http://en.wikipedia.org/wiki/Oxalis A characteristic of members of this genus is that they contain oxalic acid ( whose name references the genus), giving the leaves and flowers a sour taste which can make them refreshing to chew.[3] In very large amounts, oxalic acid may be considered slightly toxic, interfering with proper digestion and kidney function. It should be noted, however, that oxalic acid is also present in more commonly consumed foods such as spinach, broccoli, brussel sprouts, grapefruit, chives, and rhubarb, among many others.[4] General scientific consensus seems to be that the risk of sheer toxicity, actual poisoning from oxalic acid, in persons with normal kidney function is " wildly unlikely".[5] While any oxalic acid-containing plant, such as Oxalis, is toxic to humans in some dosage,[6] the U.S. National Institutes of Health note that oxalic acid is present in many foodstuffs found in the supermarket and its toxicity is generally of little or no consequence for people who eat a variety of foods.[7]
When pulling, make sure you get the bulbs/bulblets/bulbils, which is the primary reason it is hard to eradicate this kind of oxalis. For chemical control, one can try herbicides with glyphosate (e.g. Roundup) or at least 8% triclopyr (e.g. Ortho CCO) as the ingredient. Make sure you read the labels carefully as glyphosate is non-selective (i.e. kill all) and triclopyr is selective (i.e. kill some including some warm season grasses). The key is multiple applications because the bulbs may not be killed with one application, or even if they are killed, there may be detached bulbs underground that may grow later on, so it might be a multi-year project. Only apply those herbicides when you see leaf growth.