This is an updated reprint of a May 2015 article. As of 2023, eight years later, Natural Areas (now called “Natural Resource Areas”) are still being sprayed with powerful pesticides against oxalis. It’s still futile.
Five Reasons it’s Okay to Love Oxalis – and Stop Poisoning It
(May 11, 2015; updated March 8, 2023)
The oxalis season is over, and the perky yellow flowers have vanished for another year. These Bermuda buttercups will be back next year to herald the spring, bringing joy to those who love them, irritation to those who hate them, and powerful herbicides targeted at them in San Francisco’s so-called “Natural” Areas.
THOSE WHO HATE OXALIS AND WANT TO POISON IT WITH GARLON
These flowers are so visible in spring that Bay Nature magazine did an article about them in March 2015: A Natural History of the Little Yellow Flower that’s Everywhere Right Now. It quoted Jake Sigg, the retired SF Recreation and Parks gardener who is considered the doyen of San Francisco’s native plant movement. He hates oxalis pes caprae, which he considers extremely invasive. The article quotes him as saying that, without intervention, “in X many years Twin Peaks would just be one solid mass of yellow, and there wouldn’t be any other plants there…” The article suggested that an oxalis-dominated landscape “drives away coyotes, hawks and owls that feed on grassland foragers, and the situation is especially dire for endangered Mission blue butterflies, which depend heavily on native wildflowers.” Most of those ‘facts’ about oxalis are mistaken as we’ll explain below.
Mr Sigg’s theories align with those of the Natural Resources Department (NRD) of the San Francisco Recreation and Parks Department (SFRPD), which uses the herbicide triclopyr (among others) to battle oxalis despite its dubious efficacy for the purpose.
An article on SaveSutro.com, based on a detailed study by the Marin Municipal Water Department, describes some of the issues with triclopyr (with the brand name Garlon):
- It “causes severe birth defects in rats at relatively low levels of exposure.” Baby rats were born with brains outside their skulls, or no eyelids. Exposed adult females rats also had more failed pregnancies.
- Rat and dog studies showed damage to the kidneys, the liver, and the blood.
- About 1-2% of the pesticide falling on human skin is absorbed within a day. For rodents, its absorbed twelve times as fast. It’s unclear what happens to predators such as hawks that eat the affected rodents.
- Dogs may be particularly vulnerable; their kidneys may not be able to handle triclopyr as well as rats or humans. Dow Chemical objected when the Environmental Protection agency noted decreased red-dye excretion as an adverse effect, so now it’s just listed as an “effect.”
- It very probably alters soil biology. “Garlon 4 can inhibit growth in the mycorrhizal fungi…” ( soil funguses that help plant nutrition.)
- It’s particularly dangerous to aquatic creatures: fish (particularly salmon); invertebrates; and aquatic plants.
- It can persist in dead vegetation for up to two years.
First, a little about the actual natural history of oxalis. This plant doesn’t set seed in California, and spreads entirely by sending out roots and forming little bulbils (like tiny potatoes) underground. It’s usually found where the soil has been disturbed by activities such as road-building, gardening, or trail-building. In some cases, the disturbance come from landslides or something similar. It can’t stand frost. If we do nothing, it would tend to die down rather than spreading uncontrollably.
In disturbed landscapes, it can spread fast. For this reason it can be a nuisance in gardens. People don’t want to leave their gardens alone for years to let nature take its course with the oxalis, and not every garden design includes brilliant yellow as the dominant color for a few weeks. The only way to eradicate it in the short term is to dig it out carefully every time you see it, and make sure you get most of the bulbils. Or use strong herbicides, which may not work.
In a natural landscape, though, it’s a different story and here’s why.
1) OXALIS IS GOOD FOR BEES AND BUTTERFLIES
Oxalis is actually an excellent plant for bees and butterflies. When blooming, it provides “copious nectar.” In fact, it generously gives away its nectar. Since it doesn’t set seed, it doesn’t benefit from pollinators – but it’s a food source for honey bees, bumblebees and butterflies. (You can read a rather technical description of the plant in a 2-page PDF note from UCLA’s Barry A. Prigge and Arthur C. Gibson.: oxalis_pes-caprae_ucla_santamonicas )
In fact, a 2014 study shows that plant communities with exotic plants had more plant species as well as more pollinators, that pollinators didn’t prefer native plants, and that even some specialist pollinators depended on introduced plant species. [ Journal of Ecology – 2014 – Stouffer Cirtwill & Bascompte – How exotic plants integrate into pollination networks ]
It’s true the Mission Blue butterfly needs (native) lupine as its nursery plant. (It doesn’t depend on any other native wildflowers – only three varieties of lupine. Incidentally, one of the key nectar sources for the Mission Blue butterfly is an invasive non-native Italian thistle: Carduus pycnocephalus).
