Pistia spp.

water lettuce

SunFull Sun – Part Shade

HardinessZones 9–11

Overview

Pistia spp. is a genus in the arum family Araceae containing the single free-floating aquatic species Pistia stratiotes, known as water lettuce, now found throughout the warm fresh waters of the world. Plants float on the surface as rosettes 4-12 inches (10-30 cm) across of spongy, wedge-shaped, pale blue-green leaves covered in short water-repellent hairs and marked by deep parallel ribs. A dense tuft of feathery roots hangs beneath, drawing nutrients straight from the water. New rosettes form on short runners, so plants quickly build into floating mats. The flowers are tiny and easily overlooked, held in a small pale green spathe less than 0.5 inch (1.3 cm) long near the center of the rosette. Pistia spp. grows in still and slow-moving fresh water such as ponds, lakes, ditches, and backwaters in frost-free climates. It is killed by frost and by drying out, and its rapid spread forms dense surface mats that shade out other aquatic life, so it is listed as an invasive weed in many regions and its cultivation and movement are restricted by law in several places.

Native Range

Pistia stratiotes grows throughout tropical and subtropical fresh waters worldwide, and its exact native origin is uncertain, with Africa and South America both proposed. It occurs in ponds, lakes, slow rivers, canals, and swamps.

Suggested Uses

Used in frost-free ornamental ponds and container water gardens for surface cover and shade, and in aquaria as a floating plant. In many regions its use is limited or prohibited because of its invasive spread.

How to Identify

Recognized as a free-floating rosette of pale blue-green, wedge-shaped leaves with deep lengthwise ribs and a velvety, water-repellent surface, sitting on the water like an open head of lettuce. Long, branched, feathery roots trail below. The floating rosette habit and ribbed velvety leaves distinguish Pistia spp. from other floating aquatics such as Eichhornia spp. and from the much smaller duckweeds.

Appearance

Size & Dimensions

Height2" - 6"

Width/Spread4" - 1'

Colors

Flower Colors

green

Foliage Colors

green

Bloom Information

Bloom Period

~4 weeks

Summer

Flowering is inconspicuous and easily missed, with tiny flowers enclosed in a small pale spathe at the center of the rosette, mainly in warm weather. Reproduction is mostly vegetative, by daughter rosettes on short runners. Berries with small seeds form only occasionally.

Detailed Descriptions

Foliage Description

Pale blue-green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 5-10 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 7.5(Neutral)

357912

Drainagewet

Water & Climate

Water Needs

High

Frost Tolerance

tender

Time to Maturity

Less than 1 year

Care & Maintenance

Care Guide

Grow Pistia spp. as a floating plant on the surface of still or slow fresh water in full sun to part shade, in frost-free conditions or as a summer-only plant lifted before cold. It draws all its nutrients from the water and needs no soil, multiplying rapidly when water is warm and moderately fertile. Warm water above about 60F (15C) drives growth, while frost and freezing kill the rosettes outright. Crowded mats are thinned regularly by scooping out surplus rosettes, keeping the water surface partly open. In cold climates a few rosettes are overwintered indoors in a warm, bright tank. Because of its weedy spread, plants and trimmings are disposed of well away from natural waterways.

Pruning

Thin the floating mat regularly by removing excess rosettes by hand or net through the growing season. Lift and discard yellowing or crowded plants to keep the colony healthy. No cutting is needed beyond removing surplus growth.