Water Lettuce Facts, Care, Planting, & Hardiness (Pistia stratiotes)
Water lettuce is a floating, herbaceous perennial plant found across the globe in tropical and subtropical regions. Water lettuce, also called water cabbage or Nile cabbage, is aptly named and looks like a head of the leafy greens.
Water lettuce was first described along the Nile River around 2,000 years ago. References to the plant were made in both Ancient Egyptian hieroglyphics and the writings of Ancient Greek botanists. Water lettuce has since been found in bogs, lakes, and marshes of every continent except Antarctica.
Water lettuce is popular for those with water gardens because its presence inhibits the growth of algae and cleans the water. It has even been proven effective at removing heavy metal toxins like zinc and cadmium from a water supply.
Water Lettuce Growth & Hardiness Considerations
Water lettuce grows in rosettes on the water’s surface with thick, soft leaves that can reach 15 cm (6 in) in length. One rosette can reach a diameter of around 30 cm (12 in) and a height of 10cm (4 in). Dense feathery roots extend up to 1 meter (39 in) below the surface of the water and provide ample shelter for small fish. New growth usually occurs with the development of smaller daughter plants connected to the original growth via stolons.
Water lettuce is a floating perennial plant found across the globe in tropical and subtropical regions. It requires growth temperatures of at least 15o C (59o F), and grows best around 22 to 30o C (72-86o F). This plant is particularly sensitive to dissolved minerals and requires water with a salt content of 2.5 ppt or less and an absence of lime.
Water lettuce produces small, inconspicuous flowers in the center of the plant in late summer through early autumn. They produce massive quantities of seeds, which can sink to the bottom of the water and survive winter in temperate climates. Although not winter hardy, water lettuce seeds can survive under water at temperatures of at least 4 o C (39 o F) for two months.
How to Plant Water Lettuce:
Most water lettuce is sold by head or in small groups. Any yellow leaves should be removed before planting. To plant, water lettuce can be simply spread across the surface of your pond during late April after the last frost of the season. Water lettuce grows best if it is not constantly being moved around. You can enclose your water lettuce using a floating plastic hoop, fishing line, or rocks. Although tropical plants, water lettuce grows best in the early part of the summer when temperatures reach at least 15.5 o C or 60 o F. It is a good idea to provide partial shade during midday when the temperatures are at their peak, especially if you live in a hot climate.
How to Care for Water Lettuce:
Water lettuce grows rapidly and requires the removal of excess plants to prevent overcrowding and nutrient limitation. There are two different strategies to accomplish this. Removing the small daughter plants will promote additional growth and is ideal for propagating plants. On the other hand, removing the larger plants will slow down growth and decrease the required maintenance.
It is recommended to bring water lettuce inside over winter. To do this, place several plants in wet, sandy loam before the first frost and store in an area with bright light and a temperature of at least 10 o C or 50 o F. Some gardeners have also had success overwintering by placing plants in a clear goldfish bowl filled with rainwater and storing in full sunlight at 50 o F (10 o C) or higher.
If a pond is particularly small, water lettuce may consume certain nutrients and start to turn yellow. There are several ways to remedy this including floating the failing plants in dissolved Miracle Grow for several hours, applying the solution of the depleted nutrient to the pond, and treating the pond with a commercial nutrient booster.
Additionally, if you use a pond skimmer, be sure to regularly check that no water lettuce has floated into the system, which can damage the pump. Water lettuce may also be eaten and killed by pond fish. For this reason, it may be a good idea to contain water lettuce in aquatic baskets or separate the problem fish with a solid boundary.
Is Water Lettuce Toxic or Invasive?
Water lettuce is native to most of the tropical world, but it is considered a pest because of its penchant for growing out of control. Water lettuce can create an economic toll on an area by choking irrigation and drainage systems and by preventing boating and fishing access in lakes, canals, and rivers—an important tourist draw. Water lettuce has also been identified as a risk for taro and rice fields. Severe ecological consequences have also been described. Water lettuce can block the air-water interface of a body of water, which limits gas exchange and causes dissolved oxygen levels to plummet. Additionally, the leaves, which are often filled with stagnant water, are ideal breeding environments for mosquitos.
Although there is some disagreement on whether water lettuce is native to the United States, the explorer William Bartram described large growths of the aquatic plant limiting their boat access through a creek in 1765. Regardless, water lettuce is prohibited in the states of Florida, Louisiana, Mississippi, and Texas, and the country of New Zealand.
Water lettuce is toxic if ingested by small animals like pets or even children. It contains a poisonous compound, calcium oxalate, which burns the mouth and may lead to kidney damage.
Will Fish Eat Water Lettuce?
It is not well documented which organisms enjoy eating water lettuce. However, many home gardeners have found that their fish can kill their water lettuce plants by eating its roots.