In many cases, you can grow duckweed (Lemna minor) in your pond without even meaning to. Getting duckweed started is simple, and once it starts growing, it will continue to flourish with little maintenance, making it a great plant to serve as natural fish food and to keep the interior of your pond free of weeds and algae. Some people use this weed to cover the tops of their aquariums to minimize green growth in the water and provide fish with a tasty treat.
Make sure your duckweed has a source of food. Fish waste will work. The duckweed needs decaying organic matter in the water for it to start growing.
Check the location for wind. Duckweed grows best in water that is largely undisturbed. Wind will prevent it from flourishing. If you want to grow duckweed in an area that has moving water, shelter the area from air currents or remove the source of the disturbance.
"Plant" the duckweed starters. Simply lay the duckweed starter plants on top of the water. They will begin to spread and multiply within days.
Keep the water temperature between 33 and 92 degrees F, if possible. Duckweed can survive lower and higher temperatures, but it grows best in this range.