Water lettuce (Pistia stratoites)

Aquatic, tier 2

History & Introduction

Water lettuce (Pistia stratiotes) is an invasive freshwater species known for its ability to create dense surface mats in aquatic ecosystems. This plant poses significant threats to biodiversity and public health, including the reduction of native plant species, decreased dissolved oxygen levels, and impediments to recreational activities such as fishing, boating, and swimming. Additionally, water lettuce can serve as a breeding habitat for mosquitoes that transmit St. Louis encephalitis.

Water lettuce is a pantropical species found on every continent except Antarctica. While its exact origin remains uncertain, several theories have been proposed. Fossil records indicate the plant’s historical presence worldwide. Ancient Egyptian hieroglyphics illustrate water lettuce, and Greek botanists Dioscorides and Theophrastus documented its floating growth on the Nile River over 2,000 years ago, suggesting an African origin. Furthermore, populations in Brazil and Argentina are home to numerous co-evolved specialist insect herbivores, which may indicate a South American origin.

Water lettuce was initially introduced to the United States as an ornamental plant for aquariums and water gardens. The first documented occurrence of the species was along the St. Johns River in Florida in 1765, likely facilitated by early shipping routes between Florida and South America. John and William Bartram reported finding water lettuce up to 300 kilometers upstream from the ocean inlet, indicating that it may have been introduced through ballast material from trans-oceanic vessels. However, the distance from known seaports has led to speculation about its native status in Florida. Supporting this theory, fossil records from the late Pleistocene to early Holocene eras in Florida further suggest the species may have a historical presence in the region.

Water lettuce has been introduced through various channels, including shipping containers, boats, and fishing equipment, which have facilitated its spread into new aquatic environments. Additionally, the release of aquarium plants and unintentional introductions via water gardens have contributed to its proliferation. This species is particularly invasive in regions with year-round warm temperatures and high humidity. In colder climates, it persists by reseeding itself.

Water lettuce has been recorded in twenty-eight states and U.S. territories, primarily in temperate regions; however, its range is gradually expanding due to climate change. In New York State and the Long Island Invasive Species Management Area (LIISMA), water lettuce is classified as a Tier 2 invasive species, indicating a priority for early detection and eradication. As of 2024, three known locations of water lettuce exist within the LIISMA region.

Identification & Biology

Water lettuce is a floating, monocotyledonous aquatic plant commonly found in still waters such as lakes, ponds, ditches, swamps, and slow-moving streams. It can also survive in muddy substrates.

The plant is characterized by its rosette formation of leaves on the water’s surface, resembling an opened head of lettuce. The leaves are generally pale green, featuring a spongy texture near the base and dense, soft pubescence along the clearly indented parallel veins. Each leaf blade is slightly broader than long, with the greatest width at the apex. The rosettes may occur individually or be interconnected by short stolons extending from the larger rosettes.

Water lettuce features numerous, feathery roots that hang unbranched below the water’s surface. Each root is adorned with fine root hairs and capped with long, brown coverings at the tips. The plant produces inconspicuous flowers that are clustered on small, fleshy stalks, often hidden within the leaf axils. A whorl of male flowers is positioned directly above a single female flower. The fruit develops from the female flower as a green berry containing multiple seeds.

Damage & Concern

Water lettuce poses significant ecological threats, primarily due to its aggressive growth and ability to form extensive, dense mats on the surface of water bodies. These mats are known to deplete oxygen levels, leading to detrimental effects on aquatic life, including fish mortality. They also shade and crowd out native submerged plant species, which contributes to a decline in biodiversity. Moreover, they also uproot essential emergent plants that provide habitat and resources for wildlife. Additionally, the stagnant water created by these mats serves as an ideal breeding ground for mosquitoes, increasing public health concerns.

Water lettuce poses significant ecological threats, primarily due to its aggressive growth and ability to form extensive, dense mats on the surface of water bodies. These mats are known to deplete oxygen levels, leading to detrimental effects on aquatic life, including fish mortality. Furthermore, the thick canopies created by water lettuce hinder activities such as boating and fishing, by restricting boat traffic and (and recreational activities such as swimming) while also complicating flood control efforts.

Under optimal environmental conditions, water lettuce has the potential to double its population size in less than three weeks, making it a resilient and problematic invasive species. These dense mats can increase evaporation rates, potentially lowering water levels and exacerbating flooding along rivers and canals. Its ability to produce seeds enhances its survival under adverse environmental conditions, such as freezing temperatures and drought.

Given the aggressive growth and negative ecological impacts of water lettuce, this plant is illegal to possess in Florida without a special permit. It is considered one of the world’s most problematic invasive species, having been listed on prohibited plant lists in several states, including Florida, Louisiana, Mississippi, and Texas. Moreover, it is recognized as a noxious species in South Carolina and Delaware.

Management & Control

Prevention

The most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread in the first place by maintaining healthy natural communities and by conducting monitoring of newly emerging invasive species several times each year. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem disturbances is likely to be more effective than managing solely to control any particular invasive species.

Manual Control

For small infestations, water lettuce can be manually removed by hand pulling. For larger infestations, a mechanical harvester can be proven effective. Water lettuce can also be raked or seined from the surface of the pond.

Chemical Control

In areas with large water lettuce populations, or where location or access makes hand-pulling or harvesting difficult, there are several chemical control options that may be viable and/or successful options. Consult with a licensed pesticide applicator. Always be sure to follow all label directions when using all chemical treatments. Refer to Texas A&M Agrilife Extension’s “How to Control Water Lettuce” resource page for more information.

Biological Control

Insects that effectively feed on water lettuce include: the leaf weevil (Neohydronomous affinis) or leaf moth (Spodoptera pectinicornis).

Monitoring & Reporting

LIISMA is actively monitoring water lettuce in priority conservation areas and beyond due to its status as a significant concern for invasive species management. This species is prioritized for early detection and rapid response. As reports are gathered, LIISMA collaborates with partners and landowners to identify and manage early infestations before they can spread widely. Water lettuce is classified as a Tier 2 invasive species on Long Island, emphasizing the need for timely action. LIISMA encourages the public to report any sightings of this species.

Your reports matter to us–report the species to iMapinvasives or iNaturalist as soon as you detect it!

iNaturalist is an app and website that can help you record your own observations, identify species, collaborate with others, and access the observational data collected by iNaturalist users. Already using iNaturalist? Join a LIISMA iNaturalist Project to have your invasive species observations automatically uploaded to our projects! Here are some of our projects you can join: Invasive Species in Priority Areas, EDRR Species, LIFERS, Tier 1 Invasive Species, and more!