Dealing with Duckweed in Your Pond

I have a confession to make. I really like duckweed. I think that it’s pretty, and can be really interesting to look at. The way that it propagates so rapidly is really impressive, and the free-floating roots are neat. Watermeal, a type of duckweed, doesn’t even have roots, it just absorbs nutrients and water directly through the bottom of it’s frond. Watermeal is also worth noting for being the smallest known flowering plant.
Duckweed, though, can be a huge problem in a pond. In good conditions, a duckweed plant can bud and divide once per day, meaning a daily doubling of the plant population and complete coverage of a large pond in weeks. Many pond owners have learned that duckweed can be an object lesson in the power of exponential growth. Coverage of a pond with duckweed and watermeal (they’re often found together) can cause significant problems for a pond. They block the sun’s light from penetrating the water, which will quickly kill off healthy, water-cleaning algae. Without this algae, nutrient levels will explode, creating unhealthy algae blooms and significant buildup of organic debris in the pond (the sludge layer). This will encourage growth of anaerobic bacteria, which create toxic water conditions that can kill fish, turtles, and other plants and further encourage the duckweed proliferation. That’s all bad.
Smaller backyard ponds generally don’t have much trouble with duckweed. That’s because it’s a fairly fragile plant that doesn’t do very well in moving water, and a pond with a fountain or waterfall will usually move enough to keep it down. It tends to be large ponds and even small lakes (especially manmade ones) that have the most trouble with duckweed. In a large, relatively stagnant pond, duckweed’s rapid proliferation and tendency to travel along waterways and on animals can cause it to take hold and completely take over a pond quickly, and once it’s got hold it can be difficult to get rid of. If you have a strategy, though, you can control unwanted infestations and even prevent it from taking hold.

Preventing and Controlling Duckweed

Eliminating established duckweed can be done with certain herbicidal chemicals, but this isn’t the route that I recommend, especially not at first. Rather, I’d go with a two-pronged mechanical approach to removing the pest. The first step is to simply skim out as much duckweed as you can. A simple pond net or even a pool skimmer or fine fishing net can be used. Wait to do it on a windy day, if possible, when the plants will be more compressed on one side of the pond. You’d be amazed at how quickly you can devastate the population of duckweed in your pond.
This won’t, however, be a permanent solution. If even a few tiny plants avoid removal, then they will likely come back, sometimes covering a pond in as little as a week. Once you’ve got the majority of the weeds out of the pond, though, you can go on to the second part of your attack, which will keep the duckweed from taking over again.
The same fragility that makes duckweed a rare problem in small ponds can be taken advantage of in larger ponds with one or both of two pieces of equipment. The first is a simple fountain, though a larger one than you would see in a small pond. Large pond fountains do two things: first, they agitate the water surface, making it more difficult for the duckweed to survive and spread. Second, it aerates the water, which will give the healthy aerobic algae a chance to remove some of the excess nutrients from the water that the duckweed thrives on.
The second simple piece of equipment that can help to eliminate duckweed infestations is a bubble aerator. This accomplishes the same tasks as the fountain, though it is far better at deep aeration and not as good at surface agitation. A good bubble aerator is just as, if not more, useful for keeping a pond healthy in a larger body of water, especially ponds that take in a lot of agricultural or commercial runoff that can carry huge amounts of nutrients that need lots of healthy algae to deal with.
Huge thanks to Joel, a customer who sent us an email telling us about his experience with getting rid of duckweed, along with the images that accompany this post!