St. Patrick’s day is here, and with the plastic shamrocks popping up in stores everywhere, it got me thinking about the real plant, which grows everywhere around the world, and is under attack with a vengeance by the city government in San Francisco. Oxalis, Sourgrass, Wood Sorrel, Bermuda Buttercup, Shamrock, and False Shamrock – these are just a few names for a genus of wildly prolific edible plants (aka “weeds”) which grow everywhere around the world. Even if you aren’t familiar with the name of this plant, you’ve likely encountered the clover-like leaves and pretty yellow wildflower of oxalisin a lawn before; it infiltrates grassy areas everywhere, street medians and even sidewalk cracks in cities ranging from New York and Cape Town to Sydney and San Francisco. Children love to eat it and play with it, and most school kids are familiar with “sourgrass”. In January and February, entire hillsides in San Francisco burst in vivid yellow bloom with Oxalis flowers. Whether this is a problem or not depends on who you ask. Many San Francisco residents see the hillsides of bright yellow flowers as a beautiful first sign of spring, whereas others, especially those who espouse a nativist point of view, see this plant as an “invader” that must be stopped at all costs – even when that environmental cost includes dousing entire hillsides in dangerous pesticides such as glyphosate and triclopyr.

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Yellow Wood Sorrel, Oxalis, Bermuda Buttercup, Sourgrass, lemonade grass

It’s oxalis season in San Francisco right now, which means that many San Francisco gardeners are waging a war against this prolific little weed in their backyards. It also means it is Garlon season for San Francisco’s Park and Rec Department. Garlon (chemical name Triclopyr) is a broadleaf pesticide weed killer that is used by San Francisco’s Recreation and Parks department mainly for the purposes of killing oxalis. Very little research has been done on this chemical, but it is known to be toxic to mammals and possibly carcinogenic – specifically correlated with breast tumors in rats. Glyphosate (Monsanto’s Roundup), another popular herbicide for killing oxalis, has been classified by the WHO as a probable carcinogen. In the past few months the city of San Francisco has sprayed Garlon on hillsides in public parks several times to try to eradicate oxalis; below are a few videos of these most recent offensives taped by the San Francisco Forest Alliance:

Native plant advocate Jake Sigg (former president of the California Native Plant society and spokesperson for the San Francisco Natural Areas Program) recently spoke at a public hearing on pesticides about how he thinks San Francisco needs to use much more herbicide to try to eradicate oxalis, simply because it is such a challenging task:


“Yellow oxalis is almost unstoppable –you have to kill that corm, that bulb, the only way you can do it is with herbicides. It’s impossible to do it manually. I wished I’d brought pictures of San Bruno Mountain where they sprayed the entire mountainsides of oxalis. That’s the only way they got rid of it there. I hate to hear all this unwarranted fear about herbicides. I was a gardener all my life, and I’ve used herbicides and I’m 88 now. I’ve used a lot of them, and it would seem if they’re really that bad I would have problems by now!”


Honeybee on oxalis flowers, honeybeed on yellow wood sorrel, honeybee on sourgrass

What Sigg doesn’t mention in this quote is that there are many pollinator species, including honeybees, bumblebees, and other types of butterflies, that forage on oxalis nectar during the winter time of year when no other flowers are blooming, and could be harmed by the herbicides sprayed on these flowers. So, in the interest of trying to protect one butterfly species (the Mission Blue Butterfly), San Francisco’s Park and Rec department has apparently decided it is an acceptable tradeoff to poison other pollinators that are important to local ecology and human agriculture.

In aBay Nature Magazine article, Doug Johnson, executive director of the California Invasive Plant Council says there is just no point in trying to wage war against oxalis. “It’s not a target for landscape-level eradication because it’s just way too widespread,” he said.

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In oxalis’s case, the benefits that would accrue from fighting it on all fronts aren’t quite enough to justify the costs—there’s just not enough time or people to dedicate to the effort. (Not to mention that eliminating oxalis takes a doggedness that even Sigg describes as “fanatic.” He managed to eradicate it from his garden, but it took him five to six years, and he sometimes had to comb through his plants by hand.) Instead, Cal-IPC focuses its efforts on the battles that can be won: new, potentially dangerous weeds that can be stopped, or existing weeds that threaten valuable resources.


