Aquaponics for the Frozen Tundra Part III: Choosing Plants and Fish
A look at which aquaponic plants and fish thrive in the cold, and how to raise them all successfully.
I grow in a cold house. In greenhouse language, this means I allow my temperatures to drop below 10ºF—cold enough to kill most plants. Others grow in warm (>32ºF) or hot (>50ºF) houses, which are nice and plush, but in my climate, I would have to sell my soul to the electrical utility company or burn up my woodlot to grow in a warm or hot house.
I grow in cold conditions because I want my aquaponic system to produce more in vegetables and fish than I put into it, energy-wise. My well-insulated system (see Aquaponics for the Frozen Tundra Part 1) does just that.
As you can tell, I’m proud of my energy-efficient, frozen tundra system. While my cold house limits my choices for fish and plants, the ones I like the best are the ones that love the cold.
Best Plants for Cold-weather Aquaponics
I’ve had success with the following plants in cold temperatures:
- Spinach (giant winter, tyee)
- Swiss chard
- Arugula (sylvetta)
- Lettuce (winter varieties survive down to 20ºF)
- Corn salad (a.k.a. mache, lamb’s lettuce)
Perhaps all of the Popeye watching I did as a kid has influenced me, but I love spinach more than any other food on Earth. This is lucky, because of all the plants I mentioned, spinach grows the best in the cold. With its strong susceptibility to pythium, it’s a challenging crop to grow. However, I’ve fought this battle and come out victorious. The following instructions work for spinach and will suit the other (easier) plants just fine.
How to Grow Spinach in Cold Conditions
To grow spinach successfully, you must know your enemy. Coming in many varieties, the pythium fungus will kill every single one of your winter spinach plants before you can finish your sauna and ice dip. With pythium, prevention is the only solution. Where tomatoes and lettuce will tolerate less-than-ideal seed-starting conditions, for spinach you must follow these recommendations (or their equivalent) exactly:
- Use either brand-new, sterile media, or sterilize it yourself by boiling it for 30 minutes or pressure-cooking to 15 lbs.
- Soak your trays and cells in a 5% bleach solution for 20 minutes minimum, then rinse three times.
- Dip your seeds in the bleach solution, then rinse.
- Plant your seeds at ¼-in. depth in a seed tray with a humidity dome maintained between 50-70ºF. Alternatively, you can start your seeds in a paper towel with a water/peroxide mix, and then transplant the sprouted seeds.
- Each time you water, mix 10 parts water with one part hydrogen peroxide solution.
- Provide no more than 13 hours of light. Providing only eight hours will make your plants bolt-resistant once they’ve grown to full size, although they start slower this way.
- Once they’re 4-in. tall, harden off your plants for several days, at times when greenhouse temperatures will not drop below 32ºF.
- Transfer plants to the aquaponic system.
- Once planted, the intense biological community in aquaponics, especially in a system with water temperatures at or below 50ºF, helps protect your plants from pythium.
With the hard work done, all we do now is maintain proper humidity and light. Plants need to transpire to grow, and many do so most effectively between 50-70% relative humidity. But under the high humidity conditions common in winter greenhouses, water can also condense and drip on your plants, encouraging disease.
During the daytime, I manage humidity in the low tunnels over my grow beds by bringing in cold, dry air from the outside and pre-heating it using a low-wattage hair dryer, controlled by a 120-V dehumidistat. A heat recovery ventilator would do the job even better.
At nighttime we get a free pass from humidity. In fact, the more the better! As temperatures fall below 40ºF at night (i.e. in low light conditions), humidity becomes a resource rather than a problem. Because plants stop transpiring at these temperatures, growth is not a factor and diseases are rare and largely dormant. Water condensing on plant roots and greenhouse walls releases heat that keeps your plants warmer than the air.
