Native American Uses: Yellow Marsh marigold, C. palustris L.  Leaves were considered poisonous by many tribes. Root could be put in a skin, a hot rock dropped in the skin and the decoction used to colds as an emetic (makes you vomit).  Poultice of root powdered or mashed for treating skin afflictions such as scrofula.  Leaf infusion used as a laxative.
Caltha palustris L.    Marsh marigold or American Cowslip.
  Abnaki, Chippewa, Iroquois, Menominee and other tribes boiled leaves and ate them, sometimes cooked in animal lard.  Stems were also cooked and eaten in spring.  My experience is that these plants are too bitter to eat, even after a couple of changes of water.  Anyway, come spring throughout the United States creek side, bog, marsh and lowland is blooming with marsh marigolds (also called American Cowslip).  Some say edible, I say get a life.

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Above is Caltha palustris L. found in the East and filmed in Michigan. 

Right is White Marshmarigold, Caltha leptosepala DC. ssp. leptosepala var. leptosepala filmed in a seep on the lower slopes of Mt. Baker, Washington.  Western tribes chewed plant and used it as a poultice over inflammations.