Backyard Aquaponics
A place to discuss aquaponics

We live at Bundamba near Ipswich where we're putting together a demonstration micro-farm We design and build our own food production systems and we plan to build a backyard aquaponics unit.

I am keen to trial the use of duckweed in our system - as feed for fish and for removing excess nutrients from our tanks.

Duckweed is the World's smallest flowering plant. In optimum conditions, it is capable of doubling its mass every 24 - 48 hours and it has a protein content of 35 - 40%. Fish will definitely eat it - so much so that you'll have difficulty keeping it in a tank containing fish. You'll need a separate pond in which to grow the stuff.

We've had two sub-species of duckweed - one with very short roots and one with longer fibrous roots. I suspect the one with short roots is the better one - less fibre.

Australian scientists have already done valuable research on duckweed - check out http://www.cipav.org.co/lrrd/lrrd7/1/3.htm for more information or just do a web search on duckweed.

Thanks for the info Gary.

I found this document last night on the web, its HUGE probably ANYTHING ANYONE ever wanted to know about duck weed.

http://www.fao.org/ag/AGAinfo/resources ... DW/Dw2.htm

I would have thought same as you about the roots and fibre, but i'm sure when i read the document last night it mentioned somewhere about protein in the roots as well, will read it again and confirm

Does duckweed ship well? I'd love it if you could send some to get me started. PM me if this is ok and i'll work out a way to get you some postage money.

Thanks

Steve

Gary, do you grow a fair bit of duck weed? Do you grow it in special ponds or tanks, or rather just in existing water bodies? Do you dry it out or use it wet as a feed? I'd love to hear a little more about how your growing it and using it.

Steve, you should be able to get duck weed pretty much anywhere, try a pond shop or an aquarium shop, my local aquarium shop sells little bags for $2.

Steve/Joel

We currently grow it in a small 250 litre plastic tub.

At the moment, we've de-stocked pending some earthworks in our back yard so we are only feeding the duckweed to our worms.

Many birds and animals will resist eating anything new once they've become used to proprietary pellets so it's best to feed duckweed or other fodder to them from their earliest days.

You won't have any problem with fish, however. We've got a small aquarium in the house and the fish make short work of any duckweed that we put in the tank.

Every week or so, we remove most of the weed from the top of the water. About a week later, it has covered the top of the tub again.

I'll probably kill off our existing plants because, as I indicated earlier, I want the short-rooted variety.

Once we crank up our own Aquaponics set up, we'll incorporate a duckweed pond into the system.

You can buy it from any aquarium shop and all you do is spread it on top of the water in any container large enough not to get too hot in the sun. Our current tub is in semi-shade.

On a number of occasions, I've almost killed it off (which is actually quite hard to do) and I've bred it back up from half a dozen tiny plants to many thousands in just a couple of weeks.

It's worthwhile putting a few plants in a shallow dish under some light and watching it multiply. It really is a fascinating plant.

Good Luck!

G'day Gary, love you website.

I haven't read all the info yet, so please excuse my question.

How deep would you reccommend the water be for the short rooted variety of duckweed?

Cheers

Mike

Growing conditions are the same regardless of the type of duckweed.

The tub that we've had ours in is about 600mm deep and it only gets the afternoon sun.

The important thing is to make sure that the water doesn't get too hot.

We'll probably end up up using large circular plastic stock troughs when we want to grow it in larger quantities. Reln Plastics make a 1000 litre trough which is about 2400 in diameter.

Of course, you could just dig a pond and line it with some butyl rubber or PVC liner that you get from a garden centre or pond shop.

I've seen how quickly duckweed can move and grow... My dog drinks out of some of my various fish tanks and systems and a couple of them had duckweed. One day I noticed that a single duckweed plant appeared in a different system and within no time it multiplied and grew. I couldn't figure out how it was moving from one place to another until I found some on the face of my dog. Since then I've noticed it pretty much grows anywhere in my back yard, including the dogs water bowl.

On a couple of occasions, I've referred to the perception that we used to have a short-rooted variety of duckweed that has since been taken over by a long-rooted kind. This belief was reinforced by the fact that we got duckweed from two different sources.

If what I've discovered today (through wading through the literature on the web) is correct, it appears that they might be one and the same thing.

The researchers (Le Ha Chau et al) have determined that there is a relationship between root length, biomass yield and crude protein.

In effect, the longer the roots, the lower the biomass yield and the lower the crude protein level.

My existing duckweed tub has not been fed anything in the way of nutrients (other than what washes off the occasional cane toad) so it's reasonable to assume that the nutrient level is not very high. That might explain the long roots.

I might try spiking the tub water with some hydroponic nutrients and see what happens.

Another way to spike your water would be to use worm juice or a muslin bag with some manure or compost in it, suspended in the water... It just sounds a little nicer than hydro nutrients, and the growth rate of plants grown hydroponically in diluted worm juice can be quite amazing...

Thanks Joel. We use both at different times. As it turns out, we've got a nice batch of worm castings happening at the moment. I might use some of that to make up some worm tea.

Hi all,

I'm sure that in one of the documents on duckweed that i've read it went into some detail about duckweed being able to utilize AMMONIA directly, i think it may have even gone as far as to say that ammonia was preferred by the plant over nitrate.

Was reading the post by STEVE M and remembered this.

Duck weed may be useful to have in a newly established system For many reasons, It should ease the stress on the fish by keeping ammonia levels low, it should reduce algae blooms (i think the algae like ammonia too ) and as a plus to both of these you should have a plentiful supply of duckweed to feed to your fish!

As a side note if you are waiting for your system to cycle (bacteria to colonise) then you should feed the fish very little, especially if you have "sensitive" fish. Any feed is excreted as ammonia by the fish. also uneaten food decomposes and also produces ammonia.

Steve

Hi,

I did a web search and at http://www.p2pays.org/ref/09/08875.htm#Nitrogen I found the following information in an article called "Duckweed Aquaculture" by Skillicorn, Spira and Journey:

"Nitrogen Ammonium ion is the preferred form of nitrogen for duckweed species. The main source of ammonium for wild colonies of duckweed is from fermentation of organic material by anaerobic bacteria. Duckweed plants reportedly utilize all available ammonium before beginning to assimilate nitrate, and appear to grow more quickly in the presence of ammonium than with nitrate. In contrast to duckweed unicellular algae prefer nitrate."

That seems to bear out what you're saying Steve.

Excellent...... Thanks for that info guys...

Thats one hell of a link for duck weed info Gary.

I have read that the very common "water fern" -Azolla (another water weed that can be invasive) is an alternative to duckweed - can be fed to fish like silver perch, tilapia and chooks, pigs and ducks etc. and, because it is a good nitrogen fixer can be used as green manure.

http://biotech.tipo.gov.tw/plantjpg/1/A ... innata.jpg

I just crushed some between my fingers to mash it up a bit and my fish seemed to lap it up.

that is the stuff we tried ! I couldnt remember its name but thought it started with A

it just slowly turned brown and died ........ couldnt hack the sun

frosty