Flower:

1 to a few whorls of 3 stalked flowers on a naked stem. Flowers are about ¾ inch across with 3 broad, white petals that have a spot of yellowish green at the base. The lower stem has mostly perfect (both male and female parts) flowers, with a ring of yellow stamens surrounding a bulbous yellow-green center of tiny carpels. Male flowers, with only the yellow stamens in the center, may be at the top of the stem. At the base of a whorl are 2 or 3 small, boat-shaped bracts that soon wither away.

Leaves and stems:

Leaves are all basal, up to 8 inches long, hairless and toothless. Submersed leaves, when present, are primarily linear and stalkless. Emersed leaves are long stalked, the blade either nearly triangular to arrowhead-shaped, or elliptic to narrowly egg-shaped. The basal lobes of arrowhead-shaped leaves spread outward and may be shorter or longer than the rest of the blade. Leaves without the basal lobes are often floating on the water surface. Leaf stalks are spongy and easily compressed. Flowering stems are single or multiple from the base.

Fruit:

Fruit sets quickly, the flower stalks turning down to bury the carpel head in the muck while seed is still developing. The sepals persist, and are pressed close to the fruit. Seeds have a beak that projects horizontally from the top of the seed.

Notes:

While researching this species, I came upon various references and photos showing this to be quite a large, robust species with very broad, nearly triangular leaves, flowering stems with up to 15 whorls of flowers, flowers up to 2 inches across, and growing in fairly deep waters (and surprisingly named Giant Arrowhead). These are likely regional differences, the more southern populations faring better than the dwarf populations here on the northern fringe of its range. The specimens we came upon were all under a foot tall with mostly a single group of 3 flowers per stem, no obvious staminate flowers (at the time), and leaf blades not more than 2 or 3 inches long. Of the 6 Sagittaria species in Minnesota, this may be the easiest to distinguish: look for the ring of stamens around the carpel head, the arching, stout stalks on the fruits, and growing in mucky river floodplains. There are two additional subspecies of S. montevidensis: subsp. montevidensis, an introduction from South America to the Gulf Coast that has a purple spot at the base of the petals, and subsp. spongiosa, a native to northeastern US coastal areas that typically has spatula-shaped leaves.

Hooded Arrowhead, also known by synonym Sagittaria calycina var. calycina, was historically only known from a handful of populations in a few Minnesota counties, all of which were likely wiped out in the last 50 years. In 2006 the DNR surveyed over 100 potential sites and came up with only 7 additional populations. This species apparently requires a specific habitat to survive in MN: the fluctuating water levels of seasonal flooding in the backwaters of the Minnesota and Mississippi Rivers. They estimate up to 90% of this species' habitat has been degraded or destroyed, largely due to human activities including damming and the construction of flood control structures. Invasive species pose an additional threat to its existence. It was listed as a MN Threatened Species in 2013. It is a Special Concern Species in Wisconsin, where we stumbled upon a previously unknown population in the backwaters of the Kinnickinnic River at Kinnickinnic State Park, in the mucky, receding waters of a summer flood.