Description: This herbaceous aquatic plant is usually a summer annual (rarely a perennial plant) that has both submerged and floating leaves. Each plant extends about 5-35 cm. (2-14") in length, and it occasionally branches. The slender stems are light green and glabrous (hairless), and pairs of opposite leaves occur along them. Submerged leaves are 5-20 mm. in length and they are linear in shape with a single longitudinal vein. Floating leaves are 5-25 mm. in length and up to 3-15 mm. across; they are obovate in shape with rounded tips. Submerged leaves are light green and glabrous, while floating leaves are light to medium green and glabrous with 3 veins (less often 5). Unlike the submerged leaves, the floating leaves form a rosette. On stranded plants, the submerged leaves often grade gradually into floating leaves along the stems.

Large Water Starwort is monoecious, forming male (staminate) and female (pistillate) flowers on separate plants. These flowers occur in the axils of the leaves and they are sessile; the flowers are either solitary or they occur in clusters of 2-3. Each male flower has a single stamen, while each female flower has a single pistil; there are no sepals nor petals. The ovary of each flower has 2 chambers (carpels), and each chamber has 2 ovules. Cross-pollination can occur by either wind or water; the blooming period can occur several months from spring to autumn. At maturity, each female flower forms a slightly flattened fruit that is about 0.5-0.9 mm. long and similarly across; it is indented and rounded on both ends, but more so toward the tip. Each fruit divides in 4 parts to release the tiny light brown to brown seeds. The root system is fibrous, white to light brown, and slender; adventive roots can develop from the nodes of the stems (where the leaves occur).

Cultivation: The preference is clear shallow water that is stagnant or slowly moving. The substrate can contain rocky material, sand, or mud.

Range & Habitat: The native Large Water Starwort occurs sporadically in all areas of Illinois, except the east-central area. Habitats include slow-moving springs and streams, shallow ponds, rocky sinkhole ponds (in other states), small watery depressions in sandstone glades (rarely), ditches, and seasonally flooded fields. Stranded plants can be found on wet sand and muddy shores of wetland areas. This plant prefers higher quality habitats.

Faunal Associations: There is very little information available about floral-faunal relationships for Large Water Starwort and other water starworts (Callitriche spp.). A polyphagous aphid, Rhopalosiphum nymphaeae (Waterlily Aphid), sucks plant juices from the floating leaves during the warmer months (Blackman & Eastop, accessed 2013). The foliage of these aquatic plants is eaten by the Muskrat (Martin et al., 1951/1961). Ducks and possibly other waterfowl feed on the foliage and fruits; the tiny seeds reportedly can pass through the digestive tract of ducks unharmed (Yatskievych, 2013). Some species of fish also probably feed on the soft foliage of this rather small aquatic plant. Its submerged foliage provides some protective cover for small fish and aquatic insects.

Photographic Location: A sandy wetland at Bonnie's Prairie Nature Preserve in Kankakee County, Illinois.

Comments: The only other water starwort that occurs in Illinois, Callitriche terrestris (Terrestrial Water Starwort), is less common and smaller in size. Unlike Large Water Starwort, it is usually, but not always, found on land in drier locations. This starwort has monomorphic leaves that have the same size and shape whether they are submerged or not. For Large Water Starwort, plants with floating leaves are more noticeable because of their distinctive rosettes; submerged plants are inconspicuous and easily overlooked. At one time, the water starworts were assigned to their own family, Callitrichaceae (Water Starwort Family), but they have been reassigned to the expanded Plantaginaceae (Plantain Family). Other common names for Callitriche heterophylla include Two-headed Water Starwort and Greater Water Starwort.
 

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