Short-lived perennial, often behaving as an annual or biennial in cultivation, from the family Phrymaceae, native to the western part of North America, from coastal regions to the Rocky Mountains. Naturalized in Europe, it has spontaneously established itself on numerous banks and seepages, notably in Great Britain and France. It colonizes stream banks, springs, wet rocks, and marshy meadows, sometimes at altitude.
It forms upright clumps with hollow stems, reaching 20 to 50 cm depending on conditions, with bright green, fleshy foliage, with slightly hairy, toothed oval leaves. The plant willingly branches and can spread in very humid environments.
The flowers are bilabiate, of a bright and clear yellow, with inside the throat spots and reddish-brown dots whose arrangement is extremely variable from one individual to another. Some plants bear almost uniformly yellow flowers, others display large burgundy patches almost invading the entire corolla. In its natural habitat, flowering extends from May to August. In cultivation, it can bloom from May to September.
It requires constantly moist to waterlogged, rich soil, in full sun to partial shade. It reseeds spontaneously and abundantly as soon as conditions suit it, thus ensuring its presence from one season to the next without particular intervention.