For the past several days, I have been pulling oxalis seedlings from my garden. There are 800 species of oxalis, some of which are desirable ornamental plants, but the species in my garden is no prize.
The pest in my garden is Oxalis pes-caprae (Bermuda Buttercup or Sour Grass), a South African plant that has naturalized in the Monterey Bay area’s similar climate. The plant has clover-like leaves and a bright yellow flower. It’s not unattractive, but it reproduces rapidly and could take over the entire garden in time.
It reproduces by multiplying its small bulbs, which are around one-quarter inch in diameter. One bulb can produce ten in a single season.
It’s theoretically possible to excavate 100% of the bulbs, but this is time-consuming and unlikely to be successful.
The most effective approach is “old bulb exhaustion,” which involves removing the top growth before it can flower. In this area, flowering occurs in February, with sunnier spots blossoming earlier than shadier spots. The idea is to deprive the old bulb of nutrients that would be provided by the leafy growth above ground, so that the new bulbs will die off.
It is best also to disrupt the new bulbs by cultivation, but this might be difficult if the pest has started close to desirable plants.
In any event, this process requires tilling to remove any new growth that appears in about two weeks. A Dutch hoe, which is good for shallow cultivation, would make short work of the new growth.
Several products are marketed as controls for this pest and other broadleaf plants: Weed-B-Gon Chickweed, Clover & Oxalis Weed Killer, Roundup, Finale, Oxalis X, etc. I do not use synthetic chemicals in my garden, because of concerns that they will harm plants, beneficial insects, microorganisms and perhaps myself and other mammals, in addition to the plants or pests they are intended to control.
There are less toxic concoctions for controlling this plant. For example, one foliar spray recipe calls for two cups of white vinegar, one teaspoon of baking soda, and one teaspoon of liquid detergent. Reportedly, this spray will kill the oxalis plant’s top growth but not the bulbs. It should not be sprayed on desirable plants.
Another control strategy is let chickens snack on this weed. Reportedly, they like it a lot.
The gardener engaged in weed control adventures can find confidence in the knowledge that no plant will survive the persistent removal of its top growth.
Finally, a frustrating encounter with Oxalis pes-caprae should not bias the gardener against less invasive species from the wood sorrel family. One of my favorite nurseries, Annie’s Annuals and Perennials, lists seven Oxalis species as desirable garden plants.
Enjoy your weed-free garden.
More information on this weed from the California Invasive Plant Council.