Oxalis

There are two types of Oxalis weeds found in Santa Clara County:

Creeping Woodsorrel (Oxalis corniculata)
Bermuda Buttercup (Oxalis pes-caprae) also known as Buttercup Oxalis

Information about how to control both of these weeds can be found in the UC Integrated Pest Management Program Creeping Woodsorrel and Bermuda Buttercup Pest Note.

Bermuda buttercup

This grows in a rosette pattern. It is a Southern African native, originally brought here as an ornamental in the early 1900s. Reproduction is mostly by bulbs and bulblets, but pieces of the root if left in the soil can also grow into new plants. This is what makes this plant so hard to control. All parts of it seem to be able to reproduce! Even cultivating the soil can unintentionally disperse the plants and herbicides will often leave parts of the plant below ground still able to reproduce. Oxalis pes-caprae does not produce seed.

Note that while the Pest Note (linked above) mentions solarization as a possible control, the solarization pest note says that this method is best for plants which have roots in the upper six inches of soil. Bermuda buttercup can often have roots or bulblets that go deeper and therefore may not be killed. If you don't have a lot of other plants in an area and can cover the area for at least four weeks during very warm weather, it may be worth a try.

The pest note indicates that Bermuda buttercup is not usually a problem in lawns that are mowed regularly because repeated cutting of the plant eventually depleted the bulb. We have heard from some Master Gardeners that after many years of pulling the weeds in a specific area, the plants stop producing and are eventually eradicated. Persistence is the key. Keep pulling and digging those bulblets!

Creeping woodsorrel

This is a low growing, creeping plant that reproduces by seed and by creeping above-ground horizontal stems that root at stem joints (nodes). It does not have bulbs or bulblets. The seed capsules explode when mature, spreading the seeds 10 feet or more.

Mowing, fertilizing, or irrigating to control creeping woodsorrel isn’t effective; the more vigorous the turfgrass, the more vigorous the creeping woodsorrel. Creeping woodsorrel survives and sets seed even when mowed as close as 1/4 inch. After using a lawn mower where creeping woodsorrel grows, wash or air spray the machine to remove all seeds and clippings before mowing weed-free turf.