Plant Biology

Side view of water lettucejpg

Side view of water lettuce.

Mat of water lettucejpg

Water lettuce mat.

Water lettuce is a free-floating aquatic plant originally native to northern Africa and South America [2, 8, 9]. It is considered an invasive or noxious weed in many countries, including the US [8]. These plants have the tendency to grow in close proximity to one another, forming dense mats [5, 8, 9]. Water lettuce is often found in still waterbodies, such as irrigation canals, ponds, and swamps [8].


Identification

Overhead view of water lettucejpg

Figure 1. View of water lettuce from above.

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Figure 2. Water lettuce with roots shown.

Water lettuce has light green, velvety leaves and resembles an open head of lettuce or cabbage (Figure 1) [5, 8, 9]. The undersides of leaves are covered in small hairs, which trap air and help the plant support itself on the water’s surface [5]. The roots of the plant are long and feather-like (Figure 2) [5, 9]. Water lettuce flowers grow in small spike-shaped clusters in the middle of the plant (Figure 4) [5, 9].


Reproduction

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Figure 3. Water lettuce stolons.

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Figure 4. Water lettuce flowers. Image courtesy of EDDMaps.

This aquatic plant can reproduce both sexually and asexually [4, 6, 9]. During warm, wet seasons, water lettuce spreads mostly by vegetative (or asexual) reproduction, with the plant essentially creating clones of itself that are attached to the original plant by a type of modified stem called a stolon (see Figure 3) [4, 5, 9]. Wind, animals, and boats can break these stolons, which allows the new plants to disperse and infest new areas [9]. In subtropical and temperate regions such as the southeastern US, winter freezes can cause drastic diebacks of water lettuce and halter vegetative reproduction [4, 7, 9]. Populations of water lettuce in these regions rely on seeds deposited on the floor of the water body from sexual reproduction to rebound after harsh winters [4, 6, 9]. In the spring, these seeds will readily germinate in shallow waters that receive copious amounts of sunlight [4, 9].


Distribution

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Distribution of water lettuce in the US. Image courtesy of EDDMaps.


Water lettuce is found on every continent except Antarctica [8]. The origins of the plant are unclear, but most experts agree that it is native to northern Africa and South America [8]. However, some evidence suggests that it may also be native to southern Florida as well [8, 9]. Due to its popularity in the aquaria and horticultural trade, the plant has been introduced to many localities worldwide [6]. Currently, the plant is found in at least 26 states in the US, with high densities occurring in the Gulf Coast region [8].


Author: Seth Spinner

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The LSU AgCenter and the LSU College of Agriculture

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