Water Lettuce   (Pistia stratiotes)

DESCRIPTION

Water Lettuce (Pistia Stratiotes) is an invasive weed found in many areas of the southern United States.  Water lettuce is a perennial monocot of the Araceae family.  The plant consists of thick, soft, light green leaves (usually 6 inches in length) that form a rosette.  The rosette conceals a small female flower and the seed bearing fruit of the plant.  Water lettuce is supported by a large number of feathery roots submersed in the water beneath the leaves of the plant.

Reproduction
Water lettuce reproduces using seeds and vegatatively.  Vegetative reproduction involves daughter vegetative offshoots off of mother plants on short, brittle stolons. Rapid vegetative reproduction allows water lettuce to cover an entire lake, from shore to shore, with a dense mat of connected rosettes in a short period of time.  In Florida water lettuce has been known to have densities of up to a 1,000 rosettes per m^2.

^Habit
In order for water lettuce to survive it requires a wet, temperate habitat.  It is usually found in lakes and rivers, however it can survive in mud.  Water lettuce can endure temperature extremes of 15° C (59° F) and 35° C (95°).  The optimal growth temperature range for the plant is 22-30° C (72-86° F). 

Identification Water lettuce is very easy to identify: it resembles a floating open head of lettuce and may be found singly or in abundance; its light green leaves are thick, hairy and ridged; its flowers are not seen except by close examination monocot, perennial free-floating except when stranded in the mud; singly or massed in large numbers; mother and daughter plants attached by short stolons ( ) thick soft leaves are formed in rosettes, with no leaf stems; leaves to 6 in. long; light green; with parallel ridges (veins) ( ), covered in short hairs; leaf margins wavy, top margins scalloped flowers ( ) inconspicuous (not observed in Florida till the 1980s though they had been flowering all along); nearly hidden in the center amongst the leaves; on small stalk, single female flower below and whorl of male flowers above roots hanging submersed beneath floating leaves; feathery, numerous fruit a green berry

(Source: 1. Identification & Biology of Non-Native Plants in Florida's Natural Areas, http://aquat1.ifas.ufl.edu/pistpic.html; 2. Non-Native Invasive Aquatic Plants in the United States, http://plants.ifas.ufl.edu/seagrant/pisstr2.html)

IMPACTS

Water lettuce can have a severe impact on the environment and economy of infested areas. The dense mats created by connected rosettes of the plant lead to the majority of problems encountered with water lettuce. These mats can have a negative economic effect by blocking waterways, thus increasing the difficulty of navigation and hindering flood control efforts. Mats of water lettuce can also have a severe impact on the environment. They can lead to a lower concentration of oxygen in covered waters and sediments by blocking air-water interface and root respiration. Extremely thick mats of water lettuce can even prevent sunlight from reaching underlying water. The cumulative effect of these negative characteristics of the plant is a loss of biodiversity in invaded habitats. Water lettuce mats can also serve as a breeding place for mosquitoes.

(Source: 1. Identification & Biology of Non-Native Plants in Florida's Natural Areas, http://aquat1.ifas.ufl.edu/pistpic.html; 2. Non-Native Invasive Aquatic Plants in the United States, http://plants.ifas.ufl.edu/seagrant/pisstr2.html)

Water lettuce was first identified in Australia in 1946/47, although it is now known to occur throughout the warmer regions of the world. Water lettuce was first reported in Florida by John and William Bartram in 1765. The most commonly accepted pathway of this species into the United States is in ballast water in ships from South America. Ships that travel through mats of water lettuce can carry fertile plant segments to new areas. The popularity of water lettuce as a garden plant has also led to its spread, in a more controlled manner.

(Source: 1. Identification & Biology of Non-Native Plants in Florida's Natural Areas, http://aquat1.ifas.ufl.edu/pistpic.html; 2. Non-Native Invasive Aquatic Plants in the United States, http://plants.ifas.ufl.edu/seagrant/pisstr2.html; 3. Courtesy of Nonindigenous Aquatic and Semi-Aquatic Plants in Freshwater Systems, http://aquat1.ifas.ufl.edu/mcplnt1c.html)

Water lettuce is present in the states of the southeast U.S. and north to New Jersey and New York, and westward to Texas, Arizona and California. Also present in Hawaii, Puerto Rico and the Virgin Islands.

