Sunny little pests

Growing up in the Bay Area, I have fond memories of sitting in my aunt’s side yard with a couple of cousins chewing on stems of sourgrass, whose perky yellow flowers brightened the landscape every spring.

Now sourgrass — formally known as Oxalis pes-capre, Cape sorrel or by a number of other names — is everywhere, and considered an invasive weed that crowds out native plants and wildflowers in a way that makes it hard for the natives to come back (see, for instance, this info from the California Invasive Plant Council.) It arrived from South Africa in the early 1900s and has now spread up and down the state. Once it settles in, it’s incredibly hard to eradicate, because each plant forms a number of bulbs that mature and send up plants of their own, quickly covering wildlands and gardens alike. It also spreads through contaminated soils. Two of my neighbors have been fighting a 20-plus-year war against the oxalis in their adjacent yards, and are only gradually seeing some success.

What to do?

One approach is to patiently dig them up, getting as many of the roots and bulbs as possible, as I saw a woman doing a few years ago in a big patch of oxalis at the lovely Fort Mason community garden in San Francisco. The mature bulbs are small and brown, with immature white bulbs forming alongside, as seen below.

The brown bulb at left is mature; new bulbs are forming at right.
Two freshly dug-up oxalis plants.

There’s also some evidence that persistently pulling or chopping off the above-ground parts of the plant when they’re young will gradually deplete the mature bulbs so they’ll eventually die off.

Although some of my neighbors have whole yards full of oxalis, I have only a few springing up here and there, and manage to keep them under control by diligent weeding. One thing I won’t be doing is spraying them with herbicides; that would not be The LEAF Way!

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