Re: Duckweed

> From: Andy Den Tandt <Andy.DenTandt at rug_ac.be>
> Date: Mon, 22 Apr 1996 14:03:09 +0200 (MET DST)
> Subject: Duckweed
> Hi to you all,
> > This sounds very much like Duckweed. It is a prolific floating plant which
> > will probably need thinning out regularly, it is usefull for extracting
> > nitrates from your water.
> I have high nitrates levels, so this could be a help to me. How useful is
> duckweed to lower nitrate levels, compared to other fast growing plants?
> But I also see a major drawback: as it is a floating plant, the
> light-levels will drop rather hard, and I am already low on light (I use
> 40 watt), so I wonder if the duckweed will be spreading to the whole tank,
> or if you can limit it to stay on the right hand side of the tank. Are
> there any experiences concerning the light levels? 
> <snip>

Duckweed is the only plant I use for nitrate removal so I can't comment on any
alternatives, I have other fast growing plants such as Vallisneria but its
growth rate does not come near that of duckweed. I can easily remove 500g (wet)
of duckweed some weeks when growing conditions are good. I think that any plant
growing in a tank is contributing to nitate removal, either by directly using
nitrate compounds or by extracting N in its other forms. The usefull thing with
duckweed is that it can be easily and regularly removed and it is very fast

Its main requirement, that I have found, is good light. I have 80 W over a 4'
tank containing 38 (US) Gal. That is still not a lot of light but it matches 
my lo-tech set-up. I find the duckweed a usefull indicator as to when to change
the bulbs and because of this replace the tubes about every 10 months. Each
tube change is accompanied by an explosion of duckweed growth but the rest of
the plants grow at the same rate all the time. 

As for your own tank, I would recommend doubling your light anyway, duckweed
will survive in any light condition coz its just about indestructable, but it
will do better and more importantly so will your other plants if you increase
your lighting. I usually have a blanket cover of duckweed and it does not cut
out as much light as you may think. It seems to 'filter' the light more than
block it, so the submerged plants get the kind of filtered light they might
naturally find growing in the margins beneath trees, shrubs tall grasses or 
whatever. This is also good for keeping algae in check.

Your sugestion about trying to grow it in one part of the tank only, would be
quite difficult to achieve. When it gets a grip it really grows! You could
perhaps try some sort of floating frame to contain it but if a few leaves get
on the other side you'll be back to square one. I find it easier to let it 
grow all over and then you can get a good sweep of the net every week. I 
remove about 95% each week and it is usually thick again a week later.

My fondness for duckweed puts me in a minority within aquatic plant enthusiasts
as a whole, so if you want a balanced picture it would be a good idea to try
asking for other opinions.