Hygrophila is a genus of plants found primarily in tropical regions, particularly in Asia, although some species venture into the subtropics. This genus is a member of the Acanthus family and notably features a number of aquatic species, around one third of the genus. Species domesticated for cultivation primarily comprise aquatic species, grown for the aquarium trade, as well as water gardens. Nurseries and companies that stock aquatic plants may carry Hygrophila species and can make a special order by request.
Known as temple plants, Hygrophila species tend to be relatively small, with an upright growth habit, and they grow very quickly. Depending on the species, the leaves may be relatively uniform, with a simple appearance, or they may be lacy, lobed, or toothed. The flowers are small and can be white or purple, typically appearing at the apex of the plant. The rapid growth of many species makes them potentially invasive plants, a concern in some areas where these plants have been introduced.
Of the aquatic members of the genus, several species are very popular with aquarists, including H. difformis, known as water wisteria. Hygrophila is very easy to grow, making it a good choice of plant for people who are just getting started with water gardens or aquariums with living plants. The plants have foliage varying from green to almost pink in color and have moderate fertilizer and light requirements. Plants can be grown in medium to bright light. Dimmer light and lower fertilizer tends to promote slower growth.
Aquatic Hygrophila propagates by budding, as well as from cuttings. Mature plants will produce miniature buds that eventually break off and float away. These plants can be rooted in an aquatic garden or allowed to float freely, depending on taste. Hygrophila can take over an aquarium very quickly and people are sometimes advised to remove mature plants after they have budded and replace them with the smaller juvenile plants to prevent the tank from being overrun.
Outside their native habitat, aquatic species have become invasive plants in some waterways. They are introduced to the water by accident or intentionally, and because they grow quickly, they can overtake native plants. They may grow more quickly than native predators can eat them, and in some cases become a navigational hazard, snarling in boat engines as they float through the water. In some regions, pests have been introduced to keep populations of these plants low.