A water lettuce mat
A water lettuce mat

Water lettuce (Pista stratiotes) is a widespread floating plant in Florida. This leafy plant can be observed from north to south and tends to grow and reproduce rapidly, form very dense mats, and cause numerous problems. Because of these issues, water lettuce is one of the most targeted plants for management in Florida state waters.

The guiding principle behind aquatic plant management is to reduce the population of invasive plants to low levels and promote the growth of native plants. The controversy lies in experts disagreeing on whether water lettuce is native or invasive. Some excellent genetic data seems to imply it originated from the near east region many millions of years ago and ancient fossil records show the plant occurring in India, Germany, and other European locations[1]. Conversely, both John and William Bartram clearly documented this plant in Florida in the 1700s and fossil records have also been found in Wyoming. So, which is it? Native? Invasive? Or does it really matter? To discuss this, let us start with an understanding of what invasive plants really are.

What Are Invasive Plants?

Invasive plants are not just troublesome, they are the most damaging plants in any ecosystem. By definition, an invasive plant originates from a foreign habitat and is known or likely to cause environmental or economic harm or harm to human health. Researchers have suggested that second to habitat loss, invasive plants are the greatest threat to natural environments and biodiversity. This sentiment is believed because invasives tend to take over and dominate ecosystems (e.g. environmental harm). Conversely, native plants originated in a given ecosystem and are often observed to coexist with a great diversity of other species and provide many ecosystem services.

Based on these definitions and the desire to promote biodiversity, biologists and ecologists have usually promoted the concept that invasive plants are bad and native plants are good. It is a simple, clear-cut message. Although this message is generally true, reality is often more nuanced than this.

Okay, So What Are Nuisance Plants?

Are all invasive plants 100% bad with no redeeming qualities? Well, not necessarily. Certain invasive plants can be used for bioenergy production while others are able to support native pollinators. Can some native plants be weedy and troublesome? Sure. Crabgrass seems to be present in lawns and sidewalks in every neighborhood, and poison ivy is a common problem for many nature lovers.

Rather than just seeing plants in terms of native equals good and invasive equals bad, an additional concept should be considered: the nuisance species. A nuisance plant is one that causes management issues, poses a threat to public safety, or is an annoyance. Nuisance plants often require management and some native plants fall into this category. These also include Carolina willow and cattail, which often require management in aquatic systems.

A illustration of water lettuce
A depiction of water lettuce by William Bartram

Water Lettuce: Native or Invasive?

So back to water lettuce. Is it native or invasive? Honestly, this fact is unknown and may never be known. A better question is this: is water lettuce a nuisance species? To me, this is much clearer and easier to answer. Let us circle back to Bartram, where he describes the plant like this[2]:

“It is remarkable that at the entrance of the river into the great lake there floats prodigious quantities of the pistia, which grows in great plenty most of the way from hence to the head of the river, and is continually driving down with the current, and great quantities lodged all along the extensive shores of this river and its islands, where it is entangled with a large species of water-numularia, persicaria, water-grass, and saxifrage…growing all matted together in such a manner as to stop up the mouth of a large creek, so that a boat can hardly be pushed through them, though in 4 foot water; these by storms are broke from their natural beds and float down the river in great patches…”[2]

Based on Batram’s description, an infestation of this size can easily harbor millions of mosquito larvae, impede water flow, and increase the potential for flooding, while also shading out acres of desirable submersed plants – thus reducing biodiversity. So, is water lettuce native or invasive? We don’t know. Is it a nuisance species? Clearly. Should it be managed to reduce the impact on the environment? For a plant capable of such massive proliferation, management is essential.

These ideas naturally set up the question: If water lettuce has been in Florida since Bartram’s days when no management was conducted, why does it need to be managed now? Should nature take care of itself? These are excellent questions, but the answers will have to wait for the next article.

Learn More About Water Lettuce

To learn more about water lettuce, check out our podcast, “Working In The Weeds,” where we talk about all things aquatic and invasive plants. In our “Water Lettuce Deep Dive” episode, we discuss the native vs. invasive confusion, learn more from William Bartram, and explore why we need to manage this plant. Subscribe to our podcast for more episodes on all things invasive and aquatic plants.

Featured Sources

[1] Susanne S Renner, Li-Bing Zhang, Biogeography of the Pistia Clade (Araceae): Based on Chloroplast and Mitochondrial DNA Sequences and Bayesian Divergence Time Inference, Systematic Biology, Volume 53, Issue 3, June 2004, Pages 422–432, https://doi.org/10.1080/10635150490445904

[2] Diary of a Journey through the Carolinas, Georgia, and Florida from July 1, 1765, to April 10, 1766. – John Bartram and Francis Harper. P 39. Transactions of the American Philosophical Society, Vol. 33, No. 1 (Dec., 1942).

This blog post was written by Dr. Jason Ferrell, UF/IFAS CAIP director and professor. Questions or comments can be sent to the UF/IFAS CAIP communications manager at caip@ifas.ufl.edu. Follow UF/IFAS CAIP on InstagramFacebook, and TwitterSubscribe for more blogs like this one. 

UF/IFAS Center for Aquatic and Invasive Plants. Turning Science Into Solutions.

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by Jason Ferrell

Source: UF/IFAS Pest Alert

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