Marsh marigold (Caltha palustris) is a short bog or rain garden plant that grows in small clumps and is generally considered a well-behaved wetland plant. However, a similar-looking wetland plant called lesser celandine (Ficaria verna), or fig buttercup, is considered an invasive weed in Oregon and 17 eastern states stretching from Wisconsin to New Hampshire and south to Tennessee, as well as in Canada. Lesser celandine inhibits the growth of beneficial native plants, including marsh marigold.
Definition of a Weed
Regulatory agencies define weeds as plants that have the potential to interfere with agriculture or damage natural ecosystems. The Delaware Department of Agriculture's (DDA) booklet "Mistaken Identity" notes that one problem in identifying invasive weeds is that they sometimes look like beneficial native plants. Both lesser celandine and marsh marigold appear in early spring and are low-growing bog plants with heart-shaped leaves and buttercup yellow blossoms. But DDA notes that lesser celandine is so vigorous out of its native environment that it carpets floodplains in forests and threatens native wildflowers. Washington's Whatcom County Noxious Weed Control Board says that lesser celandine's dense network of tuberous roots discourages the growth of other plants.
Similar to lesser celandine, marsh marigold is a kind of ranunculus -- a name that means "little frog" in Latin and indicates that the flowers grow in wet places inhabited by frogs. Marsh marigold reproduces by vegetative division and by seed. Its roots are fleshy but not tuburous. One way to differentiate it from lesser celadine is to count petals. Lesser celandine has seven to 12. The marsh marigold has five to nine petals that are actually leaflike structures called sepals, which support the plant's reproductive parts.
Most invasive weeds are not native to the habitat they infest. Invasive plants can become aggressive due to avoiding the insect predators, diseases and plant competitors that keep them in check in their native environments. Lesser celandine is native to parts of Europe, Africa and Asia. Lesser celandine is a perennial that reproduces from seed, tuberous roots and bulblets that develop on its stems. It is vernal, which means that it blossoms and dies back in spring.
Marsh Marigold Problems and Benefits
All parts of the marsh marigold are poisonous to humans and irritating to the skin if touched. Iroquois Indians once brewed a tea from its roots to ward off love charms. However, the New York State Department of Environmental Conservation notes that it mostly just made the drinker of the tea ill. However, marsh marigold is a major nectar source for bees and hover flies that help pollinate other plants. Its seeds are also a source of food for game birds, songbirds, chipmunks, squirrels and mice.
Eradication of marsh marigold is not a concern for managers of public lands. But homeowners who are worried about pets or small children consuming it can easily dig it up. In contrast, lesser celandine is tenacious. Manual removal of its bulblets and mat of tuberous roots is difficult if there is a major infestation. So far there are no known biological controls, according to the University of Pennsylvania's Morris Arboretum. It suggests applying the herbicide glyphosate when lesser celandine is fully leafed out and before other plants emerge.