Flower:

Flowers are whorled in groups of 3 or more in a raceme up to 20 inches long, often with a whorl of 3 branches at the base of the main cluster. Separate male and female flowers are on the same stem or branch with female whorls at the base and male above. All flowers are up to 1 3/8 inch (3.5 cm) across with 3 broad white rounded petals. Female flowers have a bulbous green center, covered in tiny carpels.

Male flowers have a group of yellow stamens in the center.

Behind the flower are 3 light green, cup-shaped sepals shorter than the petals. At the base of the whorl are 3 bracts that are up to about 1½ inch (1 to 4 cm) long, light green to straw-colored, barely fused at the base, tapering to a pointed tip, initially boat shaped but flattening out some with age and persisting through fruiting. Each branch has 5 to 14 whorls of flowers, the stem usually rising above the leaves.

Leaves and stems:

A rosette of basal leaves surrounds the flowering stem. Leaves are up to 16 inches (to 40 cm) long, toothless, hairless and arrowhead shaped with the basal lobes usually longer than the remainder of the blade. The main part of the blade is triangular to lance-elliptic and pointed to somewhat rounded at the tip.

Leaf stalks are 10 to 40+ inches (25 to 100+ cm) long, spongy, usually strongly angled especially on the lower half. Flowering stems are very stout and strongly ridged below the flower clusters.

Fruit:

Fruiting head is a round cluster up to 1 inch (2.5 cm) diameter with hundreds of seeds that mature from green to brown.

Seeds are 2 to 3 mm long, flattened, winged, with a straight to slightly curved, ascending beak.

Notes:

Short-beaked Arrowhead is very rare in Minnesota, where it reaches the northern fringe of its range. According to the DNR, its primary habitat is prairie wetlands, though it is not strictly a prairie species. It is, however, vulnerable to pollution and, of course, habitat degradation and destruction. It was listed as Endangered in 2013.

Short-beaked Arrowhead is most likely to be mistaken for Broad-leaf Arrowhead (Sagittaria latifolia), but there are some key differences. S. brevirostra is a more robust plant—I was taken aback when I first saw it at how large it was. A clump of S. latifolia was also nearby and about half the size at this particular site; its stems were round or weakly angled at best and the leaf stalks more D-shaped in cross-section, where S. brevirostra had much more prominent ridges and angles. The floral bracts on S. brevirostra are also much larger and persistent, as long as or longer than the flower stalks, where those of S. latifolia are much shorter than the stalks, turn brown and shrivel up early. The beak on S. brevirostra seeds is ascending, horizontal on S. latifolia. The arrowhead leaf shape may be similar to Arum-leaved Arrowhead (S. cuneata), but it is a far smaller plant, usually well under 2 feet.

Note that nearly every reference describes leaves' basal lobes as being shorter than the rest of the blade, which is not true. Per the DNR's description, the basal lobes are between 20% and 80% longer than the rest of the blade. In the population where the photos were taken, the lobes looked to be at least as long, mostly longer. Seeing is believing.