Controlling Duckweed and Water Fern

Duckweed and water fern are the most invasive and noxious of the floating plants. Do not be fooled by the cute little display in the store with a pot or a tiny pond covered to overflowing with duckweed or water fern. It can look so pretty, all piled up. There is so much of it that the top of the water looks textured, almost hilly with duckweed on top of duckweed. Water fern or azolla looks even prettier. It can look like solid ground and has been mistaken for it many times, contributing to the deaths of children who stepped out on it.

Duckweed (lemna) is the smallest flowering water plant with little green leaves. Frogs and other pond critters love to hide in it. It brings shade for your fish and probably stops the growth of your anacharis completely. Water fern (azolla) looks like a duckweed, but is actually a fern.

Both duckweed and water fern are actually hundreds to thousands of little tiny plants, all floating together. They help to filter the water, and many fish and frogs enjoy nibbling on them. They live up to their names. Ducks love it. Both can double in mass in three days. Be careful.

The duckweed and water fern can form a nearly solid shelter, giving cool spots to your water garden and giving your frogs and fish places to hang out. By blocking some of the sun, duckweed also helps to keep algae from growing. It also keeps your anacharis from growing.

Controlling Duckweed and Water Fern
Beware of these plants, because they can get out of control in a few days. Your pond will be covered with them. As many times as I have encountered these plants, I have never been able to rid the pond of them. I have seen them in natural ponds, drained the pond, removed the stuff and it was back in three days. In the lined or concrete pond, I have drained the pond and then scrubbed it with Clorox and was reasonably successful. That method of noxious plant removal will work most of the time, but be sure you clean out between rocks and under any statuary in or out of the pond.

if you have spitters or lights in the pond, remove them and clean with Clorox and pump the Clorox water through the spitter and any other tubing from the pump over the waterfall to remove any plant material that might be hiding and multiplying inside the tubing. If you have tubing and connections either attached to the spitter or in the waterfall, wash those with Clorox as well. If you have a filtration system, clean it thoroughly with Clorox. When you reinstall it, be sure to jumpstart the bio filtration process with a starter dose of bacteria. I have also tried introducing koi to the pond because koi will eat most anything from carrots to cabbage. They eat both plants voraciously, but even they cannot keep up with it's growth.

I strongly suggest, no matter how much you might be tempted, if you see aquatic plants you would like to have and there is any duckweed or water fern near the plant, in the same container or the same storage area, pass up the plant. Find one without any hint of either plant nearby so you don't have to fight to keep it under control.

These plants are causing huge problems in tropical countries where they are not killed off by freezing weather. When you introduce them into your pond, you are taking a chance of polluting nearby water sources. We think that could never happen, so we have a cute little pot full of duckweed and water fern, just to see what it looks like. Then Hurricane Katrina hits and the cute little pot is destroyed and the plant material is scattered over three states. Please treat these plants with extreme caution.

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