Dianthera americana, the American water-willow, is a herbaceous, aquatic flowering plant in the family Acanthaceae native to North America. It is the hardiest species in the genus Dianthera, the other members of which being largely tropical and subtropical, and it is able to survive as far north as USDA zone 4. It is common throughout its range.
The grows partially submerged in still or flowing water, reaching up to tall from a creeping rhizome. The leaves are, opposite, sessile, linear or lanceolate, and slightly crenulated. The flowers are bicolored, borne in opposite arrangement on spikes long coming off a peduncle long. Color ranges from white to pale lavender with the upper corolla lip pale violet or white, arching over the lower lip mottled in dark purple. The lateral lobes are unadorned or slightly blushed. The anthers are purplish-red rather than the usual yellow. Flowering is from May to October.[1] The fruit of this plant is a small brown capsule.
The creeping rhizome allows Dianthera americana to form large colonies on or near the shorelines of still or slow waters in lakes and rivers, and on rocky riffles and shoals in faster flowing rivers. Its rhizomes and roots provide important spawning sites for many fish species and habitat for invertebrates.[2]
Dianthera americana is a listed threatened species on Schedule 1 under the Canadian Species at Risk Act.[3] Unseasonal and extreme anthropogenic water level fluctuations and competition from invasive species have contributed to its decline, particularly in the Rivière des Mille Îles, Quebec, and it is today represented by 13 populations in southern Ontario and southwest Quebec.[3]