Water Plantain family (Alismataceae)
Description: This perennial plant is up to 3½' tall, consisting of a rosette of basal leaves and one or more flowering stalks. The leaves have blades that are up to 16" long and 10" across, but they are usually closer to half this size. The leaf blades are sagittate and smooth along their margins; they have conspicuous parallel veins and smaller lateral veins. The petioles of the leaves are up to 2' long and rather stout; they broaden toward the base and become sheath-like. There is considerable variability in the width of the leaf blades and the length of their basal lobes. The flowering stalks are about as tall as the leaves or slightly taller and more or less erect; they are terete, becoming more stout toward the base of the plant. Both the leaves and flowering stalks are glabrous and contain a milky sap. Each of these stalks terminates in whorls of 2-3 flowers. Most plants are monoecious; they have staminate flowers located on the upper portion of each stalk, while the pistillate flowers are located below. Occasionally, dioecious plants occur, and sometimes perfect flowers are produced.
Each flower is about 1" across or slighter broader, consisting of 3 white petals and 3 green sepals. The staminate flowers have numerous stamens that are golden yellow, while the pistillate flowers have multiple carpels that are green and form a bur-like mass. The petals are rather broad and well-rounded; their upper surface has a satiny lustre. The filaments of the stamens have a smooth surface.The pedicel of each flower is up to 1" long. At the base of each whorl of flowers, there are 2-3 green bracts that are linear-lanceolate. The blooming period occurs from mid-summer to early fall, lasting about 1 month. The pistillate flowers are replaced by bur-like fruits up to 1" across; these fruits consist of a dense cluster of achenes. Each achene is flattened, 3-angled, and winged; it has a straight beak that projects laterally from the center of the bur-like fruit. The root system consists of a tuft of coarse roots, which often develop starchy tubers; long rhizomes or stolons are also produced. Reproduction is by seeds or rhizomes/stolons. This plant occasionally forms colonies.
Cultivation: The preference is full or partial sun, wet conditions, and a rich organic soil. This is an emergent aquatic plant that usually grows in shallow water. Sometimes it can spread aggressively.
Range & Habitat: The native Common Arrowhead is common in central and northern Illinois, while in the southern section of the state it is occasional to absent (see Distribution Map). Habitats include swales in open woodlands, swamps, marshes, bogs, seeps, low-gradient edges of ponds and reservoirs, and low-gradient edges of slow-moving streams and drainage canals. This species is reportedly tolerant of polluted water; it can be found in both disturbed and higher quality wetlands.
Faunal Associations: The flowers are pollinated by various insects, including bumblebees, Halictid bees, small butterflies, wasps, flies, and beetles. The bees suck nectar or collect pollen, the beetles suck nectar or feed on pollen, and other insects suck nectar. Some insects feed on the foliage and other parts of Common Arrowhead and other Sagittaria spp. These insect feeders include several semi-aquatic leaf beetles (Donacia spp.), the semi-aquatic weevils Listronotus delumbis and Lixellus lutulentus, the Waterlily Aphid (Rhopalosiphum nymphaeae), and the larvae of Bellura obliqua (Cattail Borer Moth). Young larvae of the latter moth mine leaves or petioles, while older larvae bore into the root crown and rhizomes. Ducks and other waterfowl eat the seeds or tubers (see Waterfowl Table). Muskrats also feed on the tubers, rhizomes, and foliage.
Photographic Location: Along a drainage canal at the Windsor Road Prairie in Champaign, Illinois.
Comments: This is a highly variable species that has fairly attractive foliage and flowers. It is an important source of food to ducks. Other Sagittaria spp. (Arrowheads) occur within the state, but they are less common; some of them are difficult to distinguish from Common Arrowhead. Generally, Common Arrowhead has the following key characteristics: 1) The basal lobes are one-half as long to longer than the blades of the leaves, 2) The green bracts at the base of the whorled flowers are shorter than the pedicels (less than 1" in length), and 3) Each achene is winged along the flattened edges and it has a straight beak that projects laterally from the center of the fruit. Other Arrowhead species have unlobed leaves, or they have leaves with basal lobes that are less than one-half the length of the blades; their floral bracts are often longer than the pedicels; and their achenes are shaped differently, often with beaks that are curved or project upward from the center of the fruit.