Perhaps a long time solution would be to introduce some vegetarian
fish. I can't help with the name at this stage but they do exist. They
wouldn't be any good getting rid of large amounts of duckweed, just as a
control. I'll ask around for details of suitable fish and post back with
more information if I get any.
DD

Andrew G wrote:
Hi all.

Anyone here had any experience in getting rid of duckweed on a large scale?
I think that is what it is, I know it's not water cabbage (or lettuce?).
We have it in one of our dams at work, on a very large scale. Long story
short, it has made it's way to all dams on a small scale, including the
front dam where water is pumped from.
We have tried scooping it, and even barracading it off, to no avail. I have
tried Roundup Biactive (or equivalent), but it doesn't seem to hurt it as
it's too small for the roundup to stay on it in the water. If it does, then
my guess is that any bits missed will easily spread quicker than the rate
sprayed stuff dies.
Copper sulfate did have an effect, but it was only tested in a small area,
as it seems to affect water pH.
Someone suggested black plastic over it as it doesn't grow in shade, but we
haven't tried that as it can be found under shade from some of the rushes in
the dams.
The ducks have spread it already, and while we can filter it from the front
dam (though it's probably too late) it won't stop ducks or birds spreading
it.
I have noticed it on the banks of the dam where water level has dropped it
appeared to have died. We can drain all dams bar the front one, but it's
something we want to avoid due to lack of rain.

Thanks for any advice.