As for lighting, the choice is up to you. My latitude doesn’t provide enough light for significant plant growth. Because of this, I supplement in small amounts using fluorescent lights attached to the undersides of my low tunnels. With lettuce, you can leave the lights on all night long if you want, which allows for fewer lights. For spinach, however, 13 hours is the maximum to prevent bolting.
If you choose not to use supplemental lighting, you should grow your plants to full size prior to November 1. While they won’t grow much over winter, you can still harvest all winter. Carbon dioxide supplementation helps with growth in low light conditions, and the CO2 released from decomposing fish waste also helps.
Harvesting greens that have frozen and thawed improves the taste. However, it’s a bad idea to harvest while your plants are still frozen. It’s also a bad idea to let your lettuce freeze too hard (below 25ºF) or too often, or your plants will die.
Avoid harvesting more than 30% of any plant you want to keep growing. This is an important practice because in late winter as temperatures warm, your plants, which spent the winter building an impressive root structure, will take off like rockets!
Raising Fish in a Cold-weather Aquaponics System
Just like with plants, raising fish in the frozen tundra in an energy-efficient way also limits your options, which include the following:
- Shut the system down over winter.
- Switch fish seasonally.
- Raise fish that can survive both warm and cold water, year-round.
- Grow (or breed) indoors in winter.
Each choice offers benefits and drawbacks. We’ll discuss them each here briefly.
Shutting down the system is the simplest route. You don’t need to insulate or air seal your system as thoroughly or shovel the path out to the greenhouse after every snowfall. You cut heating expenses by at least 50%. On the downside, you miss out on succulent winter spinach, many months of fish growth and the option to raise fish, which takes multiple seasons to grow. It also requires you to re-introduce the nitrogen cycle in the spring.
Harvest and Switch
This is what I do. In early October, I harvest all my summer fish like tilapia, catfish and perch, and add winter fish like rainbow trout. In early June, I switch back. To do this, I have to buy larger fish, about 4-8-in. long, from the hatchery.
Advantages of switching include maximizing the total fish harvest, mixing up your fish diet and maintaining enough nutrients for vigorous winter plant growth. Disadvantages include the increased costs of buying larger fish stock, regular water changes if your plants don’t take up enough nutrients—such as if you don’t supplement with light—and the inability to grow truly huge fish, although you can tell whatever stories you want. Many people choose not to go this route due to some urban fish legends, which I hope you’ll ignore because they are untrue, such as:
- Trout are finicky.
- Nitrogen conversion stops at low temperatures.
Some fish types survive in both cold and warm water. These include perch, bluegill, catfish and largemouth or hybrid bass. A fairly straightforward and simple strategy, these fish grow slowly in winter (cold water) and faster in summer (warm water). Advantages include the option to purchase smaller fish, which is cheaper, and the ability to grow more fish types including perch, considered by many as the best-tasting freshwater fish. The disadvantages include only one fish harvest per year and limited nutrient production in winter, a problem if you’re hoping for maximum winter plant harvest.
Imagine cozy and romantic nights by the fire…with your fish. If this appeals to you, you might like to bring them indoors in winter. Doing so—with a good air-sealed system design—allows you to forego much of your winter heating bill. You can raise warm weather fish in winter, and grow them out year-round at fast growth rates.
It also allows you to breed your own fish. Many fish are difficult to breed, though tilapia are easy. An easy solution is to raise tilapia together with a couple of largemouth bass. Tilapia breed prolifically and the bass eat all their fry. When the mood strikes, you simply take a few tilapia into a separate tank for conjugal visits. When the babies grow large enough, you bring them back in with the bass.
Disadvantages to this method include the requirement that you build either two aquaponic systems or a portable one, the fact that aquaponic systems require a minimum of 100 gal. for stability, which requires some space in your home, and the need for significant supplemental light to grow plants. Or, you can simply raise fish indoors in winter without plants, which would require water changes.
Growing in the frozen tundra is a great way to beat cabin fever and eat healthy, local greens year-round. Keep it frosty!