Caltha palustris (Kingcup, Marsh Marigold) is a herbaceous perennial plant of the buttercup family, native to marshes, fens, ditches and wet woodland in temperate regions of the Northern Hemisphere.
It becomes most luxuriant in partial shade, but is rare on peat. In the UK, it is probably one of the most ancient British native plants, surviving the glaciations and flourishing after the last retreat of the ice, in a landscape inundated with glacial meltwaters.
Height is up to 80 centimetres (31 in) tall. The leaves are rounded to kidney-shaped, 3–20 centimetres (1.2–7.9 in) across, with a bluntly serrated margin and a thick, waxy texture. Stems are hollow.
The flowers are yellow, 2–5 cm (1–2 in) diameter, with 4-9 (mostly 5) petal-like sepals and many yellow stamens; they appear in early spring to late summer. The flowers are visited by a great variety of insects for pollen and for the nectar secreted from small depressions, one on each side of each carpel.
Carpels form into green sac-like follicles to 1 cm long, each opening to release several seeds.
Caltha palustris is a highly polymorphic species, showing continuous and independent variation in many features. Forms in the UK may be divided into two subspecies: Caltha palustris subsp. palustris, and Caltha palustris subsp. minor.
It is sometimes considered a weed in clayey garden soils, where every piece of its root will survive and spread. In warm free-draining soils, it simply dies away.
As is the case with many members of the Ranunculaceae, all parts of the plant are poisonous and can be irritant. Skin rashes and dermatitis have been reported from excessive handling of the plant.