Monday, June 1, 2015

How to build duckweed ponds

Here on our farm we built 5 duckweed ponds after reading about this fabulous plant. We had not seen this plant growing locally but luckily a friend of ours, found some in a pond in the mountains near here. He brought us two types, on with smaller leaves and one with larger. He was studing botany and told us the smaller leaf variety did better in a shady spot and he was right as our ponds were in the full glare of the sun and it was the larger leafed variety which grew well in our conditions.
View across duckweed ponds

We hired a backhoe and a driver to dig our lakes which were 30 m long by 2 m wide (98' x 6.5'). We then put clay down and lined them with a plastic liner. These are shallow, only about 2' deep.
Clay lined duckweed pond

 As you can see these take a bit of organising but in the end, they aren't that difficult to create. If you were to build them there are a few things to keep in mind.
The liner is crucial. Here we had to take several things into consideration.
  • Ability to cope in extreme UV conditions. (we have over 300 days of sun a year and more often than not our level of UV (Ultraviolet light) is in the extreme category. Your local weather forecast can help you determine what your UV index is for your area. 
  • Non-leaching. We had to make sure that no toxic or unpleasant chemicals would leach into the water which could cause problems for the duckweed and subsequently the fish.
  • Robust. We had to have a strong plastic liner. Often bits of twigs might end up in there. Also we have chickens. We needed something that was tough enough to resist being damaged.
Various stage of completion
We first lined ours with clay as you can see in the pictures. This was delivered and we spread it manually and flattened it down to prevent any pebbles from damaging the liner. The reason we used clay is if any water were to seep out from the liner, it would be a slow leak. We have sandy soil here and it sucks up water better than a sponge, hence our need for clay.
Overflow system for duckweed ponds
Because this was only designed by us, we had to solve problems as we went. What you see pictured above is the ingenious (and cheap) solution my husband came up with. The problem was, we needed an overflow system for water to run out if the ponds got too full. We can get heavy downpours in our wet season which would have sent our duckweed up and over the edge in a rainstorm. We could have lost it all if that happened. Remember, duckweed is a floating plant, it isn't rooted into anything.

My husband place plastic pipes from one lake to the other. Then to keep the duckweed in and allow the water to flow out, he simply cut the bottom off a 2 liter soft drink bottle and insert this onto the plastic pipe. The excess water would escape and the duckweed would stay in the pond.  Sometimes the solutions to problems are easier than we think.

We started these ponds with just a few duckweed plants and yet you can see how quickly and how full they became. It can double itself in about 16 hours!

Feeding your Duckweed Pond
You will need to help your duckweed to grow by adding manure. We used chicken manure which a friend of ours sells. He buys it in from a factory farm and it is extemely strong. My husband would half fill a plastic trash can with manure and add water to soften it. This he would then throw into the duckweed ponds with a bucket. This ensured it was spread throughout the pond. If you have put enough manure in, you will see a growth spurt. If you don't, look at the roots of the duckweed plant to see how long they are. If they are long, perhaps more than 2 inches, you need more manure. Duckweed loves mucky water. Infact there are countries, Palestine for example, which use duckweed to clean their water supply. It is one plant which has so many uses. For more information about some of the other uses, I have written another article on Hubpages.

Cleaning your duckweed pond

I know what you are thinking, that this is going to be difficult but it isn't and it is necessary from time to time. If your duckweed seems to be slowing down with its reproduction it might be time to change the water and start again. We had to do this when we had a build up of sawdust in our ponds. This came in with the manure as this is what was used on the floor where the chickens were kept.
There are a couple of ways to do this, either let the water evaporate or drain it with a bilge pump, or a siphon. If you are going to be siphoning this, using your mouth to start the flow of water, remember how much manure you have been putting in there.

Remember keep some duckweed as a starter  

This starter can be kept alive in water until your pond is ready to go again. Once dry, sweep and remove any twigs, leaves or other debris from your pond. This is a good time to inspect your liner below the water line.

Havesting your duckweed

Harvesting is easy. We used a swimming pool net on an extendable aluminum pole. We briefly let the water drain from the net and then turned the duckweed into a large plastic box. We filled this full. This was then loaded onto our kayak  to take to the tilapia which we had transfered to cages.
Because we were harvesting every other day we were able to check the liner for any problems above the water line as a matter of course. A quick once over as we were scooping it out meant any problems were dealt with early and were thus easier to control.

Cost vs Savings

The cost for us was perhaps more than it would be for yourselves as we had to hire a machine to dig the holes for us. But compare this to the savings. If you calculate that your fish are only eating commercial fish food every other day, you have halved your food bill. If you only have a small number of fish, maybe this isn't commercially viable. We had bought 10,000 and had it definitely made financial sense to do this.
We produce healthy, weighty fish at half the cost of other fish farmers in this area.


  1. I need more clarification on adding manure. Amount added (size of the trash can). Did you add additional manure after first harvest?

    1. Keep observing the plant growth when you first add some fert. Then add a bit more, watch the growth. It is a hands on process I believe :)

  2. Great going guys!! We have been doing the same for last 3 years. Duckweed grows in a pond of non-plant eating fish, is harvested to plant eaters. Thus both ponds are benefitted. The plants attract tiny critters which the non plant eating fish love! You should also try feeding your plants with mild cowdung slurry. Don't use too much, might cause an algal bloom but eventually the duckweed will Triumph. Follow me on Instagram @nikzboz , Namaste!