How to Grow and Care for Duckweed

Welcome one of the world's smallest plants to your pond or aquarium

Common Duckweed (Lemna minor) growing in water in Italy

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Duckweed (Lemna spp.) is one of the smallest plants on Earth. The Lemna genus often produces by budding, but it rarely produces flowers. When it does flower, blooms are tiny, green, simple, and either male or female. The Library of Congress reports that the Wolfia genera of duckweed has the world's smallest blossom. Naturally, duckweed carpets quiet waters, floating gracefully and protecting the aquatic creatures below. Nodding to its common name, duckweed is a beloved food for ducks and also for turtles, goldfish, tilapia, carp, and koi fish. Native to much of North America, duckweed grows atop ponds, surviving winters in USDA plant zones 4 through 10. Adaptable to a wide range of temperatures, it can also be grown easily outdoors on home ponds and indoors, near a sunny window, in large aquariums. Duckweed is native to North America and Central America, Asia, Africa, and Europe. Plants have also been introduced into Australia and South America.

Common Duckweed (Lemna minor) keeps water clean, prevents the overgrowth of algae, and generously feeds fish. Plants measure 1/16 to 1/8 of an inch long. Each plant has one to three light green leaves that are flat and oval in shape. Each leaf then has one hair-like root that hangs down in the water. Miniature air sacs keep the plant afloat. Common Duckweed has a short lifespan, but it carpets rapidly and can even become invasive. When it does produce flowers, each simple, cup-like flower measures 1mm, only visible with a magnifying glass. A sticky secretion comes from the pistol, attracting flies, mites, spiders, and bees, which then contribute to the plant's pollination (otherwise the plant is able to self-pollinate or pollinate by the wind). Tadpoles, beavers, and birds are also fond of the plant. In fact, duckweed is grown in the commercial sector as a protein-packed animal feed for fish and livestock.

Common Name Common Duckweed, Lesser Duckweed
Botanical Name  Lemna minor (Previously known as Lemna cyclostasa, Lemna minima)
Plant Type Aquatic plant
Mature Size Starts 1/16 to 1/8 of an inch long, rapidly spreads, potentially invasive
Sun Exposure  Full sun
Water Type  Pond water
Water pH  Neutral (6.0-8.0)
Bloom Time Summer (if it blooms)
Flower Color Green
Hardiness Zones  4-10, USDA
Native Area  Central America, North America except Hawaii and South Carolina
Toxicity  Reportedly non-toxic

Duckweed Care

Purchase a handful of duckweed from a pet store. Because this aquatic plant can be nourished by the air, it is a very easy plant to grow indoors and outdoors.

To grow Common Duckweed indoors, cover the outside of the aquarium with black contact paper. Install an airstone and a small pump at its lowest speed to oxygenate the water. Fill the aquarium with pond water or, if pond water is not available to you, use tap water. Let the tap water stand overnight so the chlorine evaporates.

To grow Common Duckweed in a garden pond, be sure to monitor its growth. If the plant covers the entire pond, it can cause oxygen depletion and kill the fish swimming beneath its fast-growing carpet. Rake or skim any excess duckweed off the top of the pond. While algae will likely thrive in an aquarium in the light coming in through the glass, Common Duckweed tends to outgrow algae in ponds.

Invasive Species

Duckweed is known to be an invasive species in North Carolina. Common Duckweed spreads so rapidly that it can deprive ponds of oxygen and can cause fish kill and the death of beneficial algae in still waters. To maintain the health of one's pond and aquatic life, learn to prevent such dangerous spread as early in spring as possible. Ponds set in motion by the wind or calmly moving waters prevent fewer problems and require less duckweed control.

Light

Full sun is needed for best results, but the plant will tolerate low to high light and soft or hard water. Set the aquarium in a warm, sunny location where it receives at least six hours of sunlight every day if possible. High-quality, full spectrum light and the addition of trace minerals during water changes will encourage denser growth.

Water

Common Duckweed grows, floating on the surface of calm water. Keep the water calm with little to no current; if the water moves too much, the plant will not grow quickly.

If desired, cultivate it separately in a rectangular container that is at least five inches deep, 18 inches long, and 12 inches wide. Fill with dechlorinated water. Clean the container, but do not use chemicals or soap. Add water. If treated tap water is used instead of pond water, add plant fertilizer. Blow air in the water using a drinking straw about every 10 minutes until the water shows signs of oxygenation (or use a proper water oxygenator). Check the pH level with a pH meter. It should have a neutral pH between 6.0 and 8.0, preferably just over 7. Add duckweed. Handle the plant gently. To harvest, use a fish net or coffee filter to scoop the growing duckweed and transfer it to a fish tank, aquarium, or pond.

Fertilizer

Add a balanced 10-10-10 liquid fertilizer that has iron, likely found at an aquarium store. Dilute the fertilizer with four to five times the normal amount of water.

Temperature

Common Duckweed is adaptable to temperature. For best results, maintain a temperature between 63 to 79 degrees Fahrenheit.

Toxicity Tolerance

According to the NCBI, some studies suggest duckweed plants tolerate environmental toxicity. Other studies suggest duckweed could be as sensitive to toxicity as other aquatic species. Some studies also show that duckweed plants are especially helpful in testing herbicide pollution in an aquatic environment, "lake and river pollution, sediment toxicity, and the like."

Pruning Common Duckweed

Because duckweed grows so quickly, complete control of its invasiveness is impossible. Start prevention early in spring before the spread becomes a nuisance. Here are several ways to control and prevent invasive growth:

  • Rake or net small ponds repeatedly and compost the removed weed
  • Use a floating boom to sweep larger pools from end to end, regularly from early spring to winter dormancy
  • Put stop-boards at any upstream inlets to prevent duckweed from spreading to other ponds or lakes
  • Welcome weed-eating water birds like domestic or ornamental ducks, moorhens and coots
  • Welcome grass carp fish to quickly eat any of the Lemna species
  • Shade duckweed beneath tall, bushy plants, water lilies or other plants with floating leaves to reduce growth
  • Use a fountain to gently disturb the surface of the water

Although duckweed's spreading nature is considered invasive, many gardeners would agree the benefits outweigh the maintenance. This nutritious plant keeps many creatures well-fed, and it removes pollutants from the water, making it an ideal specimen for any aquarium or pond.