herbicides that can cripple Sahara mustard without harming native plants.
“We keep hoping to find the silver bullet,” said Michael Rodriques, coordinator of volunteers for the Anza-Borrego Desert State Park.
In the meantime, Borrego Springs is relying on sweat equity to keep the mustard under control. Volunteers and park officials organize brigades to fight the weed, joining forces with university researchers to seek long-term solutions.
Residents take on plots individually in their spare time, logging their efforts on a smartphone app that maps mustard distribution. Service groups lend a hand, and a local school has formed an anti-mustard club, Garmon said.
“If there’s mustard there and people see it, they’re going to pull it,” said Sam Webb, chairman of an eradication task force.
Scientists and community members hope to exploit relatively dry years like this one to battle the mustard, knowing that the future of the desert is at stake.
“When they’re in full bloom, there is no Monet painting more beautiful than hundreds of acres of desert primrose, desert verbena and desert sunflower,” Garmon said. “To see those lost forever is really heartbreaking.”