Hooded or Mississippi Arrowhead (Sagittaria montevidensis ssp. calycina or Lophotocarpus calycinus) is a rare species in Ohio occurring in a couple dozen wetlands throughout the state. In late summer of 2003, I had my first encounter with this species. I found a few plants growing in a shallow pool at Little Cedar Point in Lucas County.
In seasons of high water, no plants may be seen but in years when water levels drop, exposing mudflats, you may see hundreds even thousands of plants.
Hooded arrowhead is distinguished from other Ohio arrowheads by sepals appressed in fruit and its thick, spongy, decumbent flower/fruit stalk. Leaves are variable but look similar to the common arrowhead (Sagittaria latifolia).
In E. Lucy Braun's The Monocotyledoneae [of Ohio]: Cattails to Orchids, she reports it from 5 counties. It is now known from 17 counties. Habitats include mudflats of river oxbows, marshes, lakes, and ponds.