About aquatic plants
The worldwide trade in plants for the aquarium and pond hobbies is a multi-million dollar industry. Along with fish, aquatic, semi-aquatic and amphibious plants are exported largely from tropical and subtropical regions to various countries around the world. While many of these plants are harvested from the wild, many more are cultivated in large nurseries in warm regions where the growing period is long and plants are not costly to cultivate. Aquatic plants come from a wide variety of taxonomic groups and association with aquatic habitats is not restricted to any single plant evolutionary lineage.
Many aquatic plants are not suitable for cultivation and commercial trading for aquaria or pond gardens; some are protected by various governments worldwide as nurseries for ocean fish and shellfish or as feeding grounds for marine mammals; or because they are of large size, specialized growth requirements, or even a lack of vegetative production.
Plants that are associated with water have a remarkable variety of growth forms regardless of their natural history. Strictly aquatic plants (hydrophytes) have an obligate submerged, emergent or floating growth habit and do not grow well out of water. Semi-aquatic plants have aerial leaves and stem but require roots to remain submerged or in moist ground. Amphibious plants grow equally well submerged or emersed, and often with distinctly different growth morphology or phenology between the two habits. Helophytes are essentially terrestrial plants that tolerate extended periods of time submerged in water but usually live a terrestrial existence.
Aquatic plants live in a highly dynamic and hostile environment characterized by the ephemeral availability of water, constant predation, intense competition for light and nutrients from other plants and algae, and shading by planktonic algae, sediment and water turbidity. Consequently, to be competitive in these environments, aquatic plants are characterized by rapid growth rates, strong chemical allelopathy, and remarkably effective mechanisms for dispersal, including seeds, rhizomes, stolons and detached leaves and shoot nodes which can develop into separate, clonal plants.
Invasive aquatic weeds: quarantine considerations
The movement of aquatic plants across international borders is of considerable quarantine concern. Owing to the strong competitiveness of many aquatic species, serious ecological consequences can result if they are released into waterways, where they often become dominant, displacing native species. The most common pathway of entry for aquatic weeds into new areas is through discarded aquarium material. Many aquarium plants, with origins in the aquarium trade, have subsequently become serious environmental weeds in various countries, including water hyacinth (Eichhornia crassipes), Salvinia (Salvinia molesta), East Indian Hygrophila (Hygrophila polysperma), Cabomba (Cabomba caroliniana) and Asian Marshweed (Limnophila sessiliflora). Preventing the introduction of invasive aquatic weeds (resulting from the aquarium and pond plant trade) into the United States, and slowing their dispersal once introduced, requires correct identification by authorities at entry points, and by local managers once a weed is introduced but still containable. The sheer diversity and phenotypic plasticity of aquatic plants makes their identification difficult.