Lythrum hyssopifolia (orth. var. L. hyssopifolium) is a species of flowering plant in the loosestrife family known by the common names hyssop loosestrife and grass-poly. It is native to Europe but it is known elsewhere, including parts of Australia and eastern and western North America, as an introduced species and sometimes a weed. It often grows in moist habitat, such as marshes and wet agricultural fields, rice paddies, for example. This is a mostly upright, branching annual or biennial herb growing 10 to 60 centimeters tall. The oval leaves are arranged oppositely lower on the plant, and often alternately toward the top. They are up to 3 centimeters in length. The inflorescence is a terminal spike of flowers with pinkish petals up to half a centimeter long. The fruit is an oval capsule