Obtaining and Culturing Duckweed
To start a laboratory culture of duckweed, you can collect plants from a local pond or order them from a biological supply company. If you collect your own duckweed, make sure to choose a relatively unpolluted source to avoid collecting from a population that is adapted to living in contaminated water.

Duckweed cultures grow best in broad, open containers in a sunny location. If containers are covered to reduce evapotranspiration, they should be placed in indirect sunlight or under fluorescent lighting rather than in direct sunlight to avoid overheating and scorching of the plants.

Duckweed can be grown in pond or spring water, with occasional fertilization using a houseplant fertilizer. To avoid buildup of salts, it is necessary to replace the water every week or two rather than just replenishing evaporation losses. Duckweed thrives in nutrient-rich waters, so adding a scoop of pond sediment or compost will help to provide organic as well as inorganic nutrients.