Side view of water lettuce.
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 . 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 .
Figure 1. View of water lettuce from above.
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 . 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].
Figure 3. Water lettuce stolons.
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 . 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 of water lettuce in the US. Image courtesy of EDDMaps.
Water lettuce is found on every continent except Antarctica . The origins of the plant are unclear, but most experts agree that it is native to northern Africa and South America . 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 . Currently, the plant is found in at least 26 states in the US, with high densities occurring in the Gulf Coast region .
Author: Seth Spinner
The LSU AgCenter and the LSU College of Agriculture