The following is an excerpt from Robert Pavlis’ book Building Natural Ponds and Garden Myths
Combining plant and fish cultivation is far from new; societies have been growing food on lakes and rivers for a long time. The Aztecs created chinampas, large artificial islands and peninsulas on which they grew trees as well as chile peppers, squash, corn, tomatoes, and beans. In modern-day Iraq, the Marsh Arabs still grow food on rafts that are large enough to hold their homes and even large meeting halls.
The idea of growing food on water makes sense. An aquatic ecosystem can provide all the moisture and nutrients plants require and eliminates soilborne pests, while the plants keep algae blooms at bay and provide cover for fish. Despite all these benefits and the long history of growing vegetables in natural water systems, the concept is rarely applied to backyard ponds.
Nutrients naturally accumulate in a pond when fish waste and dead organic material decompose. Nitrogen, the nutrient that’s most critical for fish health and plant growth, cycles through the pond with the help of a variety of decomposers. See the diagram in the slideshow for an example of how this works.
Organic matter, such as dead animal and plant material, accumulates in the pond. Bacteria then start to decompose the organic material. The initial decomposition produces ammonia, a chemical that’s very toxic to fish. Fortunately, bacteria will also convert ammonia to nitrite and then to nitrate, neither of which are as toxic to fish. Some nitrogen will escape into the air, but most of it will remain in the water as nitrate until plants use it again as they grow.
Algae requires higher levels of nitrate to grow than most other plants, so the key to controlling algae growth is controlling the level of nitrate in the water. In a natural pond, this is often done by including a lot of decorative plants in the water and at the edge. Instead of decorative plants, try growing vegetables right in the pond water. They will not only control algae, but will also provide food for your table while they work.
Why Grow in a Pond?
Hydroponics is now a common way to grow vegetables, such as lettuce and tomatoes, commercially. In hydroponic systems, plant roots are suspended in water that contains fertilizer while the rest of the plant grows normally in the air.
How does a pond compare with a commercial hydroponic system? Provided you have some way to suspend the plants over the pond water, pond growing is virtually the same as a hydroponic system, with one important difference: You won’t need to fertilize the pond, because fish, insects, bacteria, and decaying plant material will do this for you.
Commercial hydroponic systems are relatively complex, with pumps to move the water past the plants’ roots and equipment to test the oxygen and nutrient levels in an effort to maximize productivity. You won’t need all of this for a pond hydroponic system. A simple raft that holds the plants in place is all that’s required.
Choosing Plants for Aquaponics Systems
In aquaponics systems, the roots of the vegetables will mostly be immersed in water instead of soil, so this system isn’t a good choice for root crops. Most other plants will grow quite well directly in water.
If you’re just starting out with aquaponics, try growing any of the leafy greens, such as lettuce, spinach, or Swiss chard. If you’re an experienced aquaponic grower, you might branch out to more adventurous options, such as peas, beans, tomatoes, cucumbers, or even peppers.
Whichever plants you choose, start your seeds in soil, as you would for any garden, and after the seedlings are a couple of inches tall, transplant them to their water environment.
Growing Vegetables on a Raft
The easiest way to grow vegetables in a pond is to use a floating raft system. Dense polystyrene is the cheapest and most readily available material to make a floating raft, but be aware of the environmental risks: when it breaks apart, animals (including fish) can eat and choke on the pieces, and the material takes at least 500 years to decompose. Take a piece of dense polystyrene, cut some holes through it, insert your plants, and float the whole structure in your pond.
Dense polystyrene will withstand floating on a pond surface better than the cheap stuff that easily breaks into little balls. The lids from containers used to ship fresh fish to stores and restaurants work well, and you can easily get them free of charge by asking the staff of places where fresh fish are sold. You can also buy larger sheets of polystyrene in building supply stores.
The problem with flat pieces of polystyrene is that they leave no air space between the polystyrene and the water. Plants grow better if some of the roots are exposed to air, so it’s a good idea to have a small air gap between the foam and the water. You can attach small strips of foam around the edge of the raft with glue or twist ties, or use the insulation foam covers sold for copper piping instead of strips of foam. Any material that floats will raise the main raft so it’s not touching the water, which will give your plants’ roots the air space they need.
You’ll have several options to hold the plants in place. You can use 1- to 2-inch lengths of the insulation for copper piping to make “collars” for each plant. The foam insulation will already have a slit along one side, making it easy to insert the seedling without damaging its roots. You can then insert the plant with the foam collar into a hole in the polystyrene by compressing the collar a bit. As the plant grows and needs more space, it’ll push against the foam, compressing it.
