Benign neglect is the keyword in duckweed cultivation. The less attention you pay to it the more it will thrive. Duckweed does not need supplemental fertilization or winter protection. It only needs still water in full to partial sunlight. Duckweed grows best in ponds that contain fish, as duckweed can be invasive if not kept in check. Fish love duckweed and will happily eat as much as they can. Duckweed is a simple plant consisting of two to three leaf- or frond-like structures and a single root or root hair. Duckweed is hardy to Zone 3 and grows on every continent except Antarctica.
Add duckweed to your pond in mid-spring by floating a small clump on the surface of the water. Duckweed can be purchased from most fish stores, garden centers or catalogs that sell aquatic plants and supplies.
Allow your duckweed to increase until it covers 1/3 of the total surface area of your pond.
Use a hand skimmer or fish net to remove the excess duckweed. Add the removed duckweed to compost piles, as it is a rich source of nitrogen, potassium, phosphorus and other nutrients.
Establish a duckweed breeding tank if your fish are eating it faster than it can reproduce. Use a bucket to remove pond water containing duckweed plants.
Place your aquarium in a sunny spot and fill at least half of it with the pond water/duckweed mix. Use pond water to top off the tank when evaporation lowers the water level.
Periodically add half of the duckweed growing in the aquarium to your pond.