Rumex acetosella

Rumex acetosella, commonly known as red sorrel, sheep's sorrel, field sorrel and sour weed, is a species of flowering plant in the buckwheat family Polygonaceae. Native to Eurasia including the British Isles, the plant and its subspecies are common perennial weeds. It has green arrowhead-shaped leaves and red-tinted deeply ridged stems, and it sprouts from an aggressive and spreading rhizome. The flowers emerge from a tall, upright stem. Female flowers are maroon in color....read more on Wikipedia.