This section is from the book "A Guide To The Poisonous Plants And Weed Seeds Of Canada And The Northern United States", by Robert Boyd Thomson, H. B. Sifton. Also available from Amazon: A guide to the poisonous plants and weed seeds of Canada and the northern United States.
Other Common Names: Cowslip, Soldiers-buttons.
As a general rule animals may be trusted to avoid the Marsh Marigold on account of its acrid properties. Cases have occurred, however, of poisoning among both cattle and horses, sometimes with fatal results. The young plants are harmless, becoming toxic about the time of flowering. The symptoms resemble those produced by Buttercups. When dried in hay the plant is harmless.
Though it is an eastern species, the plant is found in moist or swampy situations as far west as Saskatchewan. It is a somewhat fleshy perennial with hollow, grooved stems, growing in clumps with abundance of large, bright yellow flowers. The sepals are coloured but no true petals are present. There are numerous stamens and ten pistils developing flattened seed pods. The leaves are rounded, with a smooth or crenate margin. The flowers are found in early spring, but may continue until June.
Caltha asarifolia DC, Caltha leptosepala DC, Caltha biflora DC. and Caltha chelidonii Greene, are western forms with similar acrid properties and grow in marshy places among the mountains. The first named species has yellow flowers, while the rest have white.