Tips for Growing Duckweed
Where to get duckweeds?
Duckweeds are available from many sources.
How to handle duckweeds?
Duckweed plants are delicate and easily damaged by fingers, forceps and other instruments. Individual plants and small colonies may be picked up and moved without damage using a bacteriological loop. Just place the loop in the medium beneath the plant and lift up. To collect larger quantities of plants, use lightweight screening material to net the plants from below. Fiberglass screen material is available in hardware stores. Alternatively, fabric stores sell strong, light-weight netting used for making veils. Duckweed roots are sticky and will adhere to screens and nets, so it may be necessary to gently scrape the plants off the net with a knife or a thin spatula.
How to grow duckweeds?
Growing duckweeds is like growing any other plant. Moderate conditions of temperature and light and a liquid medium with the necessary nutrients are essential for good growth. Fortunately, duckweeds adapt well to a wide range of conditions and are easy to grow.
Duckweeds can be grown in the pond water from which they were collected in open containers. It is important to replace the water frequently, since evaporation will result in concentration of salts. Using open containers prevents overheating if you place the containers outside or in a sunny window. See below for more about lighting duckweeds for the best growth.
In nature duckweeds grow in water from many sources and compositions. They can be grown in artificial pond water or in diluted aquaculture media, such as Hoagland’s solution. It is important to provide a source of chelated iron and to adjust the pH to the optimal range.
It is important to keep your duckweed cultures clean. If you collect fresh duckweed specimens from nature, the water will contain a variety of other organisms. These will include bacteria, fungi, algae, protozoa, and even small multicellular animals and insect larvae. You can clean up your duckweed cultures by transferring the plants individually to clean fresh water. Remove damaged and aged (yellow or white) fronds from your cultures as they appear.
Native populations of duckweeds may be mixtures with varying genetic compositions. For serious work it is advisable to start cultures from a single clone. This will help increase uniformity for experimental work. It is easy to clone duckweeds.
Lighting duckweeds for the best growth.
Direct sunlight is a natural condition for duckweeds. Duckweeds commonly grow in open ponds or shallow wetlands with little or no shade. However, direct sunlight can be a problem if you grow duckweeds in small containers. Sunlight will warm the water and cause evaporation. Replacing the lost water frequently (not just topping off the lost volume) is important. Otherwise, you will gradually concentrate the salts in the growth medium. Duckweeds are freshwater plants (glycophytes) that do not tolerate high salt conditions. Plants grown in covered containers may not lose water from evaporation, but under direct sun the interior will overheat, bleaching and killing the plants.
Indirect sunlight, from a north window or skylight may be an acceptable light source, but growth may be slow, particularly if the days are short and there is much cloud cover. If you use indirect sunlight, remember that radiation cooling can be a problem at night during the colder months. Radiation cooling results from the difference in temperature between the plant (room temperature) and the night sky (very cold). Radiation cooling will slow duckweed growth, although most duckweed species are not damaged by cool temperatures. It may be necessary to cover the window at night to prevent excessive cooling. [ Read how greenhouses work. ]
Incandescent light bulbs are a poor choice. A major fraction of the light that they emit is in the form of infrared radiation that will directly overheat your plants. It is hard to obtain sufficient light from incandescent lamps for good photosynthesis without overheating, so they are not recommended.
Fluorescent lights are recommended if closed culture vessels are used, or if a sunny window is unavailable. Unlike incandescent bulbs, fluorescent tubes produce much less infrared energy. Most labs use two to four F40cw tubes in simple fixtures (often sold as shop lights) hung roughly 30 to 50 cm above the cultures. These conditions will supply sufficient light for photosynthesis and plant growth without overheating the plants.
Newer compact fluorescent fixtures that combine a twisted fluorescent tube with an electronic power supply in a screw-in base are especially convenient for building small duckweed growth areas. These fluorescent fixtures are also available in reflector mounts like floodlights. Plans for building an inexpensive portable plant growth stand are available.
Different duckweed species grow from the Arctic and Antarctic Circles to the Equator and from sea level to the high mountains. However, different species are better adapted to various temperature conditions. If you are going to experiment with duckweeds outdoors, you may be more successful with a locally gathered species than with a culture from a stock center.
Duckweeds can tolerate hot midday air temperatures if the water on which they rest warms more slowly than the air. Thus, a deep container (like a bucket) will be necessary if you want to grow duckweeds outdoors in hot weather.
Under cool cool conditions, duckweeds may form dormant buds, called turions. Duckweeds can overwinter in frozen ponds as turions or seeds. Freezing vegetative fronds will cause frost damage, as in other plants.
Studying the effects of stresses, like high or low temperatures, is an excellent subject for research.
Landolt, E. and Kandeler, R. (1987) The family of Lemnaceae – a monographic study. Vol. 2, Phytochemistry, physiology, application, bibliography. Veroff. Geobot. Inst. ETH, Zurich, 638 pp.