Frogbit, or Hydrocharis morsus-ranae, is an aquatic perennial plant that is native to marshes and shallow waters in parts of northern Africa, western Asia, and Europe. Water gardeners value it for its ability to provide shade beneath its floating mass of leaves, stolon-like stems, and roots. Shading a pond usually helps to slow unwanted algae growth and provides a cool area where fish may rest and hide from predators. In the summer and sometimes autumn, it produces white flowers.
Floating aquatic plants generally absorb nutrients that feed undesirable algae. Most aquatic gardeners try to keep 50 to 70 percent of a pond's surface covered with foliage. Frogbit plants are either floaters on shallow water or bog plants, often called marginal plants. Here frogbit camouflages the artificial outline of the pond and provides cover for wildlife, such as frogs.
There are two types of frogbit flowers — male and female. Generally, the flowers are white, three-petaled, and bowl-shaped, similar to a simple poppy flower. The male flowers grow in clusters of up to four blooms; the females are usually solitary. Each of them has three papery, broadly ovate petals that sport a yellow spot at each petal's base.
Cultivating this plant generally is easy. It prefers still, shallow water that is alkaline. Usually the plant excels in water that is deep enough for the plant to float, but shallow enough for the roots to set themselves into the mud on the pond bottom. They may survive in deeper water but might not thrive.
Gardeners may buy plants from aquatic nurseries. Growers may sow the seeds in shallow trays of water or divide the stolons in the spring. One of the few pests to attack the plants is the common snail, which will chew holes in the leaves.
Frogbit grows in warm climates. In cultivation, a pond usually must be deep enough to not freeze at the bottom during the wintertime. Although the mass of foliage and shallow roots float on the surface, the deeper roots and buds winter over on the pond bottom. In the spring, the plants rise to the surface and produce new plants.
As with many pond fillers, frogbit often becomes invasive. Typically, the best way to control it is to hand-pull it. As a rapidly growing perennial, it usually recovers quickly. The US government lists it as an invasive species, especially in the Great Lakes.
People often confuse frogbit with the common water lily. The plant has a smooth-edged, rounded heart-shaped leaf that is purplish on the underside. Typically, the green leaves are 1 to 2 inches (2.5 to 5 cm) across. The flowers are much smaller than water lilies and have only three petals, whereas lilies generally have more petals. Frogbit flowers may be up to 0.5 inches (1.25 cm) in diameter.