Tips for Growing Duckweed
Where to get duckweeds? How to handle duckweeds? How to grow duckweeds? Axenic (Sterile) Culture Growth Media for Duckweeds
Lighting Conditions Growth Temperature Measuring Growth Links for more tips Troubleshooting (My duckweed didn't grow!)

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.  This technique also facilitates handling axenic cultures.  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 (included in the recommended synthetic media) 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.

Axenic (Sterile) Culture.

Duckweeds can be grown in sterile (axenic) culture.  Axenic culture is useful for studying the effects of organic chemicals and nutrients on duckweeds without the complications introduced by microbial metabolism.

Axenic growth requires the use of closed containers to prevent microbial contamination.  Growing these plants in closed containers requires a cool source of light.  Fluorescent tubes (see Lighting duckweeds) work well, particularly if sugar-fortified medium is used.

To obtain axenic cultures, the plants must first be surface-sterilized.  This is easily done by placing plants in a 10% to 50% bleach (sodium hypochlorite) solution for several seconds to a minute or so.  It is important to use a fresh solution of bleach, since chlorine slowly evaporates.  Treat plants with bleach for varying amounts of time to ensure that you have at least one living culture that is sterile.  Be sure to rinse the plants in several changes of sterile medium or sterile water before transferring to the final growth medium.  Transfer the plants using a heat-sterilized bacteriological loop.

It is prudent to place each treated plant in a separate container, so that plants that are not sterile do not contaminate the rest.  This will also protect properly rinsed plants from plants containing residual chlorine.  Residual chlorine will kill the plants or prevent them from growing.

Examine your plants after rinsing them in fresh medium.  Properly sterilized plants will have a small green area in the bud zone along the center of the frond.  If there is no green bud remaining, the plant was treated too long and is dead.  Since only a small bud is left to re-grow after surface sterilization, it may take some time before sufficient plant material is available to do experiments.  The growth rate can be accelerated somewhat by using medium fortified with 1 % sugar.

You can test duckweed cultures for sterility by removing a few plants and placing them in a rich medium (by adding sugar, yeast extract and tryptone, for example).  Contaminating microorganisms will grow rapidly and shortly make themselves visible.

Growth medium can be autoclaved (sterilized in a pressure cooker) or filter-sterilized into pre-sterilized containers.  It is important to note that some organic nutrients are destroyed by autoclaving.  Some mineral salts may precipitate during autoclaving.  After it has cooled, gently swirling the container will sometimes bring the precipitate back into solution.  Empty glassware (but not plastic caps!) can be sterilized by heating in a dry oven.  For a thorough discussion read Methods of Sanitization and Sterilization by Maribeth Raines.

Growth Media for Duckweeds.

Duckweeds can be grown in a variety of synthetic media.  Duckweeds are usually grown floating on the surface of the medium, but they will also grow well on media solidified with agar.  A dilution of Hoagland's solution plus EDTA-chelated iron is most often recommended.  If a single crop is to be grown, it is not necessary to change the medium or add additional nutrients later.  Under ideal conditions on mineral media, the plants will grow to nearly cover the surface of the growth container.  [ formulas for synthetic media ]

If the plants are grown under aseptic conditions, sugar can be added to the medium.  With 1% sucrose (table sugar) duckweed plants grow very rapidly indoors under fluorescent lamps.  In fact, they will continue to grow after they cover the surface of the culture container and begin to pile up on top of one another.  Under these conditions, I have seen Lemna gibba grow until all the free liquid has been taken up and the culture bottle is filled with plants.

For a thorough, but understandable, discussion of aquaculture media, I suggest this short article:  Nutrient Management in Recirculating Hydroponic Culture by Prof. Bruce Bugbee, Crop Physiology Laboratory, Utah State University, Logan, UT 84322-48

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.

Growth Temperature.

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.

Links for more tips

Troubleshooting (My duckweed didn't grow!)

The author of this website gets messages from students, their parents and teachers.  In this section, I'll deal with the most frequent problems.  Here is one example:

Question:

"My son and I had visited your website and became interested in doing a jr. high school science experiment with duckweed.  We got some duckweed from a pond and tried to grow it under fluorescent lighting near a window.  ( We hoped to see how well different plant colonies grew under different periods of lighting.)  We added a water soluble fertilizer (Peter's plant food diluted 5:1 more than the regular concentration) to tap water that we left standing for several days and added the duckweed.  We did not have the chemicals to make your suggested growing solution.  After several days, we ended up with a bacteria film on the water and little growth of the duckweed.  We washed out the container and are ready to try again."

"The duckweed we left outside  in a mixed pond water/tap water container (with no fertilizer) seems to be doing well.  Beside going back and filtering out several gallons of pond water for the project, or adding chlorine to the water for bacteria control (which I thought might also harm the duckweed), are there any commercial products to help grow Duckweed for a short term experiment?"

Answer:
I like people who write me with a specific, defined question and have done some preliminary work:  congratulations!

You did all the right things, so why didn't your little plants grow?

Most probably, the pH of the solution with Peter's is out of the growth range.  I suggest that you measure the pH of the solution and adjust it to be in the range of pH 6-8.  I suspect you'll find the pH is below 6.  If so, adjust it with a VERY DILUTE solution of NaOH or ammonia [ Safety tip ].  Be careful: since there is no buffer in the Peter's, the pH will rise very fast once in the correct range.  If it becomes higher than pH 8, then start over with a new batch or (not recommended) lower it with dilute phosphoric acid.

The clue that suggests that pH is the problem:

"The Duckweed we left outside in a mixed pond water/tap water container (with no fertilizer) seems to be doing well."

You might just want to filter the pond water and use it!  The big problem with using pond water:  lack of chelated iron, which Peter's provides.

Some additional suggestions from Prof. Ross E. Koning, Eastern Connecticut State University posted on Bionet.



Safety tip

Adjusting the pH of a solution usually requires preparing and using a dilute solution of an acid (e.g. phosphoric acid) or a base, (NaOH, ammonia or KOH).  If you are a student, I strongly suggest that you discuss safe preparation and handling of these solutions with your teacher and make them only under your teacher's direct supervision.  For convenience and relative safety, I recommend instead that you purchase dilute solutions of acids and bases from a laboratory supply house already prepared.  These dilute solutions are still hazardous and must be handled with care. I repeat:  any solution of acid or base should be handled with due care.

As a substitute for strong acid or base, one can use solutions of a weak acid or base, which are safer to handle.  For example, if the pH of your duckweed medium is too low, adjust it with a dilute solution of dibasic potassium phosphate (K2HPO4).  To lower a pH that is too high, use a dilute solution of monobasic potassium phosphate (KH2PO4 ).  This has the advantage that the PO4 (inorganic phosphate) is a pH buffer, so it will reduce the change in pH that may occur to the medium after it is added.   Be advised that phosphate is also a major nutrient for duckweed, so adding it in varying quantities to the medium will likely affect the duckweed growth rate.  Thus, it is advisable to prepare a very large quantity of medium at the start to use for all of your experiments.


Reference

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.


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Revised:  June 5, 2005