“Five Reasons it’s Okay to Love Oxalis – and Stop Poisoning It”

The San Francisco Forest Alliance has published an excellent article about the pointless and harmful attempt to eradicate oxalis in San Francisco’s so-called “natural areas.”  We are grateful to SFFA for their outstanding research and permission to republish the article. 

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 Tier I herbicides targeted at them in San Francisco’s so-called “Natural” Areas.

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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 Areas Program (NAP) of the San Francisco Recreation and Parks Department (SFRPD), which uses the herbicide, Garlon (triclopyr) to battle oxalis despite its dubious efficacy for the purpose.  San Francisco’s Department of the Environment classifies Garlon 4 Ultra as Tier I: Most Hazardous. It’s listed as HIGH PRIORITY TO FIND AN ALTERNATIVE (their caps). Since oxalis is the main reason NAP uses Garlonthe alternative we propose is – don’t use Garlon or anything else on oxalis.

An article on SaveSutro.com, based on a detailed study by the Marin Municipal Water Department, describes some of the issues with Garlon:

  • Garlon “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 female rats also had more failed pregnancies.
  • Rat and dog studies showed damage to the kidneys, the liver, and the blood.
  • About 1-2% of Garlon falling on human skin is absorbed within a day. For rodents, it is 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 Garlon 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.
  • Garlon can persist in dead vegetation for up to two years.
Natural Areas Program uses Garlon on oxalis
Natural Areas Program uses Garlon on oxalis

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 comes 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.


Honeybee on oxalis flower
Honeybee on oxalis flower

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 HERE in a 2-page PDF note from UCLA’s Barry A. Prigge and Arthur C. Gibson.)

Bumblebee on oxalis flower
Bumblebee on oxalis flower

In fact, a recent 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.

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).

Butterfly on oxalis flower
Butterfly on oxalis flower

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 Garlon 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.


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.

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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.

coyote pouncing in oxalis field. Copyright Janet Kessler
coyote pouncing in oxalis field. Copyright Janet Kessler
coyote in oxalis field. Copyright Janet Kessler
coyote in oxalis field. Copyright Janet Kessler


The article says that oxalis leaves “bare ground during the six months of the year oxalis doesn’t flower.” That’s not true either.

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oxalis 10The spectacular yellow bloom of the oxalis – valuable because 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 grassland.

oxalis 11


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 by non-native grasses. The change occurred over 100 years ago, when these grasses were planted for pasture. So the grassland that NAP is defending with herbicides is primarily non-native anyway.

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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.


Children love oxalis, both for its pretty flower and for the sour taste of its edible stems.

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oxalis 14Even 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 Garlon to it is probably a bad thing.

Photo credit: Badjonni (Creative Commons – Flickr)
Photo credit: Badjonni (Creative Commons – Flickr)


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 like Garlon to control it is weak. We call on NAP to stop using Tier I and Tier II herbicides altogether.


Why are native plant installations often failures?

We have been watching attempts to eradicate non-native plants and replace them with native plants on public lands in the San Francisco Bay Area for over 15 years.  Few of these efforts have been successful.  Non-native plants are repeatedly eradicated, then natives are planted.  Within months the natives are dead and non-natives have returned. 

The few projects that are successful are usually fenced, irrigated, and intensively planted and weeded.  Few managers of public lands have the resources to achieve success.  We have identified here on the Million Trees blog many reasons why attempts to return native plants to places in the Bay Area where they have not existed for over 100 years are often failures.  Many of those reasons are related to the changes in the environment:

  • Higher levels of CO2 and associated climate change are promoting the growth of non-native plants.  A USDA weed ecologist (1) studied the effects of higher temperatures and CO2 on the growth of non-natives (AKA weeds) by growing identical sets of seeds in a rural setting and an urban setting with higher temperatures and CO2 levels.  Seeds grown in the urban setting produced substantially larger plants with much more pollen and therefore greater reproductive capability.
  • The growth of non-native annual grasses is encouraged by higher levels of nitrogen in the soil found in urban environments as a result of the burning of fossil fuels. (2)

 The methods used by the projects undermine success

With the exception of the project on Mount Sutro in San Francisco, all of the projects use herbicides to eradicate the non-native plants and trees.  Most of the non-native trees will resprout if their stumps are not sprayed immediately with herbicide and this must be done repeatedly to kill the roots of the trees.  Many of these herbicides persist in the ground for years and probably suppress subsequent plant growth.

This problem is illustrated by a USDA study of the effects of a one-time aerial spraying of herbicides on grassland after 16 years.  Although the herbicide is assumed to “dissipate” within a few years, the negative effect on the natives persisted 16 years later:  “…the invasive leafy spurge may have ultimately increased due to spraying.  Conversely, several desirable native herbs were still suffering the effects of the spraying,,,” 

Anyone who is familiar with native plant restorations in the Bay Area knows that most are covered in a thick layer of mulch.  When tree removals are required for a project, the mulch is usually composed of the chips of the trees that have been cut down.  The projects of UC Berkeley for which UC is applying for FEMA funding (based on its claim that the clear-cutting of all non-native trees will reduce fire hazards) say specifically that the clear-cut areas will be covered with 24 inches of mulch composed of the chips of the destroyed trees. 

