Water lettuce is a weed of waterways. Its native origins are obscure, and it is widespread throughout the tropics and subtropics. Found on every continent except Antarctica, it is spread intentionally and accidentally as a contaminant. Water lettuce was imported to Hawaii from Los Angeles in 1932 or 1933 for unknown reasons. A few years later, in 1938, it was documented as naturalizing.
It vigorously reproduces vegetatively and by seeds. Copious amounts of floating seeds are produced, which move by water and spread great distances to pollute waterways further. When tested, 80% of the seeds germinate. Even when the water body dries up, the resistant seed bank remains dormant yet viable, waiting for the right conditions to grow.
The plants connect to form a mat that reduces light, oxagen and changes the ph in water bodies rendering it uninhabitable for more desirable plants and animals. The mass mats block waterways and impede water recreation. A polluted waterway of water lettuce is an excellent mosquito habitat.
Description and Dispersal:
- A floating herbaceous herb.
- Leaves often spongy near the base, densely soft pubescent with obvious parallel veins, slightly broader than long,
- Rosettes of grey-green leaves occurring singly or connected to others by short stolons.
- Roots are numerous and feathery.
High Risk Traits:
- Grows in tropical climates
- Pantropical, and widely naturalized, including Kauai, Oahu, Molokai, and Maui
- An agricultural and environmental weed
- Forms dense cover in water bodies
- Reproduces by seeds and vegetatively
- Able to reproduce within first growing season
- Seeds and vegetative fragments spread by water, attached to boats or other water craft, and possibly externally by animals
- Prolific seed production
- Seeds may persist in the soil
Low Risk Traits:
- Unarmed (no spines, thorns or burrs)
- Palatable to animals (possible food source)
- Herbicides may provide effective control