Greetings Patricia,

Unfortunately, there are some real risks with either tolerating or treating complete coverages if fish are present.  Plants and algae conduct photosynthesis to manufacture sugars for their own metabolism.  A byproduct of photosynthesis is oxygen, and daytime photosynthesis will be the primary source of oxygen in most ponds; plants are net producers of oxygen under sunlight.  However, when the sun sets or is obstructed by weather, photosynthesis is reduced with darker conditions, ultimately terminating in darkness, and plants then become net consumers of oxygen.

Where duckweeds and/or watermeal completely cover the surface of a small pond, they concentrate all productivity at the surface, almost no sunlight penetrates, and photosynthesis by rooted species at depth is extinguished. A duckweed-covered pond can be anoxic (i.e., without enough dissolved oxygen to support life) just beneath the productive surface layer. Even if any remaining fish are crowded near the surface during the day, extreme biological oxygen demand (BOD) by respiring duckweeds can lead to low-oxygen stress over the length of night when photosynthesis stops. Not many (if any) fish will persist if complete duckweed coverage persists.

Of course, killing too much green stuff at once with an herbicide also creates extreme BOD as all that biomass stops photosynthesizing and begins decomposing. Unfortunately, the right time to try to bring duckweeds or watermeal under control is early in the season as growth is beginning, before coverage is extreme, and while water is still relatively cool. If you opt for an herbicide application at this time of year, read the label carefully regarding the risk of fish kills, relative area that might be safely treated at once in warm water, possible reference to spot or repeat treatments, etc.

Paraquat is not labeled for application to the water!  Do not apply paraquat to your pond; that would both be illegal and create liability for yourself in applying a deliberately toxic substance to a place where it shouldn't be: https://emergency.cdc.gov/agent/paraquat/basics/facts.asp.  There’s really only one consistently effective herbicide for watermeals: flumioxazin. It’s relatively new, but its patent protection has just expired. There are now three brands labeled for aquatic applications of which I’m aware: Clipper (the originally patented brand), Pond Klear, and Propeller.  There are older treatments that used to be more commonly used for duckweeds (especially diquat and fluridone); however, they won't be as effective as flumioxazin.  (Still, let me know if you'd like to discuss them.)

Feel free to drop me a line directly via the contact info below if there's more to discuss.

Luck!

Eugene

hello Eugene!
Thanks so much for answering my email!  We have owned this pond for
nearly 50 years.  I bought the property from Jerry Bidwell when he
retired from the OSU faculty in 1986 and we continued to live together
on the pond until his death this past January.  I am learning how to
maintain it now that he is gone, so I am a relative novice to pond
management.

As i mentioned in my first email, we have never used chemicals in the
pond - it maintained its balance with relatively few problems.  We
added triploid amurs (from Fender's Fish Hatchery in Baltic, OH) over
the years and they pretty well took care of any algae and pondweeds.
so I have no experience whatsoever applying sprayed chemicals to the
pond, and I really don't know how to go about it.  I know I'll
probably have to buy some sort of spray equipment, , but how to
obtain, mix and apply the chemicals is pretty much a mystery.  How
much of the pond can i treat safely at a time?  How long do I wait
between applications?  I can learn, but at age 74 with arthritis, it
might be smarter for me to pay someone to do this.  Do you have any
advice on this?

I understand that the best time to treat is in the spring, before
infestation becomes so thick.  Should I delay treatment until next
spring?  Or should I treat now and accept a less than ideal result?
Would this spare fish kill over the winter?

I wonder if aeration might help manage this infestation better?  The
flumioxazin looks like just the thing the pond needs to begin to
control these two plants.  I read an article about this just before i
contacted you.

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I am concerned about your comments regarding the fish - I have not
fished at all this year - Jerry's death, assuming total responsibility
for maintaining the property,  and the pandemic have pretty well
flattened me for fun this year.  We have had no winter kill in many
years.  The pond was built to a 15 foot depth, and although I'm sure
that sediment has lessened that, the fish survive several months of
complete ice-over each year.  I wonder if i add a bubbler for the
winter if that will help the fish manage with the oxygen depletion
until spring?  If it will be inefficient or insufficient to be worth
the cost, please let me know your thoughts.

I really appreciate your willingness to give me information!  The pond
is so important to me and to our neighboring farm friends, who enjoy
it as well.

Sincerely,

pat edwards


On 8/4/21, Ask Extension <askextension> wrote:
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