By Dr. Todd Sink, Aquaculture and Fisheries Extension
Specialist
Texas
A&M Agrilife Extension Service Wildlife and Fisheries Extension Unit
Meagan Hooker, Fisheries Extension Communications
Intern
Texas
A&M Agrilife Extension Service Wildlife and Fisheries Extension Unit
When having issues with managing duckweed, there are
multiple methods that may be used to efficiently reduce or remove this
problematic species. A combination of the biological, mechanical, and chemical
treatments below will best solve a problem related to duckweed.
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| Common duckweed (Lemna minor). Photo from Texas A&M AgriLife Extension AquaPlant. |
Herbicide may be used to control duckweed. However, there
are only a few herbicides that have proved effective. (http://aquaplant.tamu.edu/plant-identification/alphabetical-index/common-duckweed/)
Diquat has a fair to good success ratio,
it is a low cost contact herbicide, that costs between $125 and $250 per acre, but
it requires contact with every plant, so it must be sprayed over the entire
water surface. It requires one to four treatments a year, every year and will
never eradicate the problem, but it does demonstrate an attempt to control
dotted duckweed. Fluridone is a long lasting systemic herbicide that offers a
good chance of eradicating the species and can simply be poured into the water,
but is pricy at $625 to $1,250 per acre. Flumioxazin is an excellent, low cost
contact herbicide, that costs between $75 and $145 per acre, but requires
contact every plant, must be sprayed over the entire water surface, and
requires one to two treatments a year and will never eradicate the problem, but
it does help control duckweed. Penoxsulam is somewhat long lasting systemic
herbicide that offers perhaps the best chance to eradicate the species and can
simply be poured in the water, but costs $555-2,572 per acre. It may also be
good to use Carfentrazone, a good to excellent contact herbicide that must be
sprayed over the entire pond and requires multiple treatments over time, but cost
is variable but mid-range.
Aeration is a common gimmick that claims to help control
duckweed. (http://fisheries.tamu.edu/aquaculture/aeration/)
It is great for the fish and healthy bacteria and it will help deepen a pond by
digesting any organic sludge that has built up, but it will not do anything for
duckweed or watermeal. (http://aquaplant.tamu.edu/plant-identification/alphabetical-index/watermeal/).
It has been though to help control duckweed because it appears to work at first
because the surface agitation pushes the floating plants to the edges where
they are condensed and stacked up. Given enough time the plants will continue
reproducing and reclaim their former surface area. This technique often leads
to a dense mat of plants covering the entire surface after aeration is removed.
Using fish to control duckweed is a common remedy. Tilapia (http://fisheries.tamu.edu/pond-management/species/tilapia/)
manage duckweed and an remove up to 20%,
but it may not have any effect when
stocked in ponds with existing bass populations (the bass eat all offspring
before they grow to the size necessary to eat duckweed). The only plant
Mozambique tilapia will control is mosquito fern (http://aquaplant.tamu.edu/plant-identification/alphabetical-index/mosquito-fern/),
but only when bass are not present. Grass carp are not great for coontail
because they do not like to eat it, but they will control it at high densities
(>15 fish per acre). As with any fish species, check with your local
extension agent before stocking. Tilapia and grass carp require permits.
The solution to duckweed
problems is not through biological controls (grass carp, tilapia) or mechanical
controls (aeration), nor does it have to come entirely from chemical controls. Integrated
pest management practices that combine several control methods will be best
when attempting to eradicate duckweed.
Visit the Texas A&M AgriLife Extension Aquaculture,
Fisheries and Pond Management website at http://fisheries.tamu.edu/ or
download a pond management app at http://fisheries.tamu.edu/mobile-apps/
for any further information.