A few short years ago, when we first moved to Curbstone Valley, there was no Oxalis pes-caprae to be found anywhere on the property.  None.  I was almost smug about it, as I’d hear of friends closer to the coast constantly battling the thick choking mats of this plant in their gardens each spring.

Oxalis pes-caprae

Oxalis pes-caprae is also known as sourgrass, goat’s-foot, Cape sorrel, or Bermuda buttercup.  This species is native to the Cape region of South Africa.  While it’s at home in Diana’s garden, it simply doesn’t belong here. Now a common sight in California’s orchards, vineyards, and agricultural fields along our Central Coast, it’s gained a firm foothold here due in part to our mild climate. The California Invasive Plant Council (Cal-IPC) lists Oxalis pes-caprae as a moderately invasive species in California [1], and its range continues to spread.

Oxalis pes-caprae

If we didn’t previously have Oxalis pes-caprae, how did it get here?  Well, it’s at least in part my own fault that this plant found its way to the farm, and a perfect example of how a gardener can carelessly, unknowingly, move invasive plant species around.

These plants first arrived here as stowaways in one gallon containers brought onto the property from a University plant sale we attended soon after we moved here.  Clearly, unbeknownst to us at the time of purchase, the soil surrounding some Ribes viburnifolium that we acquired contained small bulblets of this invasive species.

When the leaves first appeared, we didn’t think much of it at first.

The clover-like leaves of Oxalis pes-caprae

We have various clover-like weeds and plants here on the property, including bur clover (Medicago polymorpha), creeping wood sorrel (Oxalis corniculata), and our native, and very beautiful redwood sorrel (Oxalis oregana).  However, as soon as these plants bloomed there was no question we’d been invaded.

Emerging flower buds of Oxalis pes-caprae

You don’t have to drive far around coastal California at this time of year to see lawn after lawn, flower bed after flower bed, completely entombed in Oxalis pes-caprae.  I used to find the gently nodding swaths of yellow flowers each spring to be charming, even in the most unkempt gardens, but as I’ve come to know this plant better I’ve realized it’s a sinister wolf in sheep’s clothing, and now very much regret that it has found its way to Curbstone Valley.

Buds of Oxalis pes-caprae

Impact

Why, of all the weeds here, is this species of concern?  In cultivated gardens O. pes-caprae can quickly establish itself and rapidly choke out other ornamental annual and small perennial plants.  I’ve seen a number of gardens within Santa Cruz County that are wall-to-wall with this plant, swallowing anything in their path, with stray plants drifting into adjacent properties.

Oxalis pes-caprae

Extensive swaths in farm land can become potentially harmful to livestock.  Oxalis pes-caprae contains oxalates and,if consumed in significant quantity, may result in oxalate poisoning.

This species can grow almost anywhere, in the poorest of conditions, as evidenced by this plant growing on the face of a concrete wall.

Oxalis pes-caprae has no trouble growing between the joints of this concrete wall

Regionally, one of the most alarming impacts in recent years is the invasion of this plant into California’s native coastal dunes.  In fragile dune habitats Oxalis pes-caprae is successfully excluding native plant species, enriching and stabilizing soils, and altering nutrient cycling, making the dunes unfavorable to re-vegetation with native plant species [2].

Reproduction

Part of the reason this species is so successful is that it spreads primarily via undetected underground bulbs.

Bulblets of Oxalis pes-caprae are already forming on plants here this year

Each bulb potentially can produce more than 20 bulblets each year, resulting in explosive growth and colonization in just a few short seasons, which accounts for the increased spread in just this small area in the past two years.

By the time this plant begins to bloom, the bulblets have already begun to form

Oxalis pes-caprae has apparently not been documented to form viable fruits/seeds in North America [3].  However, when it arrived at Curbstone Valley, it was presumably contained within the containers of Ribes viburnifolium.  I didn’t plant the Ribes immediately, and noted a few O. pes-caprae that grew and bloomed within the 1 gallon pots, so I waited until after the following spring before finally planting out the plants in the orchard, in an effort to clear the soil within the containers of this weed.  Subsequently this infestation has occurred, not where I planted the Ribes in the orchard, but where I held the plants that year near the coop.  It’s difficult to believe that the Oxalis that has grown there since could have resulted only from one or two bulbs falling through the containers, but if this species doesn’t set seed here, that must be the case.

This Oxalis pes-caprae is growing where the infested plants were held prior to transplanting elsewhere

Control

So now what do we do?  Reportedly the best control for Oxalis pes-caprae is prevention.  “Don’t move soil from an infested site to one that is free of the weed“.  Well, for us that’s clearly too late, it’s here, and it’s spreading.  We now need to stop it before it spreads further, and escapes into our surrounding woodland.

Oxalis pes-caprae

We don’t use herbicides here, and hand weeding has only limited effectiveness, as removing the top of the plant doesn’t kill the bulbs.

Handweeding alone is rarely successful

We’re fortunate thus far in that the infestation of this plant is in a relatively small area, in a strip approximately 10 feet long x 4 feet wide. Repeated pulling of the plants, preferably before bloom, helps to deplete the bulb’s nutritional reserves, but this method is time consuming, and can take years to be completely effective.  I pulled the Oxalis last spring, but the area of infestation has actually increased this year, so I likely pulled the plants too late, after they had set new bulblets.  We now need to redouble our efforts if we’re to be effective at eradicating this species from the property.

Oxalis pes-caprae

Additional control can apparently be obtained by sifting the soil to remove bulblets, unfortunately, where this particular patch is growing, the soil is very shale-like, which will make sifting challenging.  An additional method that may in fact be more beneficial here, is soil solarization with transparent or clear plastic [4]. The area this plant has colonized is fallow, so after hand-pulling all visible plants this weekend, our next step will be to attempt to solarize the soil, hopefully limiting regrowth next spring.

We hope to see less of this next spring...

If Oxalis once again rears is pretty yellow head next spring, we’ll repeat the process of hand-pulling, followed by solarizing, and later replanting with some native plant species.  We’re crossing our fingers that this method will prove successful, but recognize it may take a few growing seasons to bring this invader under control.  We’ll let you know how it goes.

——————

[1] California Invasive Plant Council – Oxalis pes-caprae

[2] Nature Conservancy Global Invasive Species Team Weed Alert – Oxalis pes-caprae

[3] Ornduff, R. 1987. Reproductive systems and chromosome races of Oxalis pes-caprae L. and their bearing on the genesis of a noxious weed. Annals of the Missouri Botanical Garden 74: 79-84

[4] UC-IPM – Soil Solarization for Gardens & Landscapes