Choosing Plants For Winter Aquaponics
By Jeremiah Robinson, Madison, Wisconsin
Over the past eight months we’ve been learning how to do aquaponics in greenhouses in cold climates. For the last installment in this series, we look at plants and fish that thrive in the cold, and how to raise them.
I grow in a cold house.
In greenhouse language, this means I allow my temperatures to drop below 10˚F—cold enough to kill most plants. Others grow in warm (>32˚F) or hot (>50˚F) houses, which are nice and plush but in my climate require you to sell your soul to the electrical utility or burn up your woodlot.
I grow in a cold house conditions because I want my aquaponics to produce more (in vegetables and fish) than I put into it (in energy). My super well-insulated aquaponics system does just that.
As you can tell, I’m proud of my energy efficient frozen tundra system.
While my cold house puts limits on my choices for plants, the ones I like the best are the ones that love the cold.
I’ve had success with the following list of plants in cold temperatures:
• Spinach (Giant Winter, Tyee);
• Swiss Chard;
• Arugula (Sylvetta);
• Lettuce (Winter varieties survive down to 20˚F); and
• Corn Salad, a.k.a. Mache and Lamb’s Lettuce.
Perhaps the Popeye watching as a kid did it to 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.
In growing spinach, 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:
1. Use either brand new sterile media, or sterilize it yourself by boiling 30 minutes or pressure-cooking to 15 pounds.
2. Soak your trays and cells in five percent bleach solution for 20 minutes minimum, then rinse three times.
3. Dip your seeds in the bleach solution, then rinse.
4. Start your seeds in the seed tray with humidity dome—maintained between 50-70˚F—by planting them at •-inch depth. (Alternately, you can start your seeds in paper towel with water/peroxide mix, and transplant sprouted seeds.)
5. 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, though they start slower this way.
6. Once they’re 4-inches tall, harden off your plants for several days, at times when greenhouse temperatures will not drop below 32˚F.
7. Transfer plants to the aquaponics.
8. Once planted, the intense biological community in aquaponics (especially with water temperatures at or below 50˚F) helps protect you from pythium.
With the hard work done, all we do now is maintain proper humidity and light. Plants need to transpire to grow, and most do so most effectively between 50 and 70 percent Relative Humidity (%RH). Under high humidity conditions (common in winter greenhouses), water can also condense and drip on your plants encouraging disease.
During the day, 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-volt dehumidistat. A heat recovery ventilator (HRV) would do better, but they’re expensive.
At night we get a free pass from humidity. In fact, the more the better!
As temperatures fall below 40˚F at night (i.e. in low light conditions) humidity becomes a resource rather than a problem. Because the 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 (or low tunnel) walls releases heat that keeps your plants warmer than the air.
With regard to light, 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.
Depending on the temperatures you maintain based on your climate, and the amount of light you supplement, you get anywhere from 0 to 100 percent growth rates. If you choose not to supplement light, 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 (CO2) helps with growth at low light conditions, and the CO2 released from fish waste decomposition helps with this.
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 25˚F) or too often, or they’ll die.
Avoid harvesting more than 30 percent of any plant that 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!