Water gardening is composed of plants that grow in deep water, plants that grow in shallow water, and plants that float. Plants that grow in deep water and shallow water still grow in the soil located beneath the water and send up stalks to the surface to absorb sunlight. But floating plants do not grow in soil at all. Instead they absorb the nutrients that they need from the water itself. Although these plants may be ornamental in some ponds, in others they are considered invasive.
Duckweed grows in dense colonies in quiet water. Each plant thrives from a single root hair, and has two to three leaves or fronds that may be as small as 1/16 of an inch wide. Ducks feed off of the plant and unknowingly spread it from pond to pond in the wild as they travel, according to Texas Agrilife Extension. Excessive colonies of duckweed can spread over a pond and deplete it of oxygen, killing all fish and submerged plants in the pond. In many states, including Ohio and Florida, duckweed is classified as a noxious weed.
The unusual shape of water lettuce has caused many home gardeners to purchase the plant for water gardens. Water lettuce is a free-floating, cup-shaped plant with spongy leaves that are covered in fine hairs and arranged in a spiral from the center of the plant. The leaves may grow up to 6 inches wide and have large veins that run their entire length. In the wild, water lettuce is an aggressive invader and can form dense surface mats. If the mats cover the surface of a lake, they will deplete oxygen and kill all life in the water body, says Texas Agrilife Extension.
Parrot feather is a South American native used in the U.S. in fish aquariums and garden ponds as a submerged plant. Though the plant is not seen above the water's surface, it provides food and habitat for fish. Its name comes from a profuse mass of feathery leaves that grow in whorls of 4 to 6 leaves along the stem of the plant. The stems trail along the surface of the water body, or along the ground. If grown in shallow water, the plant will put down roots and grow out of the water in an upright method, according to North Carolina State University Extension. In the wild, parrot feather can be found in ponds and lakes as well as rivers and streams. The plant may multiply to the point where it becomes invasive and chokes off flowing water.