Will My Koi Pond Freeze in the Winter?


koi under iceWinter is here and temperatures in Alabama have already started to drop. Soon, we’ll hit below-freezing temperatures, which leads many koi pond owners to wonder if their ponds will freeze over – and how this will affect their fish and pond if that happens.

We’ll address how winter weather impacts a koi pond and the koi below, and we’ll discuss what you can do to protect your animals and equipment.

How Winter Affects a Koi Pond

First, a pond can stay unfrozen even if the air temperature is below the freezing point (32 degrees F). That’s because water is a good reservoir of heat, like the ground around and beneath it. So, the air can be freezing but the water can still be liquid.

But let’s say that the water temperature falls below the freezing point. That doesn’t mean the entire pond will freeze solid. What happens first is that ice will accumulate on the surface. As the surface freezes, the layer of ice will serve as an insulator that keeps the heat in the water below from leaving the pond. This is why your pond probably won’t freeze solid, especially in Alabama. (Koi fish can’t survive a completely frozen pond, so if your pond will freeze – particularly if it’s a shallow pond – you should move your fish elsewhere.)

Water getting colder after the pond starts to freeze isn’t necessarily bad for your koi, either. In fact, it can actually foster torpor (hibernation), which is what your koi will start once the temperature drops low enough. They’ll stay in hibernation mode until winter is over. During this time, you won’t have to feed them at all.

You can determine if/when your koi pond will freeze by consulting this map. Created by the USDA to determine plant hardiness, it can also be used to evaluate the likelihood of a hard freeze. The top half of Alabama is in zone 7a while most of the lower half is in zone 8a, with the exception of the extreme lower counties, which are in zone 9a.

Generally, in zones 10 or below, you’ll want to take steps to winterize your pond. Since that includes Alabama, we’ll talk about what you can do to protect your koi.

frozen ponds

Winterizing Your Pond

Start preparing for the winter by removing all the leaves and debris from the pond. You can then install a pond net to catch anything else that falls into the pond.

If ice isn’t accumulating enough to freeze over the entire surface of the pond, you can oxygenate the water with an air pump and air stone(s), which can also help keep parts of the surface clear for oxygen to enter. This will keep a hole(s) open for oxygen to enter the pond, which is what your fish will need.

It’s possible that you can run your external pond pump throughout the winter, but be careful: as the temperature falls, it becomes more likely that your pump will freeze solid and stop working. As the air temperature approaches freezing, cover all external pumps and filters with a heavy tarp to use heat from the pump motor and keep them from freezing. Be sure to leave a corner of the tarp open so the equipment can breathe on warm winter days. Koi ponds rarely freeze solid in Alabama so any internal/submersible pumps will be fine left running. You may choose to turn off your waterfall or stream to keep them from freezing and diverting water from the pond.

frozen waterfall

Caring for Your Koi Pond

Winterizing a koi pond and keeping it maintained throughout the winter can be a chore, and it’s probably not something you want to do yourself. If you’d like some help, contact the koi pond maintenance pros at Alabama Aquarium & Pond Services. We’re ready to serve you – and your fish – during the cold winter months.