Non-native plants are sometimes accidentally attached to other horticultural species and discarded unintentionally or intentionally. Individuals who do not follow strict protocols for animal management may accidentally release specimens. Accidental release may also occur when those protocols do not exist. Intentional release and escape from confinement are also possible. Ships and boats can introduce organisms through ballast discharge and hull fouling.
Ballast water can contain aquatic plants, animals and pathogens.
Scientists consider ballast water discharge a major pathway for aquatic introductions because of the huge volume of water carried as ballast. This water carries millions of single-celled and larger organisms. Fouling of vessel hulls by encrusting organisms also provides a mechanism for transfer of species. Recreational boats and boat trailers transport nuisance species including Eurasian watermilfoil and zebra mussels. Aquatic plants, in particular, are easily transported when plant fragments get tangled on boat propellers and fishing gear.