Lupine has been planted on Twin Peaks as NAP attempts to reintroduce the Mission Blue butterfly there. But lupine is also a plant of disturbed areas, which means that NAP must maintain it or it will die out as the area stabilizes. They have to keep planting it, weeding, and trimming the grass around the lupine patches to make it attractive to the butterfly. An SFRPD report on the reintroduction project said “unmanaged habitat deteriorates quickly.” Presumably, they don’t use pesticides near the lupine patches, since it would likely kill that too. Despite what is implied in the Bay Nature article, it’s not oxalis that’s the issue. The real problem is another native plant, the coyote bush which takes over grasslands in a natural succession.
2) OXALIS IS GOOD FOR WILDLIFE
Oxalis bulbils are a food source for wildlife. Gophers and other rodents eat them. In fact, the Bay Nature article says, “Their spread is abetted by pocket gophers and scrub jays, which have been spotted carrying the bulbs and caching them in the ground—effectively planting them in new areas.”
Since gophers are a foundation species in the food web, being dinner for predators from hawks to coyotes to great blue herons, these plants actually provide habitat benefits whether or not they’re flowering, because the bulbils are there all year.
Where there are gophers, the predators follow. Like the coyotes in these pictures, which clearly haven’t been driven away by a landscape dominated by oxalis.
3) OXALIS DOESN’T LEAVE THE GROUND BARE
The article says that oxalis leaves “bare ground during the six months of the year oxalis doesn’t flower.” That’s not true either.
The spectacular yellow bloom of the oxalis – valuable because it the mass of color attracts honey bees and bumblebees – gives the impression that it’s the only plant there. But though it visually takes over the landscape when it’s in bloom, it naturally grows interspersed with grasses and other plants. Like in the picture above.
In fact, oxalis tends to enrich the soil with phosphorus, which is good for grass.
So when it finishes blooming, as it has by now – you don’t get bare ground. The picture below shows the same area as the first picture in this article – but it’s after the oxalis bloom is over. It’s a grassland.
4) OXALIS HAS LITTLE IMPACT ON “NATIVE” PLANTS
One argument – related to the ‘bare ground’ argument – is that oxalis takes over grasslands and destroys them, particularly the native grasses. However, grasslands in most of California including San Francisco are dominated non-native grasses. The change occurred over 100 years ago, when these grasses were planted for pasture. So the grassland that NRD is defending with herbicides are primarily non-native anyway.
But anyway, what’s the evidence that oxalis is actually damaging native plants?
It’s true some European studies do suggest that an increase in oxalis is associated with a decrease in native plants diversity -though whether it’s a cause is unclear. It may just be benefiting from human activities that disrupt the landscape. Another study put oxalis head-to-head with a native annual grass, lolium rigidum. The native grass tended to dominate. Their conclusion: “Oxalis is a poor competitor. This is consistent with the preferential distribution of Oxalis in disturbed areas such as ruderal habitats, and might explain its low influence on the cover of native species in invaded sites.”
The California Invasive Plant Council rates its invasiveness as “moderate,” considering it as somewhat invasive in sand dunes and less so in coastal bluff areas.
In San Francisco, every place where oxalis grows is already a disturbed environment, a mix of non-native grasses and plants with native plants (some of which have been artificially planted). Here, oxalis appears to grow happily with other plants – including, for instance, the native California poppy in the picture above.
5) KIDS LOVE IT AND IT’S EDIBLE
Children love oxalis, both for its pretty flower and for the sour taste of its edible stems.
Even small children love gathering posies of Bermuda buttercups (though picking flowers is technically prohibited in Natural Areas). The flowers are surprisingly hardy for wildflowers, and in a glass of water last quite well as cut-flowers.
The plant is edible, and its tart leaves make a nice addition to salad. People enjoy snacking on its sour stems. Besides Bermuda buttercup, it’s also called ‘sourgrass’ and ‘soursob.’ It does contain oxalic acid (as does spinach, for instance), and so you probably wouldn’t want to make a meal of it. Though in South Africa it’s made into soup.
Adding pesticides to it is probably a bad thing.
From our current evidence, there’s no sign that oxalis has a negative impact on wildlife, and plenty of evidence it’s already part of the ecological food web of our city. The evidence also suggests it’s not having a negative effect on other plants in San Francisco either. Lots of people find this flower attractive; one writer described it as the city smiling with Bermuda buttercups.
In any case, even Doug Johnson of the California Invasive Plant Council doesn’t think it’s worth attacking at a landscape level: the payoff isn’t worth the expense. Removing it from the hundreds of acres in Natural Areas isn’t as simple as eradicating it from a small yard where it’s clashing with the garden design. It requires a lot of work, a lot of powerful herbicides, a multi-year effort – and for what?
The justification for using strong pesticides to control it is weak. We call on NAP to stop using Tier I and Tier II herbicides altogether.
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