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 is rare in the United Kingdom, with occasional isolated populations. It often grows in moist habitats, such as marshes and wet agricultural fields, rice paddies, for example.
It is a mostly upright, branching annual or biennial herb growing 10 to 60 centimeters (4" to 24") tall. The oval leaves are arranged oppositely lower on the plant, and often alternately toward the top. They are up to 3 centimeters (1") in length. The inflorescence is a terminal spike of flowers with pinkish petals up to half a centimeter (¼") long. The fruit is an oval capsule containing many minute seeds.
- "Lythrum hyssopifolium". Natural Resources Conservation Service PLANTS Database. USDA. Retrieved 27 January 2016.
- BSBI List 2007 (xls). Botanical Society of Britain and Ireland. Archived from the original (xls) on 2015-06-26. Retrieved 2014-10-17.
- California Dept. of Food & Agriculture: Lythrum
- Helen Briggs (28 November 2020). "Surprise discovery of rare plant at Norfolk 'ghost pond'". BBC. Retrieved 28 November 2020.
- James Donn, Hortus Cantabrigiensis: or, a Catalogue of Plants, Indigenous and Exotic (1809), p. 5