Plant growth and aquaculture work together in Aqauponics

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Plant growth and aquaculture work together in Aqauponics

Plant growth in an Aquaponics system is intended to remove waste nutrients that would otherwise accumulate to toxic levels in the Aquaculture component of the system. This means that the plants are growing hydroponically, but don't be fooled, this is very different from conventional hydroponics, since there are no other added nutrients. The plants here have to be able to grow on what is available.

Also, plant growth in aquaponics has other important functions and it is important not to loose sight of these. When we think of plants in aquaponics, we generally think of the plants off to one side, in a hydroponic-looking trough or 'bed'. However, there should also be plants growing on the water in the aquaculture tank. These mainly help to provide shelter for the fish, which goes a long way to keeping their stress levels down. It also helps moderate temperature changes (or reduce energy costs if you really want to get into heating and cooling), as well as reducing evaporative losses from the tank.

Both forms of plant growth are important and need to be carefully considered. For example, I use Water Fern (Azolla sp.) on the water in my tank, because it is cheap, easy and occurs locally. Although in my location, a catastrophic loss of water could not possibly let any of the water fern into a local stream or pond, it would not cause any environmental problems if it did.

On the other hand, a plant like Water Lettuce (Pistia stratiotes) could become a problem if accidentally released and others like Water Hyacinth (Eichhornia crassipes) would be an absolute "No-No"!

If you use a pond for your fish (rather than a tank), Water Lilies could be a good alternative, since they grow on the bottom, with leaves floating on the surface. This could help reduce the build up of nutrients in the sediment collecting on the bottom and provide additional shelter for crustaceans like 'Yabbies' that would also help with this nutrient recycling.

Of course, once you have this part of the system worked out, it becomes a something of a 'set and forget' component. All you need to do is remove excess plant growth to the compost bin every now and then (mostly so you have some clear water for feeding your fish).

The main form of plant growth in aquaponics is still outside the tank. In my system, I use simple airlift 'pumps' to bring water over the side of the tank, which then gravity-feeds itself through the growing bed and back into the tank. I find this is the most energy efficient method and it also has the benefit of making sure the water is always well aerated.

The growing bed is a Food-grade PVC trough mounted in a frame that holds it up next to the top of the tank. Inside, it is partly divided into two sections.

The water enters the smaller section at one end, where I have set up a version of my original bio-filter unit (except for the big chunks of Limestone, since the gravel in the other large section now serves their function). So the water filters down through a bucket containing straw inside some fine netting, and then wells up within this section of the trough to spill over the section divider into the part containing the gravel.

The gravel is held in a 'cage' of stainless steel wire netting and a supporting frame, so that it can be easily removed for cleaning.

Remember that stainless steel should be the only metal used in your system as others can contaminate the water with
toxic minerals like zinc or copper.

The outlet pipe is at the bottom of the trough at the end furthest from the inlet and set just above the point in the side of the aquaculture tank where the water goes back in. Not being much on an engineer, I also made the outlet a bit bigger than the inlet, with a control valve so I could tweak the flow and keep the water level in the trough just right.

Most cleaning of the plant growth trough will occur at the smaller bio-filter end, by changing the straw and rinsing the net - and occasionally stopping water flow and cleaning out the whole filter section. However, you will also want to be able to remove the gravel easily and clean out the whole thing at least between every plant growth crop.

As to what can be grown in the gravel section, it could be almost anything, but since this is a food site, we will focus here on growing food plants.

The selection is still quite broad, but at the same time, it is probably best to also keep things simple. Some plants produce a toxic exudate from their roots. For example, Marigolds produce compounds that are useful in controlling nematodes in the soil. There is little or no information on whether these compounds would also be harmful to fish, but you are probably better safe than sorry. Some reading about Companion Planting would probably be the best way to avoid choosing the wrong kind of plant growth for your system.

Many vegetables that grow well in containers could be used here , especially those that enjoy plenty of water. Lettuce is one that makes very useful plant growth in Aquaponics. Others that have long growing or producing times, such as Chillies or strawberries, could also be of value in aquaponics. Another plant that could be very useful - having begun as an ornamental for planting around ponds - is Rhubarb. On the other hand, those requiring a lot of nutrition (since you cannot use any fertilizer here at all) like tomatoes, are probably not suitable.

The ultimate aquaponic in plant growth, of course, would be a semi-aquatic food plant and two examples spring to mind - Chinese Water Chestnut and Wasabi.

There is still a lot more information to be added here so please stay in touch. One good way to do this is by subscribing to our Free email newsletter - The Food Gardener.