The Innocuous Looking Weed Saturating the Bay Area

It's spring, and the hills are alive with the sounds of … oxalis.

by Jake Sigg

It's everywhere.

••••• March 2023 •••••

Yellow oxalis, Oxalis pes-caprae, appears after the onset of autumn rains. It persists through the wet season, then disappears until the next season. Perhaps your awareness has unconsciously become part of the shifting baseline syndrome—it is so much a part of the landscape that it is part of your baseline view of the world. You can't remember a time when it wasn't here.

Yet oxalis has not always been here; it was not common a mere 20 years ago. The fact that it has increased dramatically and that there are some areas where it is the only kind of plant—what does this tell you? It says that it is displacing what was there before. In many cases, what was there was what co-evolved with the landscape—that is, the native animals and the plants they depended on.

And, because its increase is surreptitious, your baseline view is likewise gradual—and may lack awareness of the damage it is doing.

The shifting baseline syndrome anesthetizes us to the assault on California's biodiversity by its insidious spread. The grasses and wildflowers—and the animals needing those plants—will be surreptitiously transformed into a flash of yellow in winter/spring months, and then we're left with bare ground the rest of summer and autumn. No birds, no foragers.

And no solution in sight.

Biodiversity is not easy to define and is poorly understood. A quick and dirty attempt: The greatest number of biological organisms in an interacting biological community persisting over time. You may introduce a plant—like this oxalis—and that temporarily increases the diversity. However, if it expands and displaces the other plants and animals it drastically decreases diversity.

1. As with many introduced plants, the organisms that kept it in check in its native range are missing here, so the indigenous plants and animals native lack defenses against it, not having evolved in its presence. For a discussion of this and two other problem plants—Cape ivy and ehrharta grass—see Triple Threat From South Africa in the October 2003 Fremontia, online.

Worldwide, biologists and land managers are gravely concerned that the balance that provides the necessary stability to ecosystems is falling apart, and plants and animals that formerly provided the balance have weakened or displaced.

2. No other plant, in my experience, has occupied new territory and so much of it in such a short time. This mystery deepens when we learn that it doesn't set seed and produces exclusively by bulb-offset. That is highly unusual in nature—I know of no other plant doing this—and not a recipe for success in natural competitive situations.

Some plants, such as quaking aspen, seldom flower or set seed because aspen is so good at spreading via root runners that it has little need for seed. However, nature doesn't carry all its eggs in one basket, and aspen retains the ability for sexual propagation—it flowers and sets seed on rare occasions.

3. How do oxalis bulbs get around to new territories? How does it jump a hundred feet, or colonize new places miles away? How does it get to all the window boxes?

We do know that gophers cache bulbs. But more numerous, harder to explain, and more damaging are the single plants that pop up in the middle of a grassland—how did that happen? Finding this out is valuable information we need, and surprisingly, no botanists or ecologists I know of have ever remarked on it, much less researched it.

One credible witness said he had seen scrub jays deliberately move a bulblet from an above-ground mother plant and plant it in another part of his garden, so we have a beginning to solving this mystery. Two native (!!) animals promoting an exotic plant that displaces a native plant they co-evolved with? What irony. Did nature make a mistake? This deserves scientific research.

4. This oxalis not only spreads quickly, but it displaces other vegetation. Once it invades a grassland, the reproduction of existing plants slows, and by the second year will show stress. By the following year, it is in dramatic decline, subsequently becoming an oxalis monoculture. A 100-year-old native bunchgrass clump (bunchgrasses tend to live a long time, sometimes centuries) succumbing to this invasion says one thing: chemical warfare.

To my knowledge there has been no research on chemical warfare (aka allelopathy) by oxalis. There is no question in my mind but that's what is happening.

Oxalis is active only in the rainy season. When rain stops in April the plant dies down to the offset bulblets, producing a dozen or more bulblets before dormancy. That is how it multiplies.

5. Once it forms a monoculture when the rain stops and it dies down to the bulb, the soil is bare until the next rain season. People who rejoice at its cheerful beauty must consider this.

6. When native grasslands/wildflowers are displaced, they cannot be recreated; they are gone forever. It took eons to create these intricately-woven balanced communities—and we carelessly let them disappear overnight. We stand in the grim prospect of losing nearly all our grasslands in San Francisco - and many other coastal areas in California. Here is a scene from Glen Canyon in 2016, on its way to becoming a monoculture:

That is bad news for us but a tragedy for all the foragers whose lives depend on the native plants: myriads of insects and other creepy-crawlies, the birds and others that feed on the insects, and other hunters higher up the food chain—coyotes, hawks, owls and the like. These charismatic hunters are loved by people who count them and log their comings and goings. These folks are not aware of the coming danger posed by this pretty plant.

7. Controlling oxalis is a Herculean challenge. Hand removal, even on a small scale, is difficult, perhaps impossible. The deeply-set bulb must be killed, and that is challenging. The best long-term hope is control by biological agents: researching how nature keeps it as part of a balanced community in its native range. This research is expensive, rigorous, and time-consuming, and not without risk. It is already late to start research, and today US Department of Agriculture is not convinced it should be a priority. But it may be our only hope, and I will approach USDA again in view of the biological crisis oxalis has created.

There are presently no large-scale technologies other than herbicides for addressing this frightening weed. As a consequence, the City's Natural Resources Division—charged with responsibility of preserving our biological heritage—must spray in order to save our grasslands; there is no other technology. The choice is to save a whole biological community or knowingly let it perish, an intolerable thought.

8. Some waifs of the plant world survive in marginal orphaned pockets of land, like along sidewalks. These are being monopolized by oxalis. The native miner's lettuce has survived here, but is losing out to oxalis. Miner's lettuce, so numerous—even weedy—is being displaced by this oxalis. That would be tragic. Unseen, many native animals need this plant.

9. The United Nations, the United States, California, and San Francisco have all passed Biodiversity Resolutions because it is now recognized that this diversity is intimately related to climate warming, and you can't have one without the other. As for San Francisco, our native biological communities are under assault from this one oxalis—an arresting statement that deserves close attention. The San Francisco Recreation and Park Department's Natural Resources Division is understaffed and under extreme challenge from this plant, on top of assault by other invasive plants.

Each oxalis leaf has three leaflets, and the leaflet is like the imprint of a goat's foot, hence the specific epithet, pes-caprae.

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February 23, 2023