[Wikipedia] Europe Native: 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. (link added by Mary Ann Machi)

[Cal-IPC] Invasiveness: Lythrum hyssopifolia (hyssop loosestrife) is a perennial forb/herb (family Lythraceae) that invades wetlands. It occurs in seasonal wetlands, ditches, and cultivated fields, especially rice fields. L. hyssopifolia often grows on exposed mud, tolerates some salinity, but is sensitive to heavy frost. Cal-IPC Rating: Moderate (link added by Mary Ann Machi)