I visited some of the areas that had been sprayed with Triclopyr recently and the results were not impressive. The fields of bright yellow flowers were not gone, just missing in little patches here and there. It is easy to see how it will immediately grow back.

Two species of oxalis in San Francisco, Oxalis oregana, oxalis pes-caprae, redwood sorrel, South African Wood sorrel, invasive wood sorrel, natiive wood sorrel, invasive bermuda buttercup

The question around what is “native” and “non-native” seems like an arbitrary and potentially slippery debate as it often taps into deeply held xenophobic sentiments about what is valuable, and what should be allowed to thrive in a given location. That said, I find the discussion around “native plants” versus “invasive species” to be particularly fascinating and confusing when it comes to oxalis. It is often the claim of native plant advocates in any location, that oxalis is an non-native invader that needs to be eradicated.

The truth is that oxalis grows all around the world, and there are many species of oxalis that are native to California, including Oxalis Californica (Yellow Wood Sorrel), and another forest-dwelling species with whitish lavender flowers called Oxalis Oregana (Redwood Sorrel). Oxalis has been growing in California for thousands of years, and the original native people of this country – the American Indians – widely ate both its leaves and bulbs. There is a species of oxalis from South Africa (Oxalis Pes Caprae), which is the invader that native plant advocates will tell you that they are doggedly fighting in San Francisco parks, but to an untrained eye (like mine), this plant looks exactly the same as the native yellow oxalis. When I was living in New York City, we had a yellow flowered oxalis “weed” growing everywhere that looked pretty much the same as both of these other species, but naturalists in that area called it Oxalis Stricta, which is native to North America.

Oxalis species comparison, oxalis stricta, oxalis californica, oxalis pes-caprae

I suppose plant experts can distinguish between these different types of oxalis, but can your average gardener or pesticide applier? And what specifically makes a native plant “a weed”? It seems there is no scientific definition of the word “weed” – it is just a term used to designate prolific plants that reproduce quickly and sprout up in locations where they are not wanted. And in public lands – who determines if a plant is wanted or not? That is the heart of the fierce battle now waging between native-plant advocates and anti-pesticide activists.

Oxalis Pes-Caprae (South African Oxalis) reproduces underground with little teardrop shaped bulbs, so just killing one plant doesn’t kill the underground bulb, which just spreads and pops up somewhere else – much to the dismay of gardeners who like to keep their gardens oxalis free. This plant is literally everywhere – including sidewalk cracks and highway medians, so it really is impossible to get rid of. And is that necessarily really a bad thing, I would ask?

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Wood Sorrel doesn’t just have aesthetic value with its sunny yellow flowers, but is also useful as an edible plant. I first learned about this cute little weed from renowned New York City foraging guide Wildman Steve Brill, and then discovered my kindergartner was picking and eating it every day at school in New York City. “Oh that stuff? We call it sourgrass, mom” he told me.

bermuda buttercup, yellow wood sorrel, oxalis, Preschool sourgrass picnic, kids playing with sourgrass, kids eating

Now that I live out in San Francisco, both of my children are very fond of oxalis and encounter it every day; in our backyard, surrounding sidewalks and parks in our neighborhood, and at their outdoor schools. We see both the native lavender variety (Redwood Sorrel) and the yellow flowers. Both of my kids are in an outdoor forest school in San Francisco’s parks, so they spend their all of their days playing in nature. Kids are naturally drawn to the vivid yellow flower, and I’ve found them making buttercup daisy-chains, using sourgrass as currency in some complicated grade school game, and, of course, chewing on it. I am personally concerned about pesticide use on oxalis, mainly because San Francisco’s “sourgrass” is in my children’s hands and mouths on a daily basis, and I don’t want them ingesting cancer-causing pesticides.

false shamrocks, edible oxalis, oxalis, sourgrass, bermuda buttercup, pesticides in san francisco, san francisco oxalis

As soon as the weather gets warmer than about 70 degrees, which happens by April, the Oxalis withers and dies back until next season. So, what is the point – I would argue – of wasting money, time, and damaging our local ecosystem with poison, in order to wage a futile war against this useful, beautiful and clearly unstoppable plant.

What are your thoughts on oxalis? Experience with this plant? I’d love to hear about it in the comments!

+ Why it’s okay to love oxalis and to stop poisoning it

+ A history of the little yellow flower that is everywhere