Oxalis, commonly known as Wood Sorrel, is hardly one of the ugliest of weeds. On the contrary, many species are favored perennials in garden beds, with their delicate, clover-like leaves, and attractive blooms. Yet one species of Wood Sorrel, Oxalis pes-caprae, can be in certain circumstances, one of the most annoying and difficult weeds to eradicate.Oxalis has two properties that make it particularly problematical. One is the vigorous network of bulbs that it develops, rendering ineffective, hand or mechanical weeding. Secondly, while most perennial weeds are active during the summer, the primary growing season for Oxalis is the winter. Why is this a problem?It is often the case that perennial weeds need to be treated with a systemic herbicide in order to eradicate them. The most suitable systemic weed killers available to gardeners are those based on glyphosate, such as Roundup. The trouble is that Roundup’s systemic properties (that is the capacity to reach down to the plant’s roots) are only effective during warm weather, and when the weeds are actively growing. Considering that the plant is mainly active during the cooler months of the year, it follows that the bride and the groom; in this case Oxalis and Roundup, “miss” each other.Wood Sorrel is not a disaster in all situations. If it grows in a small flowerbed, hand weeding as a part of routine maintenance, will not eliminate the weed, but keep it under reasonable control. As mentioned before, the plant is not unattractive.Heavy infestations in a lawn can be more disagreeable, unless mowing is carried out very frequently – say every three days or so. If eradication is the goal, then it is possible to apply a selective herbicide, that poisons broad-leaved plants (like Oxalis) without damaging then lawn. The trouble is that the weed killers in this category, which are based on the dreaded 2-4-D, evaporate easily, and by so doing, can seriously damage neighboring plants, including those in adjoining properties. While their use may be reasonably safe where there are no plants in the vicinity, in the average private garden, they are liable to be very dangerous.Without a doubt though, Oxalis comes into its own as a noxious weed, in areas of the garden planted with low ground covers. If it is growing with the ground covers, it is virtually impossible to get rid of without damaging the desired plants. It is vitally important therefore to eliminate the Oxalis prior to planting. However, here it is worth remembering the case of the unhappy bride and groom who discover that they are unsuited to each other!What should you do in the summer for instance, if you were planning to plant ground-hugging plants, in soil that may contain Oxalis bulbs? The answer is to wait for winter and the sprouting of the Oxalis weeds, and then treat them with the afore-mentioned selective herbicides, which unlike Roundup, are effective in cool weather. Needless to say, the work should only be carried out by a qualified professional.