Water lettuce consists of one species in the genus Pistia (Pistia stratiotes) in the Araceae. You may read about several different other Pistia species of plants called water lettuce and examples in other genera such as Limonensis, Apiosperium, and Zala but all have been reclassified previously misidentified and are now considered as synonyms of Pistia stratiotes.
Water lettuce is pantropical, but most widely distributed in the southern hemisphere. Other common names for the plant include shellflower, Nile cabbage, and water cabbage.
The plant was first identified in the Nile delta but now has been distributed worldwide. In many freshwater locations, it is considered an invasive weed, and attempts to curtail its spread have been extensive.
Pistia stratiotes is a buoyant plant and a perennial monocot, meaning it overwinters in warm climates as an adult plant and sprouts offspring which start with only one primitive leaf. Water lettuce is a dioecious plant, which means there are separate male and female plants, each bearing male or female flowers, respectively.
Water lettuce is a beautiful and visually stunning plant, although it is all green with no other coloration, many people regard the plant as very aesthetically pleasing.
It is a large floating plant and leaves in mature specimens can grow up to 14 cm long. The leaves are light green in color like cabbage, have wavy margins with parallel leaf veins, and are hirsute (hairy).
The surface hairs form concentrically like baskets, are covered with a thick wax, and help to form air bubbles for buoyancy. Flowers are hidden down between the leaves in the middle of the plant.
After fertilization and maturation of the ovaries, they develop into small green berries in the center. Water lettuce can also reproduce through asexual propagation (budding), with daughter plants remaining attached to the parent plant by an abbreviated stolon (a type of plant runner).
This reproductive and growth characteristic causes water lettuce to form very dense surface mats in favorable conditions and has been known to reduce gas exchange at the water surface both by restricting surface area access to the air and by reducing contact of wind currents with the surface of freshwater bodies.
Water lettuce is considered one of the most productive freshwater plants in the world and because of the vigor of its growth and broad growth condition tolerances, it is a pest weed in many places. Pistia stratiotes thrive in eutrophic (nutrient-enriched) waters and will readily grow in heavily eutrophic conditions such as fertilizer- or sewerage-rich environments.
Along with water hyacinth, water lettuce is a common choice for phytoremediation programs to clean waters of agricultural and industrial runoff or for use in wastewater treatment systems.
Benefits of Water Lettuce in Your Aquarium
Clean the Water
Water lettuce aggressively consumes decomposition byproducts in water, such as nitrate, nitrite, and phosphates, as well as other organics. It is a very effective plant for keeping your aquarium waters in chemical balance and healthy for fish and invertebrate life.
Reduces Algal Blooms
As with many surface-dwelling, buoyant aquatic plants, they can restrict light penetration to deeper levels of the aquarium stifling the growth of algae and also sequester the nutrients necessary for algal growth. Water lettuce is very effective at controlling algae levels in an aquarium environment.
Downsides of Using Water Lettuce
Heavy overgrowth can form a very dense, interconnected mat that reduces surface air exchange and has been known to contribute to fish kills in the wild. In aquarium systems with bubble stones, however, this is not an issue.
Excess growth can be curtailed by an increased oxygenation of the water since water lettuce prefers more eutrophic environments which tend to drive toward low dissolved oxygen content.
Depending on where you live, it can be difficult to obtain water lettuce. It is considered a restricted pest weed in the wild and, in some states or countries, it is illegal to transport or possess water lettuce.
Water lettuce is currently illegal to possess or transport in the following US states: Alabama, California, Florida, Louisiana, Mississippi, South Carolina, Texas, and Wisconsin. Pistia stratioites are also banned in the United Kingdom and New Zealand and are heavily restricted in Australia and Japan where permits are required.
In the European Union, a ban has been considered but is not currently in place due to lobbying opposition.
How to Care for Water Lettuce
Despite its vigorous nature, water lettuce requires some attention to establish the plant in aquarium environments. Once established, however, growth must be managed or you will end up with a runaway train full of plants.
Water lettuce tolerates soft to moderately hard water in a pH range of 6.5-7.5. It thrives in medium to bright light (such as full-spectrum T8 or T5 grow bulbs) and prefers moderately warm water temperatures (21-27°C) since it is a tropical plant.
If you are growing water lettuce in a dry environment (we mean the air, not the tank since liquid water is, of course, wet), leaves can dry out and become vulnerable to being burned by bright light. Adding an aquarium cover usually remedies this problem without much additional effort needed.
When establishing new plants, allow your water lettuce to acclimate for a few days in lower light levels and adjust to the water chemistry of the tank. Water lettuce sold in nurseries is typically grown in lower light conditions just to try and keep it under control, so the plants will need a short period of adjustment before you transition them to brighter light levels, otherwise, the leaves will burn.
If you have other aquatic plants in your tank, you will want to regularly thin out the water lettuce so that light can reach deeper levels of the tank. Failure to do this on a regular basis (weekly) will kill off the other photosynthetic life in your aquarium.
If your tank filter system generates a substantial current, insert a floating dam to protect (and restrict spread) of water lettuce. Flowing water with disturb water lettuce plants and they will cease growing or become submerged and die.
A simple dam solution consists of plastic tubing with suction cups on the ends which can be strung across a portion of the tank.
What Fish like Water Lettuce?
Most fish get along just fine with water lettuce. Some larger fish such as cichlids may damage the plants. If you host goldfish or any large herbivorous fish, be prepared for your pets to graze on the plant roots and sometimes the leaves, which can stunt water lettuce growth or kill the plant.