On one of our jaunts through the woods and parks in NE Ohio, my wife was thrilled to see glorious blooms of intense yellows created by Caltha palustris or Marsh Marigold (MM).  The genus name “Caltha” is derived from the Latin meaning “yellow flower” and the specific epithet “palustris” means marsh-loving.  Therefore, the Latin binomial for this plant literally means “yellow flower marsh-loving”!!  This North American native plant thrives in bogs, ditches, swamps, forested swamps, wet meadows, marshes, and stream margins from as far east as Newfoundland to as far west as Alaska.  MM then slips down into Nebraska and then over to Tennessee and North Carolina and that is as far down south that it is able to tolerate the intense summer heat.

MM flowers are a cheery yellow and a welcome signal that Springtime is just around the corner!  In fact, MM is really NOT a marigold nor in the family, Asteraceae, the family to which marigolds belong, but it is a perennial in the Ranunculaceae or buttercup family.  Looking closely at the flowers, you clearly see the shiny, yellow, buttercup-like resemblance.  These plants may commonly be referred to as Caltha cowslip, cowslip, cowflock, or kingcup.

As an herbaceous perennial, MM prefers full sun to light shade and grows 1-2 feet tall and wide, with a naturally mounding growth habit.  The planting site would of course have to be consistently moist or even wet.  MM flowers are 1-2 inches in diameter, with 5-9 waxy, rich, golden yellow “petals”, which are really sepals, that appear in early Spring; specifically, they can bloom April to June depending on elevation, temperatures, and exposure to sunlight.

In fact, humans see MM sepals as yellow, but to insects, the outer half of the sepal is a mixture of yellow and the ultraviolet “bee’s purple”, while the inner most portion of the sepal is yellow.  MM flowers have anywhere between 50 to over 100 stamens.  The flowers offer an early source of pollen and nectar to insects, butterflies and hummingbirds, but they are most commonly pollinated by hoverflies (Syrphidae)!  MM can be propagated by either using “fresh seeds” (planting mature seeds immediately harvested from existing plants) or by dividing mature plants.

The more exposed MM are to direct sunlight in their site, the more quickly the soils will warm up and plants will bloom; conversely, the less exposure and more hidden or cooler the site, the flowers take much longer to mature and emerge.  While it is true that the best flowering will occur in full sun during the Spring, later in the season, especially during the heat of the summer, MM will do better if they have partial shade.  If sited in full sun in warm summer climates, the plants can actually go dormant with summer heat and dry conditions and drop their leaves!

MM’s have waxy, glossy green, basal leaves that may be round, oval, heart or kidney shaped and by mid-summer they may mature in size to about 7 inches across.  The leaves can have smooth margins or small scallops or teeth along the leaf margins.  The basal leaves of MM’s have long petioles with a deep, narrow sinus or notch where the petiole attaches to the leaf blade.  The upper leaves are alternate on thick, hollow stems with shorter petioles or no petioles at all and tend to be smaller than the basal leaves.

I found it fascinating that parts of MM are processed and used medicinally but handling the plant can cause skin irritation and blisters, and uncooked parts are toxic to humans.  WHAT?!  Now this is where plant research REALLY gets fascinating!  All plants of the buttercup family, Ranunculaceae, contain the toxic glycoside protoanemonin, sometimes called anemonol or ranunculol!  MM contains this yellow oil irritant, protoanemonin, throughout the entire plant, especially the older foliage and supporting plant parts.  Protoanemonin can be broken down or destroyed by heat!  Cattle and horses can be poisoned too by consuming raw or fresh MM, although dried plants, like those that may be found in hay, are no longer toxic to them!  That is so WILD!!  So, if you are outdoors and need to go on a buttercup binge, just look and enjoy these beautiful marshy, swamp loving plants but NO TOUCHY!!

Authors: Erik Draper

Source: https://bygl.osu.edu/index.php/node/1557