The world population crossed the 7 billion mark almost two years back. It is predicted that by 2050 (which is not so far away!) it will be 9.6 billion. India has 1.21 billion people just behind the country with the largest population, China (1.34) but will soon be at the top.  This has serious consequences concerning the worldwide availability of food, energy and fresh water. The larger number of people in future will also have higher demands per person concerning the food quality. Energy supply is getting more and more difficult as the development since last 200 years in the industrialised western countries is based on terrestrial stocks like coal and gas. It is clear now that energy supply on a long range has to be sustainable and cannot base unlimitedly on coal, oil, gas or atomic power. Moreover, burning coal and oil contributes to the increase of the global temperature with adverse effects on agriculture and water supply. At the moment, the availability of fresh water is already one of the most serious problems as cleaning of waste water or its regeneration in nature is not anymore in tune with the accumulating amount of municipal or even industrial waste.

The name of this family of plants in English, “duckweed”, tells us already a lot about the importance of these seemingly inconspicuous plants. Have you ever seen ducks or other water fowls eating duckweed? Evidently they love it. This explains the first part of the name. The second part suggests that these plants grow like a weed, i. e. very fast. This is indeed the case. We have to talk about is later.

Shall we first have a closer look at how duckweeds look like or how they grow? This is important when we think of duckweed as a potential crop plant. We have to learn that duckweed is not a specific, single plant but a whole plant family – although a small one consisting of only 37 species. In India four of them are very common. They are scientifically named Spirodela polyrhiza, Landoltia punctata, Lemna aequinoctialis, and Wolffia globosa. There are approximately eight more species but these are rather rare in India. The four common species are very different, e.g. in size. Single plants of Spirodela polyrhiza have a size of approximately 1.5 cm – and this is the largest duckweed, also called giant duckweed.  Such plants are often connected to 2 or even 10 other plants and then the whole colony looks much larger. The size of Wolffia globosa is below 1 mm, often are two plants (mother and daughter) connected to each other. The other two species are between Spirodela polyrhiza and Wolffia globosa in size, less than 1 cm.

Duckweeds are flowering plants. The flowers are tiny and hardly seen without a magnification glass. Last year on our field trip to Madhya Pradesh, we saw a lake full of flowering Lemna aequinoctialis. However, such a view is very rare. Normally, these plants propagate without flowering. This is termed vegetative propagation: the younger plants (daughters) come just out of a hidden pocket of the older plant (mother). The daughters which are still connected to the mother plant develop granddaughters inside them and even further generations. This is most probably the secret of the very fast growth of duckweed. The precise growth rate depends on the duckweed species, temperature, light conditions and the composition of the pond water. As an example, plants can double their number in 2 days or even less. Try to count for a few days: Let’s start with 10 plants. After two days there are 20 plants, after 2 more days 40 plants and then 80 plants. Of course this will not continue forever. When the space is limiting then the growth is limited. But can you see the potential of duckweed with their very fast growth? This is not only in the scientific laboratory. Meanwhile, also first outdoor results are known. For the two common species in India, Spirodela polyrhiza and Landoltia punctata, the harvest dry weight yields between 24 and 54 t ha-1 yr-1 and 69 and 117 t ha-1 yr-1, respectively, are known. This can be compared with common land crop plants in agriculture.  The comparable value for sugar cane is 23 t ha-1 yr-1, the highest in Ethiopia in 2010 was 41 t ha-1 yr-1. The best national yearly yields of sugar beet, potato, maize, rice and wheat grains in the United States were 17.1, 11.1, 9.6, 7.6 and 2.9 t ha-1 yr-1, respectively (Source: USDA). This shows that duckweeds can indeed be used to produce more biomass per area of cultivation than any land-based crop. Moreover, the cultivation of duckweed does not need fertile land. Duckweeds can be produced in aquaculture wherever the other required factors are available, also on barren land.

What are the other required factors? Duckweeds have an optimal growth temperature of around 28oC. At temperatures around 10oC, growth is almost completely stopped. Together with the requirement for sufficient sun light these requirements are almost ideally fulfilled in many areas of India throughout the year, except in the high altitudes. Consequently, duckweed can be found everywhere in India from Kerala to Jammu and from Gujarat to West Bengal.

We have not yet mentioned the requirement for fertilizers. In the present day agriculture, often large amounts of fertilizers are sprayed on the soil. A part of it finally is washed out and contaminates surface or even ground water. This is quite different in the aquaculture of duckweeds. Duckweeds can grow very well on waste water. Waste water contains large amount of chemicals which are nutrients for plants, e.g. phosphate and nitrate. Growing plants need to take up nutrients to support their growth. Instead of using large amounts of fertilizers which unavoidably also contaminate water, duckweed take nutrients from contaminated waste water and clean it this way. One precondition is: waste water must not contain too much toxic compounds like herbicides or pesticides or chromate as it sometimes is the case in industrial waste water. Such toxic compounds must be removed before they reach surface or ground water as it is dangerous for humans anyway.

Thus, we can produce very effectively huge amount of biomass from aquaculture of duckweed and during duckweed cultivation the waste water can be cleaned. The next important question arises: What can be done with this duckweed biomass? What are the harvested duckweeds good for?
To learn about these facts you will have to wait for our next article!!!

Views expressed by PD Dr Klaus-J Appenroth, University of Jena, Germany ( and Dr K Sowjanya Sree, AIMT, Amity University, Noida, UP, India (