Pond Boss Magazine: Duckweed isn't Ducky By Bob Lusk

Common Duckweed, of the genus Lemna, is a tiny plant that floats on top, or slightly below the surface, of the water. When we see Common Duckweed, we also commonly see other species with it, especially Watermeal and/or Giant Duckweed. Overall, these plants’ ability to reproduce gives them the dubious honor of one of the most invasive plants that can grow in your pond. Literally, YOUR pond. Duckweed lives in fresh, mostly still waters, but can grow anywhere fresh water is impounded in the entire world…except the coldest places.

In Venezuela, in 2004, duckweed covered a significant portion of the country’s largest lake, Maracaibo. The government sponsored a project to remove the plant with mechanical skimmers. More than 75% of the plant mass was removed.

While that sounds impressive, we need to understand that duckweed doubles itself about every 48 hours when the temperature is warm. Remember the old adage, “I would rather have a penny, doubled every day for 30 days, than a million dollars?”

That’s duckweed.

If you ever see what looks like tiny leaves floating at pond’s edge, pick them up and give it a look. Duckweed usually has two little green leaves, about half as big around as a pencil eraser. Sometimes there’s one leaf, sometimes as many as three, but two is normal. As you look closely, you’ll see tiny little white roots coming from the joint of the leaves.

Some aquarists use it for their aquarium fish. In areas of Southeast Asia, duckweed is considered a good protein source for human consumption. Some aquaculturists use it for their fish farms as a feed ingredient or to glean excess nutrients from water to clean it up. University researchers are exploring duckweed as a possible source of renewable energy.

Recreational pond managers don’t like duckweed for the same reason other people like it.

It grows and spreads fast. Way fast.

It comes to your pond by flowing downstream or it can hitch a ride on the feathers of a passing water bird. Or, your dog can swim in the neighbor’s pond and bring it home himself.

Once in your waters, it reproduces vegetatively or via seeds. One plant splits to two, two to four and so on.

So, you have duckweed?

What’s a pondmeister to do?

There are three basic schools of thought.

One is to leave it be and hope for a flood to wash it downstream to your neighbor’s or hope a killing freeze next winter might eliminate it. Another is to physically remove it by skimming or other ingenious ideas. The third is to figure out the least toxic, best way to eradicate it efficiently with approved herbicides.

Forget grass carp as an option. While some individual grass carp might figure out how to tip their lips upward enough to eat it, the stuff grows beyond this fish’s abilities to control it. Rule out tilapia, too. Tilapia will dine on watermeal, but haven’t proven effective on duckweed.

When duckweed first shows up at your pond, a realistic way to deal with it is to skim the water’s surface with a swimming pool net. But, you best be early and observe often, net in hand.

I’ve seen some pretty interesting skimmers. Float lines, like you see at the top of a seine, attached to a short, maybe 18” deep net that’s slowly pulled through the floating mat of greenery, herding the stuff near shore so it can be retrieved with the swimming pool net described above. The coolest way I’ve seen duckweed harnessed was designed by Linda Bagley on a backwater oxbow lake off the Sabine River on family property in deep east Texas. This 80 acre, shallow, blackwater lake is subject to floods, droughts and some giant cypress trees. It loads with duckweed by mid-summer…so much that it inhibits the family’s ability to enjoy the lake. With 80 acres, there’s too much area of coverage and the Bagley’s simply don’t want to use herbicides. Linda thought about it and came up with a brilliant idea, one that might work for you. The lake basically runs north and south, with an eastward, fish-hook shaped bend at the southern end. Each end of the lake, as you might imagine, is extremely shallow.

Linda took tall, straight, dead cypress limbs and cut most of the remaining branches off. The narrow-limb logs may be 20 feet long, tapered at the short end, but no more than 8 inches in diameter at the largest end. She took ski rope and tied the large end loosely to a cypress tree trunk just above the water line and uses leverage from the big tree to limit where the horizontal floating limb can move, like a one-way hinge. She left some slack in the rope so the log can float and move. Then, she went across the narrow lake and put another skinny log in a similar place, where the two can come together like a double castle door and lodge together almost in a straight line when the wind blows toward the main body of the lake. But, when the wind changes direction, waves and currents push the floating logs open, like those double doors, toward the shallowest end of the lake. Duckweed blows through the open area with the wind, directly into the shallow, non-usable areas of the lake. When the wind changes, the logs come together, effectively shutting off the path, blocking the duckweed and leaving it to its own resources in a restricted area of the lake. Take a look at the photos and you’ll get a good image of how it works. Brilliant and effective. And, natural.

Your third option, the “fast food” option…is to use herbicides. Here’s where things can get a little dicey. Since duckweed floats AND can sit just under the surface, you need to think through the choices you make for treatment. Seriously consider hiring a pro for counsel, advice or to do the job for you. But, if you are determined to do it yourself, make sure your learning curve is beyond adequate.

Key points to remember about duckweed.

It floats…some of it…and it moves around. Surface contact herbicides such as glyphosate will take repeated treatments.

Duckweed reproduces prolifically. If you eradicate some of it, that doesn’t necessarily mean you get rid of it, since it reproduces every 48 hours.

You may have some species of desirable submerged vegetation in your pond that you actually want to keep. Make sure the herbicide you choose impacts the rest of the lake in a manner you expect. In other words, prepare for collateral eradication.

Now that you know what you are doing, decide the herbicide.

According to Texas A&M’s Aquaplant website, fluridone and penoxsulam are excellent and diquat is good. Reward is a liquid diquat product approved for use and does a good job. The fluridone products are Sonar and Avast. While Reward is a contact killer, the fluridone products are systemic, being absorbed into the plant and effectively stopping its ability to photosynthesize. Consequently, the fluridone products act much more slowly than contact herbicides. Plus, systemic herbicides need to stay in contact with the plant for longer periods. Another reasonable choice might be Habitat, active ingredient imazapyr. It, too, is a systemic herbicide. Galleon is a penoxsulam, another systemic which requires several weeks to months of contact to be most effective.

Keep in mind…if you choose an herbicide, be vigilant to follow label directions and understand the consequences of dead, decaying matter in your favorite fishing hole. The last thing we want is a phone call from one of our favorite pondmeisters explaining how an oxygen depletion happened…after the duckweed disappeared.

So, there you have it. As Forrest Gump might have said it, “Duckweed is as duckweed does.”

It sure does…every 48 hours.

This article was extracted from the July/August 2010 issue.