Rumex acetosella, commonly known as
sheep's sorrel,
red sorrel,
sour weed and
field sorrel, is a species of flowering plant in the buckwheat family Polygonaceae. 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.
(Wikipedia)