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- Name also: Double Yellow Marigold, Kingcup, Cowslip (see Primula veris)
- Family: Buttercup Family – Ranunculaceae
- Growing form: Perennial herb.
- Height: 15–40 cm (6–16 in.). Stem erect–ascending–limp, hollow, sometimes rooting from nodes.
- Flower: Perianth regular (actinomorphic), yellow, shiny (outer surface sometimes green), 1.5–5 cm (0.6–2 in.) wide. Tepals 5, round-tipped. Stamens many. Gynoecium separate, with many pistils. Flowers 1–7(–15) corymbosely.
- Leaves: Alternate, stipulate, basal leaves long-stalked, stem leaves short-stalked. Blade roundish–kidney-shaped, cordate-based, with crenate–shallowly toothed margins and palmate venation, glabrous, usually thick, sometimes thin. Stipules surround stem like a sheath.
- Fruit: Arching, brown, many-seeded, 3–19 mm (0.12–0.76 in.) long follicle forming a group.
- Habitat: Shores, ponds, springs, quiet waters in streams, ditches, wetlands, wet meadows, waterside swamps and meadows which are prone to flooding, damp hollows in broad-leaved forests, snow-bed sites, sometimes underwater.
- Flowering time: May–August.
Marsh marigold is an important sign of spring. The plant’s yellow flowers and dark green shiny leaves cheer up the otherwise barren landscape as winter recedes. Marsh marigold is familiar to Finns on ditch banks, but it is equally familiar to Australians, South Americans and even inhabitants of the East Indian archipelago. The wide distribution of the plant shows that it is ancient, as does the primitive structure of the flower. Marsh marigold is not known as a fossil, but it can be justifiably termed a living fossil. It is not however encumbered by its age because it changes and adapts to ever-changing conditions.
Apart from the subspecies palustris that is known from the shores of southern Finland, ssp. radicans grows by streams in Lapland and Kainuu. It is more delicate and spindly than ssp. palustris, and it has smaller flowers and is a paler yellow. Ssp. radicans sometimes grows submerged and can also flower underwater, in which case the pollen is probably transferred to the stigmas when it is still a bud – inasmuch as it generally needs fertilization for the seeds to be produced. The seeds of ssp. palustris have a large porous floating chamber which carries it on the waves to a new habitat; ssp. radicans seeds do not float so well, which suits a submerged plant in flowing water. Small march marigold plants that grow in small fell streams possibly represent a third form, but it is good to remember that marsh marigold changes a lot with regards to its way of growing, size, leaf form and number of chromosomes. A large group of marsh marigold subspecies have been described in Europe and it is not completely clear if the Danish version of ssp. radicans is in any way like the Lappish or northern fell and cold-water version. In southern Finland marsh marigold can also be confused with flowering lesser celandine (Ficaria verna), even though it grows lower, is mat-forming rather than growing in hummocks, and thrives in averagely dry places.