1. Water Lettuce Leaf Weevil (Neohydronomus affinis) This weevil is a native species of South America and was first introduced into Australia in the early 1980's for biocontrol of water lettuce. In Australia the weevil was extremely effective, reducing water lettuce infestation by40% or more within 12-18 months. This weevil was released in Texas in 1991 in test plots at Lake Dunlap, Guadalupe County. Additional releases of this weevil for research are currently being conducted in the Brazos Bend State Park. These weevils have a very short life cycle, approximately 30 days, which allows for quick establishment of populations. The adult weevils feed on the leaf, while the larvae attack the inside of the leaf.

2. Water Lettuce Leaf Moth (Spodoptera pectinicornis) The moth is a native specie of Thailand and was imported into Florida for the biological control of water lettuce. This moth was released in Texas for research in 1991 at Brazos Bend State Park. The moth has a very short life cycle, approximately 35 days, with the larval stage lasting 17-20 days. The adult moth does not feed on water lettuce, however, the larvae does and is capable of significant damage. The larvae are fairly large which means that fewer larvae can cause more damage.

Mechanical/Physical Control
Water lettuce can be removed by raking or seining it from the pond’s surface.

Chemical Control 
Active ingredients that have been successful in treating water lettuce include endothall (G), diquat (E), and rodeo (G). (E = excellent, G = good)

1. Aquathol, Aquathol K, and Aquathol Super K are dipotassium salts of endothall and comes in both liquid and granular formulations.  The Aquathol K liquid formulation has been effective on water lettuce. Contact herbicides act quickly and kill all plants cells that they contact.

2. Reward is a liquid diquat formulation that has been effective on water lettuce. Diquat is a non-selective contact herbicide and crop desiccant. Because of its rapid degradation in water and strong adsorption onto sediments, diquat has rarely been found in drinking-water. Contact herbicides act quickly and kill all plants cells that they contact. A non-ionic aquatically registered surfactant (see the label) will have to be added to the Reward solution for good results. 
According to EPA factsheet, http://www.epa.gov/safewater/dwh/c-soc/diquat.html, diquat may cause health problems if found in amounts greater than the health standard set by the United States Environmental Protection Agency (EPA). For example, EPA has found diquat to potentially cause the following health effects when people are exposed to it at levels above the Maximum Contaminant Level (MCL) for relatively short periods of time: dehydration. In Long-term period diquat has the potential to cause the following effects from a lifetime exposure at levels above the MCL: cataracts.

3. Rodeo, Aquamaster, and Aquaneat are liquid glyphosate formulations and have been effective on water lettuce. These are broad spectrum, systemic herbicides.  Systemic herbicides are absorbed and move within the plant to the site of action.  Systemic herbicides tend to act more slowly than contact herbicides.  An aquatically registered surfactant (see the label) will have to be added to the glyphosate solution for good results.

One danger with any chemical control method is the chance of an oxygen depletion after the treatment caused by the decomposition of the dead plant material. Oxygen depletions can kill fish in the pond.  If the pond is heavily infested with weeds it may be possible (depending on the herbicide chosen) to treat the pond in sections and let each section decompose for about two weeks before treating another section. Aeration, particularly at night, for several days after treatment may help control the oxygen depletion. 

(source from:1. Water Lettuce: Control Methods, http://wildthings.tamu.edu/aquaplant/Floating Plants/Water Lettuce/Water_Lettuce_Control.htm; 2.Guidelines for drinking water quality, http://www.who.int/water_sanitation_health/GDWQ/Chemicals/diquatsum.htm; 3. EPA factsheet, http://www.epa.gov/safewater/dwh/c-soc/diquat.html)

Non-Native Invasive Aquatic Plants in the United States: Center for Aquatic and Invasive Plants, University of Florida and Sea Grant
http://aquat1.ifas.ufl.edu/lettuce.html
This websites include all kinds of useful information about Water Lettuce such as habitat, identification, origin, distribution, problems/effects, control, etc.

Aquatic Plants: Water Lettuce
http://www.thekrib.com/Plants/Plants/water-lettuce.html#0
This page shows one brief article, Growing Water Lettuce & Hyacinth Indoors by Karen Randall..   

Biological Control of Weeds in Texas: Water Lettuce
http://bc4weeds.tamu.edu/lettuce.html
This page describes water lettuce's biology, history, biological control, and links.

Floating Plants, Water Lettuce: Aquatic Systems
http://www.aquaticsystems.net/float.html, http://www.aquaticsystems.net/lettuce.html
This site introduces brief description of water lettuce and treatment to control the water lettuce.

Water Lettuce: Control Methods
http://wildthings.tamu.edu/aquaplant/Floating Plants/Water Lettuce/Water_Lettuce_Control.htm
This site includes physical/mechanical, biological, and chemical control methods.