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 produces 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. 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. (Although the plant produces oxygen during the day, it consumes it at night, which can exceed the amount produced and result in fish kills.) 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.
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 quickly, 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.
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.
