Freely floating plants are part of the natural ecosystem of freshwater bodies in both temperate and tropical regions of the world. If you relocate these plants into mild climate regions where no natural predators contain their growth, you'll soon have a weedy plant that will spread indefinitely. Some tropical floating plants like water hyacinth, water lettuce and Salvinia ferns are regarded as among the fastest growing and more difficult to eradicate once escaped into the wild, disrupting ecosystems.
Members of the botanical plant genus Lemna are commonly called duckweed. These tiny plants bear leaves 1/4 to 1/2 inch that float atop still freshwater surfaces. A thin thread-like root dangles down into the water. In winter the plants sink to the muck below to avoid freezing. Duckweed species grow naturally across much of the temperate regions of the Northern Hemisphere.
Also called water fern or fairy moss, mosquito fern is botanically known as Azolla. These tropical plants grow quickly in warm weather and warm water and spread across the surface indefinitely, often being invasive. In cold water the plants produce overwintering tissue masses that sink to the bottom and float once spring returns.
Rounded, stems filled with air allows water hyacinth (Eichhornia crassipes) plants to float on the water. Dangling roots below, stems bud from plants to create new plants, forming a large thicket or mass. This species produces alluring lavender flowers in warm weather, but is considered one of the most invasive tropical aquatic weeds in the world according to Peter Robinson, author of "The Practical Rock and Water Garden," and environmentalists worldwide likely agree. It is not tolerant of subfreezing temperatures.
Also not surviving freezing temps is the water lettuce (Pistia statiotes). Rounded rosettes of bright light green leaves float that look like lettuce plants. Spreading stems bud to yield new plants, usually creating invasive mats of foliage in frost-free tropical freshwater bodies.
About 10 species of plants in the genus Salvinia exist in the world and grow nearly as rapidly as water hyacinth in warm tropical waters. The leaves are soft and covered in silky hairs and its roots fan out like a fern frond under the water. This plant also becomes an invasive weed, spreading into an indefinite mass in frost-free regions.