The UC Berkeley projects also claim that native vegetation will return to these clear-cut areas without being planted based on an assumption that the seeds of native plants are dormant in the soil.  One wonders how these seeds would be able to germinate when covered with 24 inches of mulch, or how the sprouts could penetrate it.  Their proposal contains the fanciful suggestion that squirrels will plant the acorns of oaks in the mulch, which may be true of the oaks, but is an unlikely scenario for the many other native plants and trees which UC claims will populate their “restorations” without being planted. 

Chips of destroyed trees, UC Berkeley project.  Photo courtesy Hills Conservation Network
Chips of destroyed trees, UC Berkeley project. Photo courtesy Hills Conservation Network

These heavy mulches prevent native bees from nesting in the ground, as most native bees do.  This reduces the population of pollinators which are essential to the germination of a new generation of the plants.  If long-term sustainability is the goal of these projects, an environment that is friendlier to pollinators would be helpful.

Lack of horticultural knowledge is also handicapping these projects

One of 5 native oaks that survive on Tank Hill 10 years after 25 oaks were planted.
One of 5 native oaks that survive on Tank Hill 10 years after 25 oaks were planted.

The managers of these projects often display a profound ignorance of basic horticultural knowledge.  For example, we have seen them plant natives that require full sun in the deep shade of trees where they will not survive.  We have seen them plant native trees that will not tolerate wind on the slopes of windy hills, only to watch the trees wither and die.

The managers of these projects are apparently unaware of the fact that hundreds of species of California native plants require fire to germinate their seeds and that most of the population will die within 5 years of the fire. (3) These are examples of such “pyroendemics” that sprout after a fire and are almost entirely gone within 5 years:

 Keeley - pyroendemics

UC Berkeley and East Bay Regional Park District do not plan to plant any natives after eradicating non-native plants and trees.  Their plans say that they expect seeds that are dormant in the ground to sprout when the ground is cleared of non-native plants.  Unless they set fire to that ground, many seeds will not germinate and most of the plants that are germinated by that fire will disappear within 5 years unless another fire germinates another generation of plants.  

UC Berkeley does not use prescribed burns on its property.  East Bay Regional Park District (EBRPD) conducts only a handful of small prescribed burns every year, which they claim are solely for the purpose of reducing fuel loads.  In an article about prescribed burns conducted by EBRPD, the District’s “Resource Analyst” is quoted as saying, “’This is not a restoration project.  Our primary goal is fuels reduction.’” (4) Ironically, both of these owners of public lands claim that their objective in the eradication of non-native plants is to reduce fire hazard, yet they are trying to reintroduce a landscape that is dependent upon fire for survival. 

The Natural Areas Program in San Francisco has never conducted a prescribed burn and the DRAFT Environmental Impact Report for their plan says they do not intend to do so in the future.

New and growing evidence that soil is altered by plants

 In addition to these issues which have contributed to the failed attempts to reintroduce an historical landscape to the San Francisco Bay Area, we are reporting today on a new issue.  Plants can change the microbial composition of the soil, including mycorrhizal fungi which have symbiotic relationships with plants. 

Researchers tested soil for changes in composition after just three growth cycles.  Several species of non-native annual grasses were grown in native soils.  They reported that the non-native species reduced the population and changed the composition of the mycorrhizal fungi, which reduced the ability of native species to establish and persist in modified soils. (5)

The Berkeley Meadow is a 72-acre native plant garden on a former garbage dump on landfill.
The Berkeley Meadow is a 72-acre native plant garden on a former garbage dump on landfill.

These changes in the soil were observed after only three growth cycles.  Our local projects are attempting to eradicate plants which occupied the soil for more than 100 years.  In some cases such as the former garbage dumps in the East Bay on landfill, the soil was never occupied by native plants.  Surely, the alteration of soil composition is a likely factor in the failure of attempts to turn these properties into native plant gardens. 

How many more decades and how much more taxpayers’ money must we spend on these projects before land managers acknowledge their failures? 


(1)    Lewis Ziska, USDA Scientist, Beltsville, MD

(2)    US Fish & Wildlife, Recovery Plan for Mission Blue butterfly

(3)    Jon Keeley, et. al., Fire in Mediterranean Ecosystems, Cambridge University Press, 2012

(4)    Wendy Tokuda, “Taming the Flames,” Bay Nature, July-September 2012

(5)    Nicholas Jordan, et. al., “Soil-Occupancy Effects of Invasive and Native Grassland Plant Species Composition and Diversity of Mycorrhizal Associations,” Invasive Plant Science and Management, October-December 2012