Oxalis is one name for members of the Oxalidaceae family, commonly called woodsorrel, sorrel or shamrock. These small herbaceous plants spread quickly, and resemble clover, but are more tender and lighter in color. There are several varieties native to Europe and the United States, including creeping woodsorrel and common yellow sorrel, along with other, more regional, varieties. They are often planted as attractive groundcover, and are not always easy to eradicate, as they form underground mats with bulbs and roots.
Killing oxalis is best done with targeted herbicides like Oxalis-X or Weed-B-Gon; if these are unavailable, general herbicides like Roundup, Ally and Escort also work. Mix them according to the manufacturer's instructions.
Add dish detergent to the herbicide for more stickiness and thorough application to the small plants. A few tablespoons of detergent for a gallon of herbicide mixture is enough.
Spray the herbicide mixture onto the plants just before they flower, before new bulbs have formed. This may be from late fall to early summer, depending on the climate and oxalis variety. Repeat a few days or weeks later if needed.
If chemical controls are ineffective or undesirable, try solarization. Lay down black plastic sheeting before the start of the growing season, covering the oxalis infestation completely. Some gardeners prefer to use a layer of cardboard covered with wood chips.
After the plants are dead or the cardboard and mulch are decomposed--after at least one entire growing season--seed a competitive planting over the top. The oxalis should be unable to come back with new growth.