There are uglier weeds than Wood Sorrel, (Oxalis) but not many as difficult to treat. Within a bed of ground cover plants for example, it can be disastrous.
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.