Water-lettuce: Pistia stratiotes
This species (the only one in its genus) commonly forms dense
floating mats, with many rosettes of fuzzy-soft, pale-green leaves.
New leaf clusters form readily from stolons (runner stems) offset
from the larger rosettes.
Look for first
- Floating pale-green rosettes
- Long, feathery roots below
- Green runner stems
||Leaves: in rosettes occurring singly or connected to others by
short stolons. Leaves often spongy near base, densely soft
pubescent with obvious parallel veins; blades slightly broader than
long, widest at apex, to 15 cm (6 in) long.
||Roots: long, feathery; bearing long root caps (brown coverings
over root tips).
||Flowers: inconspicuous, clustered on small fleshy stalk nearly
hidden in leaf axils, with single female flower below and whorl of
male flowers above.
||Fruit: arising from female flower as a many-seeded green
This floating plant native to South America is considered to be
one of the worst weeds in the subtropical and tropical regions of
the world. In Florida, it was first recorded in 1765; its
introduction is linked to early shipping commerce between Florida
and South America. Today, water-lettuce is commonly found in the
central and southern portions of the state, but new infestations of
water-lettuce have been found in North Florida's spring-fed rivers
and lakes. Because of intensive statewide management efforts,
water-lettuce populations are maintained at low population
Pantropical of uncertain origin, thought to be introduced to
Florida by Spanish commerce or other early settlers.
Under optimal environmental conditions, water-lettuce can double
its population size in less than three weeks. Seed production makes
this plant resilient to adverse environmental conditions such as
freezing temperatures and drought.
Why water-lettuce must be managed
Water-lettuce populations often form large expanses of dense,
impenetrable floating mats limiting boat traffic, recreation, flood
control, and wildlife use. These dense canopies at the water
surface shade out native submersed plant species and can uproot
native emergent plants that are important to wildlife.
Environmental damage caused by water-lettuce populations
- Water-lettuce mats can lower dissolved oxygen concentrations
reducing aquatic life.
- Dense populations may lower water levels because water-lettuce
increases evaporation rates over open water areas.
- Water-lettuce mats can restrict water flow increasing flooding
along rivers and canals.
- Dense water-lettuce populations produce ideal breeding
environments for mosquitoes.
- Water-lettuce populations crowd out native plants and animals
Because of its aggressive growth rate, water-lettuce is illegal
to possess in Florida without a special permit.