Marsh Marigolds are one of the earliest spring flowers and their bright yellow blossom and dark green leaves are a refreshing splash of color in the midst of the brown and grey of early spring. These flowers are actually not marigolds at all but rather related to the buttercup and commonly found in ponds, marshes, swamps and lakes.
Use caution when touching the leaves and stems as they have a caustic substance inside. Warts may shrink up and disappear if you squeeze a bit of the stem juice on them.
Sources: Canoe Country Flora by Mark Stensaas and Wildflowers of Minnesota by Stan Tekiela