Duckweed And Watermeal Treatments

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Written By Mark Washburn

Mark has 20 years of experience as a professional pond management consultant.

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Duckweed is one of those pesky aquatic weeds that can be very hard to control once it get’s up to speed in a large pond.  It’s fast growing, durable, and often not controlled by just any old aquatic herbicide or treatment.  In this article we want to explore duckweed in more detail and help a pond owner accurately identify it and offer treatment options which may be helpful in keeping it from taking over a pond.

Duckweed Identification

common duckweedCommon Duckweed (Lemna minor) is an aquatic plant that’s sometimes confused with algae.  However it’s very identifiable and quite different in appearance that most algae.  Duckweed looks like very small, bright green beads or pellets that appear somewhat flattened.  You would find several small “roots” extending from the main body of the plant.  Duckweed colonizes in masses and will float freely atop the surface of the pond.  It’s usually found in very still or stagnant water in pond with ample nutrients to feed it.

As the name implies, duckweed is often transported from pond to pond by waterfall and they often like to eat the plant.  In very limited amounts, duckweed, or it’s smaller relative, watermeal, will not pose any problems but when conditions are supportive of it, the plant can grow rapidly and soon cover the entire surface of a pond.  Usually it’s best to avoid this if possible because there is a risk of oxygen depletion in the pond is higher.  This of course is not good for fish stocks.  Having aeration running in a pond will help guard against this issue but it also may be advisable to try and control the duckweed growth before it overtakes the pond.

Duckweed Control Options

Several viable options exist for controlling duckweed in farm ponds and large bodies of water.  Choosing a particular method would depend primarily on the size of the pond, the budget a person might be working with, or a person’s personal philosophy as to whether they would choose to use chemical herbicides or take a more eco-friendly route.

Editor’s Note: It’s usually suggested to treat any pond weed as early as possible in it’s growth cycle to get the best results. If a plant growth is well underway, there is a greater chance of oxygen depletion as it dies off following treatment.

One of the most environmentally friendly options for removing duckweed from a pond is to use some type of skimmer which can manually remove the plant from the water.  Since duckweed floats well, it is possible to skim most it off and transport it away from the pond.  This offers a chemical free and inexpensive method of control, although it may not always eradicate the plant completely from the pond.  To get more information on a useful design watch this video on duckweed skimmers.

Other mechanical skimming options exist that are more advanced than a simple rope, however many of these systems are quite expensive and would likely only be a viable tool for pond management companies to use.

In recent years, one of the more popular and widespread treatments for duckweed is Fluridone.  This chemical comes in several brand names such as Sonar and White Cap.  This concentrated liquid is relatively expensive but about 8 oz of product (at just under $200 retail price) will treat up to a surface area of 1/4 acre.  Fluridone inhibits the plant’s ability to produce carotene and limits it’s production of chlorophyll.

Under normal circumstances it’s recommended to treat the entire pond surface and not spot treat with the herbicide.  Once treated, usually the plant will die off over several weeks time and one shouldn’t see any regrowth for the remainder of the season.

Up until recently, Sonar and a few other  herbicides were about the only solutions for treating duckweed.  In some instances, the use of beneficial bacteria has helped keep duckweed under control, but only if the plant has not progressed to covering the pond.  Once it’s well underway, most people had to resort to one of the methods mentioned above, or they would simply leave it alone.  Once cool weather arrives the plant will naturally die off and disappear.

In the last year or so however a new, organic and non-chemical solution has been developed which is showing great promise with duckweed control.  The product is called eLemna8 and it contains various organic products including turpin oil and concentrated enzymes.  What’s interesting about eLemna8 is that it effectively helps to dissolve the protective wax coating which is found on the duckweed petal.  It then makes the plant vulnerable and it will begin to die off slowly as the enzyme component begins to break the plant down.  The process can be sped up by combining eLemna8 with a herbicide, however the amount of chemical necessary for control is usually much less than if it were used alone. After the duckweed has been killed, it’s not a bad idea to add some beneficial bacteria to the pond to help speed up it’s breakdown completely.

So despite it’s natural durability and propensity for growth, duckweed is a plant that can be effectively controlled in various ways.  As noted above, and regardless of what method you may choose to use, the best advice is to treat it early, before it has a chance to take over the surface of a pond.  Doing so will allow you to treat it safely in regards to maintaining fish health, and you’ll have an easier time at keeping in check with less chemical use and/or manual effort on your part.

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