Oxalis is sometimes planted deliberately for its ornamental value, but its toughness and ability to thrive in most types of soil and climate have made it a tenacious garden weed. Complete removal of oxalis is near impossible, so control is often the best option. In the United States, just about the worst type of oxalis for gardeners is oxalis pes-caprae, widely called Bermuda buttercup, cape sorrel or sourgrass. In Europe, there are three varieties that cause problems.
Solarisation is the most likely way to remove oxalis. A clear plastic sheet treated with an ultraviolet light inhibitor must cover the weed for at least four weeks during June, July, or August, so that the sun's heat can "cook" the oxalis bulbs to death.
Uproot oxalis with a garden fork and trowel just before it flowers. However, oxalis has survival techniques to perpetuate after weeding. First, it easily spreads tiny bulbs in the soil that are hard to spot. Second, small bits of stem left in the soil rapidly become new plants.
Cover oxalis plants with stiff cardboard and apply a thick layer of mulch. This will deprive the plant of sunlight and may eventually kill it.
Improve soil structure by digging and fertilising with natural compost. Oxalis prefers compacted soil, so loose, well-manured soil will deter it.
Spray weedkiller. This may seem like the easiest approach, but chemical control can kill the top growth without preventing bulb germination. Some types of oxalis, particularly those that grow in grass, are resistant to chemical weedkillers.
If chemical weedkiller is to be used, read the label carefully before buying. Go for one that has a short life in the soil. Residual weedkillers last in the soil for weeks or months and can damage the roots of other plants.
Take care when moving soil that might contain oxalis bulbs. Routine cultivation and soil recycling can spread the weed.