Duckweed (Lemna Minor) is a weed. People say that if you try to grow it, you’ll have no luck. But God help you if you have a place where you don’t want it to grow. Well, the eradication of duckweed is not the topic of this post. Duckweed makes a fine fish food and hopefully you will be able to grow it yourself using my techniques.

Below you see a photo of my duckweed tank on April 17. It’s clear the poor little things barely survived the winter.

By April 23, it seemed to have improved somewhat.

In another six days on April 29 it looked like this.

What made the difference?

Duckweed like any other plant has a set of conditions required for optimum growth. It is common for it to double its mass every 24-48 hours. What other plant can make that claim?

Duckweed grows in water temperatures between 6 and 33° C. The best temperature is 30° C.

Duckweed thrives on oxygenated nutrient water directly from your fish tank. While most plants prefer their nitrogen in the form of nitrates, duckweed actually likes ammonia instead. It may sound strange but I find it helpful to pour in a few ounces of fresh urine every other day or so.

Duckweed does well in full sun to partial shade. A 40% shade cloth is recommended in the hot desert summers.

Where many duckweed farmers go wrong is the failure to recognize that duckweed does not like surface water currents. Slight movement due to light breezes is actually helpful but duckweed in the wild grows on calm backwater ponds. Any water currents are subsurface and don’t disturb the plants.

My grow tank is about 72 inches long. The water is pumped in directly from the fish tank. It falls a short distance into the duckweed tank. This serves to add oxygen through the bubbles. To avoid surface disturbances in a the rest of tank it drips into a juice bottle. The bottle is weighted with a few pebbles so it doesn’t float. It has holes around the sides near the bottom and below the water level.

The water fills the tank and exits through a stand pipe at the opposite end from the juice bottle. I place a larger pipe with subsurface holes around the standpipe in a manner somewhat similar to a bell siphon. Of course there is no cap. We don’t want a siphon here. We just want water to pass through holes so it can flow up, over, and out of the stand pipe without taking the duckweed with it.

There is another method to protect from draining the duckweed and I use it as an emergency drain in case of blockages somewhere.

Drill a hole in the side of your tank at a level somewhere above that of the standpipe. Install a bulkhead connector.

On the inside add an elbow pointed down and a length of pipe that reaches well below the water’s surface.

On the outside, add a tee oriented up and down. Plumb the tee to your drain. Remember, this is a backup in case your primary drain is plugged.

You may want to add a short piece of pipe to the top of the tee section to prevent overflows. Be sure to leave it open or you will form a siphon and drain your duckweed tank dry.

Some folks recommend placing a few small fish in your duckweed tank as mosquito-eaters. It’s up to you.

Good luck and I hope you will soon be growing all the Lemna Minor your fish can eat.