Eradicating duckweed (Lemna minor)

[froggy]

Does anyone have a better method than frequent netting-off for eradicating duckweed from apond, or at least keeping it at bay?

[grunsven]

Reduce nutrient content. Dominance of Lemna minor indicates far to high nurient concentrations.
I do not think there are other solutions.

[ajc]

Ducks.

[froggy]

The new pond is only big enough for about 3 ducks to fit on, and I think that the local fox would make short work of them.
How can I reduce nutrient content without adding filtration (running electricity to the ponds is not viable)? Are there any aquatic or marginal plants suitable for the small pond that will mop up nutrients?

[azhael]

Introducing fast growing plants may pose a competition for Lemna and reduce it´s growth.
However i don´t really undertsnad why Lemna seems to be so annoying for you.
Elodea, Eichornia crassipes, Azolla....are some fast growing plants you could use. However, i don´t think they will erradicate Lemna, they´ll just compete with it, and if they are removed duckweed will probably strike back.
I think your best bet is to net it off, and "clean" the plants with stuck duckweed with a hose, and then net it of again.

[grunsven]

First question is how did the nutrients get in?

I agree that adding plants to compete with the duckweed would help. But I'd prefer native species. Azolla is defitely not suitable as this fixes nitrogen and makes the problem worse.
Myrrhiophyllum or Potamogeton species should be OK. Just shop around in ponds and lakes in the neighbourhood.

It is also likely to become less of a problem when the pond ages.

[froggy]

The older pond probably has a nutrient problem because of fallen leaves in the autumn; trees that are not our property have grown up to overhang the pond. We recently revamped the pond and restocked it with plants after a a major dredging, so hopefully things will balance out in time. The new pond is just a normal hole dug in the garden with a butyl liner. We don't use fertilisers in our garden. The duckweed hasn't got a hold yet, so hopefully all the other plants (Elodea, Nymphaea, Potomogeton, water forget-me-not) will outcompete it. I am also removing the little bits that I see.

The duckweed is a problem because it smothers the submerged plants byt blocking out almost all the light. The submerged plants then die, rot and cause the pond ecosystem to collapse, whilst providing more nutrients for the duckweed to grow. I don't mind having a little bit, as it provides cover for amphibians and invertebrates, but I don't want it to smother the pond by becoming too prevalent.

Thanks for the pointers.

[benw]

Hi Chris

Duckweed can be a real pain to get rid of as you say, the way i got rid of it was deny it of light by stretching a black sheet across the pond.

It took a couple of weeks, and it did set back the other plants in the pond, but they soon bounced back.

I appreciate it may not be totally practical, but it didnt involve chemicals and the hastle of netting every last bit out!!!


Ben

[froggy]

Hi Ben

Thats a novel method of getting rid of it - thanks. I may try this in the winter when the emergent plants have died off anyway.

Thanks

Chris

[Mark]

Chris, I think I may have the answer....

http://www.caudata.org/forum/picture.php?albumid=68&pictureid=602

[otterwoman]

That's an AMAZING picture!!!

[Nathan050793]

That's an AMAZING picture!!!

Dawn pretty much stole the words right out of my mouth. By the way, I like the slate bottom.

[taapua]

That is an outstanding picture that should probably be considered for the next calendar edition?

[oceanblue]

A further recommendation as a native boggy semi floating marginal - watercress. It mops up nitrogen very well in my small tanks and grows well in shallow ponds. If there are not sheep to carry liver fluke you can also eat it.

Avoid azolla like the plague. It's like duckweed on steroids.

[froggy]

Fantastic picture, Mark!
Good tip with the watercress, oceanblue. I may try it along with the sheeting idea.

Chris

[fishkeeper]

Also, Azolla gets nitrogen from the air...

[blckkeys]

Here is a publication I found on controlling duckweed. Hope it is found to be useful.

http://ohioline.osu.edu/a-fact/0014.html

[froggy]

Thanks for the link, Pat

[bobberly1]

No plants use nitrogen from the air, that's what fertilizers are for.

[caleb]

No plants use nitrogen from the air

Some plants (including Azolla) form symbiotic relationships with bacteria that allow them to use nitrogen from the air:
http://en.wikipedia.org/wiki/Nitrogen_fixation
That's what crop rotation is for.

[fishkeeper]

Splitting hairs are we? ;)

But thanks for the correction!

But the take home message is. Reducing nitrogen is not going to stop azolla the way it might phase duckweed.

[grunsven]

Adding Azolla to fight lemna does not sound like the smart thing to do.
Have you ever measured nitrogen and phosphorus in your pond?

[JWERNER]

Just start mailing it to me!;)


I would look into the placement of the pond itself. I can use duckweed in the terraium and it is nitrate lack do to the other plants in the tank but since it is in direct light it wll still grow regardless of the lack in nutrients.


If you look some ponds just get hit hard no mater what and some just dont at all. In most cases the ones that have no shade at all are the ones to get nailed by the duckweed the most.

[TristanH]

Hi,

In my experience, the only practical way to control duckweed is by netting. Fish and ducks are a non-starter with amphibians, as are herbicides. They also increase nutrient levels. Covering the pond basically involves starting again and again will cause deoxygenation and release of nutrients (much the same effect as the duckweed really).

That's the bad news. The good news is that it IS possible to get rid of duckweed by assiduous and frequent netting - but like any invasive plant, don't let quantities build up. And net the pond in autumn to intercept falling leaves.

A healthy community of pond plants will help to absorb the nutrients on which duckweeds thrive. In particular, rooted species such as pondweeds, milfoils and water lilies are good. Free-floating plants that are much easier to control include water-soldier (Stratiotes) and frogbit (Hydrocharis morsus-ranae). I have some spare if you need any (all UK native) - pm me if interested.

Tristan