Overwintering Koi In A Greenhouse

One couple's quest in keeping their koi safe for the winter.

A small pond in a greenhouse. Via Ella/Flickr

My wife Susan and I have raised koi for almost nine years. Since we live in Ottawa, Canada, where winter can last eight months or more and temperatures can go as low as -40 degrees Fahrenheit, providing winter shelter for our fish has always been a big concern. For three of those winters, we have tried to protect the fish by building temporary greenhouses over our 20,000-gallon backyard pond.


The first attempt resulted in a low-slung, oddly-shaped igloo made of PVC piping bent into arches and greenhouse plastic. It survived the winter, but it required constant snow-clearing — even during the middle of the night. Otherwise, the whole structure would collapse under the weight of the snow.

The author successfully built a greenhouse for overwintering his pond. The greenhouse’s covering not only protected the fish but gave the pond’s plants a head start when the first vestiges of stronger spring sunlight appeared in March.

We tried to improve on the first greenhouse by building the second one higher and into a consistent “Quonset hut” half-cylindrical shape. It was so tall that it cleared the pump shed! Because this greenhouse was so tall, I was unable to install a center ridge pole to connect all the hoops together (the hoops held the plastic just like a Quonset hut). As a result, they moved freely whenever snow piled up on them. The result was a big dome that had no strength and that required even more care than the first one did.

To make matters worse, the wind would roll down off the house roof and compress the dome. Its subsequent release would make a noise like a thunderclap and provide enough force to roll off the 200-pound boulders (set on planks) that were holding the plastic in place. Even worse, the plastic split like a giant zipper during a windstorm in early March.

The result was that the koi were suddenly faced with the temperature dropping some 60 to 70 degrees. The surface of the pond quickly froze solid, and when spring finally arrived, we had lost a lot of fish — even after going to the trouble of drilling air holes.

The next winter, we tried keeping air holes open and letting the pond freeze. That resulted in more lost fish as the water went below 32 degrees. Horrified, we then brought the fish back indoors to the 3,000-gallon basement holding tank we had installed the year before when the pond outside was being built. This kept the fish alive, but they subsequently grew so big that we didn’t want to haul them outside twice a year. And this is how things remained for two years: We had an empty 20,000-gallon fish pond outdoors and an overstocked 3,000-gallon fish tank in the basement of our home. This wasn’t at all what we had intended!

Then we had had enough. We laboriously hauled 47 giant koi up the stairs, one by one, and put them outside. And when the next winter came, we built another greenhouse — but this time, we got smart.

What Made this Greenhouse Different?

Instead of returning to the PVC pipe design, we decided to use roof trusses — the same kind found in any modern house. With the guidance and help of our trusty carpenter, Eric (who has seen us through more disasters than I can count), we had the trusses custom-made to span the pond’s 22-foot width. Eric then used two-by-fours to build a support wall around the pond. This was no easy feat, given that he had to include the waterfall.

With the base wall in place, a group of us worked together to put the trusses in place. Unlike with the last two greenhouses, these verticals were firmly attached to each other using horizontal boards. The result was a very solid structure.

Next, we stapled on two layers of greenhouse plastic. On the roof, we used 1-by-3-foot planks screwed into the trusses to hold the plastic in place. They proved to be an excellent idea. Had we just used staples, the plastic likely would have torn off.

To allow for easy movement inside the greenhouse, we built a plank catwalk (which our cats appreciated). We also built a door that sealed shut, and installed aerators and a cattle trough de-icer to ensure that there was always open water for venting pond gas. Finally, we added a rope light for illumination and a wireless digital thermometer that relayed water temperature information back to the warmth of our house. (To prevent pipe freezing, we added aquarium heaters to both of our settling tanks — connected to the pond by two bottom drains — and plugged in a heater to keep the indoor shed above 32 degrees).


The winter of 2010 to 2011 was relatively light by Canadian standards. This means that we did have a lot of 2-foot blizzards and very cold temperatures, but spring nevertheless arrived in April rather than late May.

The pond’s surface did freeze up in December and January, with holes being maintained by the aerators and the de-icer. However, the lengthening days resulted in the pond ice melting in February, while the outdoors remained frozen solid. By March, the indoor temperature was in the 50s (aided by the pond’s southern exposure), and the water temperature was slowly climbing. But it wasn’t until mid-April that the water itself got above 50. Meanwhile, the fish only returned to the surface to eat in late April; it had been about six months since we had seen them last.

All of the fish that were in the pond when the greenhouse was built survived the winter. The greenhouse came through with flying colors. Although it was known to rock a bit when the winds got strong, the structure remained entirely intact. The double layers of plastic didn’t rip, and the wooden frame didn’t shift. The only issue we actually faced during the winter was the freezing of the aerators’ air line, which was being fed from the shed. I solved this problem by putting the air pump in the greenhouse directly, keeping the pipe from being exposed to the frigid outside air.

Originally, our plan was to remove the greenhouse and to store it until next fall. But given that outdoor watering had been banned locally due to a broken water main, we left the greenhouse up to reduce evaporation, while covering it with cloth to reduce heat. (The unused basement 3,000-gallon tank was also converted into a rainfall run-off tank. All I had to do was extend the eavestrough downspout so that it fills the tank when it rains.) The greenhouse functions today as well as it did its first year, and it was successful in reducing evaporation.

The greenhouse is not a perfect solution — it gets in the way of our favorite swinging chair — but it does protect the koi from winter’s icy blasts.

James Careless has a 20,000-gallon pond with many koi. He has written for Koi World, Koi USA, Koi Carp and Koi magazine.

Article Categories:
Fish · Ponds and Koi