The marsh marigold is ubiquitous across Britain, and extends over much of the northern hemisphere, in marshes and wet woodlands. Marsh marigold is a perennial that dies back in autumn, growing from buds close to the soil surface in the late winter. This hairless, fleshy plant with shiny, dark-green, kidney-shaped leaves blooms in late winter and early spring; each golden-yellow flower is saucer-shaped and up to five centimetres in diameter. The generic name derives from the Latin name for a yellow-flowered plant with a strong scent, whilst the species epithet means 'of marshes'.
In marsh marigold flowers there are no petals. The attractive function of the flower is provided by petal-like sepals. Inside the flower is a dense ring of pollen-producing stamens that surround a cluster of flattened, horn-shaped carpels. Under visible light, the sepals appear to be a uniform yellow (rarely white or purple). However, under ultraviolet light, that part of the spectrum perceived by insects, the base of the sepals is almost black. The contrast created produces guide marks to attract insects to the flower's nectar and pollen rewards. Marsh marigold is pollinated by a wide range of different insects including beetles, bees and flies. Marsh marigolds do not produce seed when selfed.
The carpels eventually develop into funnel-shaped dry fruits which split along the edge closest to the centre of the flower. The effect is to produce a little cup filled with small, black, slightly spongy seeds. It is thought that when water droplets hit the centre of the cup, seeds splash out. These seeds float in water for short distances before they are washed ashore.
Given its vast circumboreal distribution, it is hardly surprising the marsh marigold is a highly variable species. There is much variation in features such as plant size and form, leaf morphology and flower colour. People have naturally wanted to try and name this variation, which has contributed to a plethora of scientific names at the species and subspecies levels. Modern classifications group the variation into five varieties of Caltha palustris based on sepal colour, plant form and habitat.
The marsh marigold has been a popular garden plant for hundreds of years. As early as 1648, Jacob Bobart the Elder was growing two forms of the marsh marigold in the Oxford Physic Garden; the wild type with a single row of sepals, and a double form where stamens are transformed to sepals.
Primack RB 1982. Ultraviolet patterns in flowers, or flowers as viewed by Insects. Arnoldia 42: 139-146.
Smit PG 1973. A revision of Caltha (Ranunculaceae). Blumea 21: 119-150.