Duckweeds are flowering aquatic plants which float on or just beneath the surface of still or slow-moving bodies of fresh water and wetlands. They arose from within the arum or aroid family (Araceae), often are classified as the subfamily Lemnoideae within the Araceae. Classifications created prior to the end of the 20 th century classify them as a separate family, Lemnaceae. The flower of the duckweed genus Wolffia is the smallest known, measuring merely 0.3 mm long.
Duckweeds have received research attention because of their great potential to remove mineral contaminants from waste waters emanating from sewage works, intensive animal industries or from intensive irrigated crop production. Duckweeds need to be managed, protected from wind and maintained at an optimum density to obtain optimal growth rates. One traditional use of Duckweeds is as a vegetable foodstuff. In many parts of the world, Duckweeds are consumed by domestic and wild (fowl, fish, herbivorous animals and humans). The smallest of duckweeds (of the Wolffia genera) has been used as a nutritious vegetable by Burmese, Laotians, and the people of northern Thailand for generations. Duckweed makes a fine addition to a salad and is quite tasty.
Duckweeds (most of genera species) comparatively to other aquatic plants, even the terrestrial, have a high binding capacity (fixation) of various minerals (cations and anions) from their growth medium. This property is exploited for cleaning water supplies (water depollution) but at the same time, this property constitutes a major restriction to use such plants as a source for human food alternative unless the cultivation environment is tightly controlled.
Symbiotic duckweed-bacteria cultures have been used to improve wastewater treatment and starch-biomass production from wastewater (production of energy/chemical feedstock) by duckweed. (http://duckweed2013.rutgers.edu/presentations/16_toyama_tadashi.pdf). JP patent pub no. S6352960 teaches fermentation of alcohol from a starch forming plant body, by cultivating a photosynthetic bacterium and microalga in an organic waste liquor, adding a rotifer and a water flea to the resultant growth liquid, cultivating Wolffia and duckweed in the presence thereof with the food chain between them, and utilizing the starch forming ability of the duckweeds, in particular, Wolffia. 
The natural habitat of duckweed is floating freely on the surface of fresh or brackish water sheltered from wind and wave action by surrounding vegetation. The most favorable circumstance is water with decaying organic material to provide duckweed with a steady supply of growth nutrients and trace elements. A dense cover of duckweed shuts out light and inhibits competing submerged aquatic plants, including algae.
Duckweed fronds are not anchored in soil, but float freely on the surface of a body of water. Wolffia fronds are oblate spheroid shaped with no visible roots or leaves. They can be dispersed by fast currents or pushed toward a bank by wind and wave action. If the plants become piled up in deep layers the lowest layer will be cut off from light and will eventually die. Plants pushed from the water onto a bank will also dry out and die. Disruption of the complete cover on the water's surface permits light intrusion and the growth of algae and other submerged plants that can become dominant and inhibit further growth of a duckweed colony. The most rapid reproductive rate occurs when the plant is at a surface concentration on 100 grams per square meter. Homogeneous dispersion, including stacking up to 20 layers, may occur at up to 700 grams per square meter. Initially, small clusters are formed held together by capillary forces. These merge into a continuous layer which can then stack to several layers.
Duckweed plants can double their mass in two to eight days under a variety of conditions of nutrient availability, sunlight, and temperature. This is faster than almost any other higher plant. Average growth rates of unmanaged colonies of duckweed will be reduced by a variety of stresses: nutrient scarcity or imbalance; toxins; extremes of pH and temperature; crowding by overgrowth of the colony; and competition from other plants for light and nutrients.
To cultivate duckweed a grower needs to organize and maintain conditions that mimic the natural environmental niche of duckweed: a sheltered, pond-like culture plot and a constant supply of water and nutrients from organic or mineral fertilizers.
The system for cultivation of aquatic plants, for example, a cultivation pond, a raceway type cultivation apparatus, a tubular type cultivation system, a liquid membrane-forming cultivation system have been well known. However, there are various shortfalls in these cultivation systems, such as requiring large water body area, expensive leveling of large areas, potential for contamination of the water, energy intensive water circulation of large amounts of water volumes, accumulation of toxins in the older plants residing in the water for protracted times, low yield due to overcrowding, inefficient use of light energy per unit of cultivation surface area, difficult to implement as mass production due to access limitations, harvesting difficulties et al. Transferring cultivation of a surface floating species to a solid non horizontal substrate is non intuitive and is particularly difficult because the duckweed does not anchor itself to a substrate but rather floats, which would cause it to fall off a non horizontal substrate when subjected to water shear flow forces occurring parallel to the substrate surface. Furthermore, if cultivating the Wolffia for human nutrition or other purpose where the objective of cultivation is for the harvested Wolffia to consistently comply with certain chemical composition requirements, then the non-horizontal cultivation would present difficulties in supplying the tiny plant proper nourishment both in terms of nutrient and of lighting conditions to avoid highly fluctuating composition, including of undesired trace elements and harmful microbiological infestation.
In view of the above, there is still a long felt need to address the challenges of solid substrate cultivation systems and methods for cultivating surface-floating plants, in particular, duckweeds in an efficient, low cost, controlled plant quality, space-saving and high yield manner.