Aquatic Weed Control
Right: Duckweed overpopulation in a constructed wetland system, Costa Brava Water Agency in northeastern Spain. Image courtesy of Sr. Luis Sala.
In case you have studied my web site, looked at the photos and explored the links, but have decided that you still do not love Lemna, this page is for you.
Simply put, if you want to get rid of duckweeds in your pond or wetland, you have three choices: herbicides, biological controls and mechanical controls (see below). Each of these has advantages and drawbacks, so read what I have written about each of them before you form your own opinion. Also read why duckweeds proliferate. Many times eliminating excess fertility from the lawns or fields that drain into a pond will solve the problem. And I would strongly suggest one more step: Get advice from a professional. Depending on whom you ask, where you live and who you are, professional advice my be free or may cost you a consulting fee. But don't even consider using chemicals, exotic biological controls or mechanical skimmers without expert advice.
One more comment: duckweeds are not only difficult to eradicate, they just keep coming back. Although actively growing duckweeds are not difficult to clear from a pond, their rapid growth means that they can re-establish their populations from a few hidden or protected fronds before you know it. In addition, dormant submerged seeds and turions will germinate when conditions become optimal. Finally, water fowl will re-inoculate your water with duckweeds that cling to their feathers when they fly in from other places.
Instead of getting rid of your duckweeds, think instead of how they are cleaning your water.
Herbicides | Biological Controls | Mechanical Controls | Get Advice
Herbicides are substances that have found use in killing one type of plant or another. Some herbicides are selective and only affect certain groups of plants, others kill most vegetation. Some are quite hazardous to humans or wildlife; others are relatively benign. To learn about the science of herbicide action, including plant biochemistry and physiology, see this [ link ].
The duckweeds are classified as monocots, so they are relatively insensitive to well-known broadleaf herbicides like 2,4-D. Herbicides that are effective on duckweeds include fluridone (Sonar), and diquat (Reward). Fluridone has been by extension service authors. See [ this link ] for a general discussion.
Once again, before using herbicides it's best to get expert
waterfowl, and certain insects and
microorganisms use duckweeds as food
sources. Each of these duckweed "consumers" is a potential for
biocontrol to prevent the overgrowth
of populations or to remove it where it has already taken place.
Biocontrol should be compared with other
means of controlling duckweed populations. A few of the
advantages and disadvantages of biocontrol are discussed here.
Benefits/Disadvantages to Biocontrol
There are many advantages as well as disadvantages to the use of biological control as part of an overall aquatic plant management program. Advantages include longer term control relative to other technologies, lower overall costs, avoidance of the use of chemicals, and the possibility of plant-specific control that leads to ecosystem stability and enhanced environmental compatibility.
On the other hand there are several disadvantages to consider, including the slowness with which some controls take effect (lag times of months or years instead of days or weeks), the limited availability of agents suitable for controlling duckweed plants, and relatively strict environmental conditions that will lead to success.
Further information about the advantages and disadvantages of biocontrol and practical tips for its use can be obtained from the US Army Corps of Engineers Aquatic Plant Information System The Army Corps created this website to provide information on biocontrol of aquatic plants. A CD-ROM version may be requested.
Herbivorous fish eat
The common goldfish and the related grass carp are good examples. Unfortunately, few temperate zone sportfish are herbivorous. Most native omnivorous fish, like catfish, are bottom feeders with little interest in floating duckweed. There are two fish that devour duckweeds with gusto:
Duckweeds can be harvested with mechanical skimmers or (with much more labor) with fine nets.
Using a skimmer is not unlike cutting a lawn. Some of the plants will be missed by the skimmer, so in time the population will re-grow to the original size. This is not a disadvantage if you have a good use for the plants that you will harvest. The harvested plants are high in edible protein. They can be used as food for poultry or fish , mixed into silage to feed cattle or used as green manure to fertilize crops.
It is not possible to make general prescriptions for controlling aquatic weeds. Weed control recommendations that are appropriate for a pond in Virginia may not be appropriate for a wetland in Louisiana. Recommendations for homeowners are considerably different from those for businesses and government installations that may have access to chemicals requiring a pesticide applicator's license for purchase and use. Certain chemicals may be harmful to the environment or to humans, if used in reservoirs used as a domestic water supply or for fishing. There are many variables that will be specific to the particular landscape, climate and weed species. Without this detailed information it is impossible to provide safe and reliable recommendations.
I suggest that you contact your local Cooperative
Extension Office for specific recommendations that are appropriate
for the conditions in your local area.
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Revised: December 3, 2005