10 Ways to Control Duckweed in Your Pond

Duckweed and watermeal can cause big problems in a pond and they can be difficult to control. This post looks at 10 different ways to control or elliminate duckweed from your pond and evaluates their pros and cons so that you can select the best method.

What is Duckweed and Watermeal?

Both of these are plants the float on the surface of water. The whole plant is very small and a single one is hardly noticed, but in aggregate they can cover the surface of a pond.

There are numerous species of duckweed that can be recognized by its small single or grouped round leaves. Each plant also makes small roots that hang down into the water. The whole plant is about the size of the end of a pencil.

Watermeal is a very similar looking plant, except that it tends to be smaller, does not have roots and is commonly confused with duckweed. It gets its name because it looks like cornmeal floating on the water.

Duckweed and watermeal are controlled in similar ways, therefore this post will use the term duckweed to refer to both plants.

Concerns for the Pond

Both plants grow very rapidly through a process called budding, whereby a piece of the mother plant can form a whole new plant.

Once a pond is covered, they block sun light, inhibit oxygen exchange and reduce dissolved oxygen all of which have detrimental consequences to the other members of the pond.

How Does Duckweed Overwinter?

The plant is not very winter hardy, and the cold and ice will kill the living plant. Some species are able to make special buds, called turions, as the weather gets colder. These are dormant and heavier, so they sink to the bottom of the pond, overwintering in a relatively warm environment.

In spring, as things warm up, turions start to grow not unlike seeds, to form now floating plants.

Prevention – The First Line of Defense

Duckweed problems occur because it is brought to the pond. Many times this happens when you introduce new plants and duckweed comes along as a hitchhiker. Once in the pond it starts to grow. For this reason be very careful where you get your plants. Inspect and wash them well before they are added to your pond.

Never introduce duckweed on purpose.

Can’t Stop Nature

The problem with duckweed is that it easily sticks to birds and animals and when they visit your pond they bring along a small sample. You can’t stop this type of introduction, so the next line of defense is to prepare your pond for invasion.

Nutrient Levels Are the Key

Biologically, duckweed is no different than other plants. They need nutrients to grow and since they can grow faster than other plants, they tend to use the nutrients faster and starve your other plants.

One way to control accidental introduction is to keep nutrient levels low, so that when the duckweed does find your pond, it has a difficult time to grow.

I’ve discussed way to keep nutrients low in Pond Pumps Are Not Needed and in my book, Building Natural Ponds.

This technique should be your first choice since it not only controls duckweed but also algae.

Physical Removal

In a small pond it is relatively easy to use a net and scoop out the duckweed from time to time. Add it to the compost pile or directly to the garden as a great source of nitrogen. Each time you remove some, you will also be reducing the nutrient level in the water. At some point the level will drop low enough that duckweed’s growth slows significantly.

Removal in larger ponds is also possible with boats ad larger nets.

Physical removal is not really a solution since it will never get every last plant, and any that are missed start the process all over again. But when this method is combined with one of the others it can provide complete control.

Aeration Slows Down Growth

Duckweed like to grow on water that is not moving. Adding aeration from either a mechanical aerator or through a waterfall can significantly slow down growth.

Aeration also moves the living plants to the edge of the pond where they are more easily removed with a net.

Fish Control

Goldfish and to a lessor degree koi, like to eat the plant, provided you are not feeding your fish. Hungary goldfish can eliminate duckweed from a pond.

A recent post in our Building Natural Ponds Facebook Group got numerous replies about fish eating duckweed so fast they could not keep it growing.

Duckweed and pH Levels

Duckweed tolerates a wide range of pH from 5 to 9. Except at high pH values the pH of the water will not control duckweed.

Biological Control

Companies sell products for control that contain bacteria. The claim is that once the bacteria are added to the water they grow and use up the nutrients so that duckweed starves and dies out. But other types types of plants will also start dying.

In order for this to work, you will end up with a bacterial bloom, which can a bigger problem than the duckweed.

Unless the bacteria are removed from the water, they will die and decompose, returning all of the nutrients to the water. Removing duckweed is easier than removing bacteria.

Scientists have been studying the relationship between bacteria and duckweed to see if the bacteria can be used to make the duckweed grow. There is an interest in duckweed as an agricultural crop and as a biofuel. What the studies have found is that many types of bacteria actually increase the growth of duckweed – they don’t control it.

I have looked at several of these products and not one has any references to support their claims that they work. Until more evidence is available, I don’t consider these products to be a viable option.

Chemical Control

A number of herbicides are available for controlling duckweed and they fall into three categories.

Type 1: whole pond herbicide. Works a long time and “kills any weeds” and probably plants you want to keep since herbicides don’t know about weeds.

Type 2: contact herbicide, which needs to be applied to the plant. New plants will be able to grow, and so you may need to apply repeatedly during a season.

Type 3: selective herbicide. It can be applied to the water and it only harms a select number of plants.

Herbicides work, but keep in mind that:

• They can also affect fish and other wildlife – check the label.
• The dead duckweed should be removed or it just decays and makes the nutrient level even higher.
• They are best used on small infestations, so that the depletion of oxygen due to dying plant matter does not kill fish.
• Use a product that is specifically targeted for duckweed. Three products you can consider are Sonar™, Fluridone or Diquat dibromide.