Another good option is to use hydroponic pots, which are designed with lots of holes for good water circulation, or, if you prefer the DIY approach, you can punch holes in plastic drinking cups to make your own. These pots can then be filled with mineral wool hydroponic growth medium or expanded clay pellets. The plants really won’t care which system you use, provided it gives them some support until they grow good-sized root systems.
Although the raft system is easy to build and use, it does suffer from one limitation: Fish can get at the plant roots. Most fish will nibble on plant roots, but koi and other carp, such as goldfish, may eat enough to affect your plants’ productivity. To keep your plant roots safe from piscine predation, you can add netting below the raft.
Using a Bog Garden
Bog gardens are frequently added to natural ponds as a way of cleaning the water and keeping nutrient levels low. The diagram above shows how a bog garden connects to a pond. Water is pumped from the pond into the bottom of the bog garden, where it percolates up through the sand layer and past the plant roots, and then finally returns to the pond. The plants benefit from the nutrient-rich water being pumped in from the pond, and the low-nutrient water that returns can’t support algae blooms.
Normally, gardeners grow ornamental plants in a bog garden, but there’s no reason you can’t use vegetables. Planting in sand is easier than using individual pots, and the natural look of the pond won’t be disfigured by floating rafts. Bog gardens also solve the problem of fish eating the plant roots, since the two are kept completely separate.
Planning an External Growing Area
The two methods described above are fairly simple to implement, but you might not like the idea of seeing your vegetables in or near your pond. You may also want a larger growing area to increase the number of vegetables you can grow. The solution to either of these issues is to create a separate vegetable growing area.
There are many variations to this option, but in general, you’ll need to add a pump that will move the pond water to an external tank. The water will flow through the tank, eventually returning back to the pond. The tank will hold sheets of polystyrene with plants inserted; most growers using such a system prefer square tanks, because polystyrene sheets can then be easily cut to the right dimensions. This tank system is very similar to commercial hydroponic systems, except your pond will be providing the nutrients.
In a normal pond, keeping fish levels low is important so ammonia levels don’t get too high. But with a larger external growing area, you’d actually have to increase the fish load in the pond to provide enough nutrients for the plants. This is great news for people who love to have lots of fish in their ponds and for people who raise fish for food.
The moving water and the large surface area of the external tank also provide a great way to oxygenate the water, which is needed to support a higher fish load.
Small backyard ponds tend to get fairly warm, and traditional goldfish, koi, and mosquitofish are good choices. A midsized backyard pond could support tilapia, which require cooler water. A larger pond can support game fish, so the whole system could provide your table with meat and vegetables.
Balancing Fish and Plant Load
Aquaponics systems are easy to set up, but for a commercial operation, the difficulty is in getting the fish-to-plants balance just right. If the fish level gets too high, the plants can’t keep the nutrient levels under control and the fish die; alternatively, too few fish can’t generate enough waste to provide the plants with enough nutrients.
Keep in mind that the number of fish won’t matter as much as the total fish weight. Large fish produce more waste than small fish. This is all complicated by the fact that fish grow, so a system in balance today may be out of balance a few weeks later. The key is having a progression of differently sized fish in the system so the overall weight of fish is kept constant.
A noncommercial operation, such as the one in your backyard, won’t be as complicated, because you won’t need to maximize productivity. Just keep fish loads low, and you won’t have any problems. If nutrient levels dip, your plants will just grow a bit slower.
Cannabis and Tilapia
Canada is on track to legalize recreational cannabis in 2018. (Medical cannabis has been legal in Canada under certain circumstances since 2001.) This has spawned a number of commercial growing operations. One of the most interesting new growing operations is located in southern Ontario.
Green Relief grows medical cannabis in an underground facility that provides about 2 acres of growing space lit by LED lights. The plants grow in rafts floating in tanks supplied with water from fish tanks containing tilapia. The fish waste in the water provides plenty of nutrients for the plants. The circulating water keeps both the fish and the plant roots well-oxygenated. The company harvests both cannabis and fish from the system.
In a demonstration area, Green Relief uses the exact same system to grow a variety of flowers and vegetables; see photo in slideshow.
Robert Pavlis is the owner and head gardener of Aspen Grove Gardens, a 6-acre botanical garden, and has more than 40 years of gardening experience. He is the author of Building Natural Ponds and Garden Myths. Find his blogs online at Garden Fundamentals and Garden Myths.