Winterizing Ponds and Water Features

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Robert Pavlis

Winterizing ponds is important for anyone who lives in a climate where water freezes. Even in warmer areas were temperatures get close to freezing, you should consider some preparation steps for cold weather.

This post looks at things you should consider before winter sets in.

Winterizing your pond - a small air pump is preventing the pond from freezing over, by Robert Pavlis
Winterizing ponds – a small air pump is preventing the pond from freezing over, by Robert Pavlis

Why Winterize Ponds?

Ponds in nature don’t have a helping hand to get ready for winter so why does your pond need help?

  • Equipment – native ponds don’t have equipment that can get damaged when water freezes.
  • Fish – native ponds don’t have an excess number of fish like many backyard ponds. Native fish are also more adapted to winter.
  • Plants – native ponds don’t support non-hardy plants that need special care in winter.

No matter how natural your pond is, it is not a native pond and it requires some special attention.

Winterizing Small Water Features

Small features that are less than one foot deep (30 cm) will freeze solid in many cold climate locations. It is best to empty them completely for winter since ice expands as it forms, cracking many containers. Once empty, lay the container on its side or cover it to prevent water and snow from accumulating in it during winter.

What Happens in Winter?

Except for non-hardy animals and plants, there is not much concern about cold weather. Properly selected fish and plants will have no trouble overwintering in water that reaches the freezing point. There is also no concern about equipment. The problem starts when temperatures drop below freezing and the pond chemistry starts to change.

Soil Science for Gardeners book by Robert Pavlis

Below freezing, ice starts to form on the surface of the water. Cold air temperatures cause ice to form, while the sun helps to melt it. Wind keeps ice from forming, provided that it does not get too cold. Moving water also prevents ice from forming. All of these forces fight with one another to control formation of ice.

As temperatures drop more, the forces making ice win and a layer of ice covers the pond. Several things happen now. The layer of ice traps warmth in the water – it acts like a big blanket and slows down further ice thickening. Heat from the center of the earth move up through the soil to the bottom of the pond and into the water. This heat is trapped in the water by the ice layer.

The ice layer also stops gas exchange with the air. Excess CO2 produced by living organisms like fish, and bacteria can no longer escape the water. Oxygen from the air can’t enter the water. As organic matter in the bottom of the pond decomposes it releases toxic gases like sulfur dioxide and ammonia. The ice prevents these from escaping into the air and they slowly build up.

Over time the top layer of ice does get thicker, reducing the amount of liquid water in the pond. This results in the concentration of the gases in the water. Carbon dioxide, ammonia and sulfur dioxide reach toxic levels. In extreme cases the water freezes solid but that rarely happens in most ponds, except in extremely cold climates.

Colder climates tend to have thicker layers of ice that stay on the pond longer. What this means is that the toxic gases are more critical in these climates.

Keep in mind that no matter how cold it gets, the temperature of the water never gets colder than the freezing point.

Building Natural Ponds book by Robert Pavlis

Why Do Fish Die in Winter?

Many people think that they just freeze to death, but that is rarely the cause. Fish and other animals die because of gas poisoning (carbon dioxide, ammonia or sulfur dioxide). This statement assumes that the animals are suitable for the climate. Gold fish and Koi will survive winter in a pond with ice on it provided the gases don’t become toxic.

How do you keep gas levels from becoming toxic? The best way is to ensure that the pond never freezes over completely for more than a week and keep decomposing organic levels low.

How Thick Does Ice Get?

You probably think that in cold climates like zone 5 (Chicago and Toronto) pond ice is very thick, but that is not true. The following is taken from my book Building Natural Ponds.

“A pond that is 2 feet deep will rarely freeze solid, except in extremely cold climates.”

“How thick will the ice get where you live? That is a difficult question to answer. The rate of ice thickening depends on temperature, wind, snow cover, and the current thickness of ice. Thick ice gets thicker more slowly than thin ice and a cover of snow slows the process even more. As temperatures get near freezing, the ice gets thinner so on mild winter days you actually lose ice.”

“As a general guide you can use the following information. It represents ice growth without the protection of snow.”

ice formation on a pondTo use this table, get an average temperature for each week, by taking the low and high each day and averaging the values for the whole week. Be sure to include day and nighttime temperatures.

Should You Keep Water Flowing?

Assuming you have flowing water through a pump, you need to make a fundamental decision. Should you keep the water flowing or not?

Reasons to keep water flowing all winter

  • Less work; you don’t have to dismantle equipment and take it inside.
  • Prevents the top from completely freezing over.
  • Increases gas exchange with the air.
  • Keeps the pond and waterfall interesting all winter.

Reasons to stop the moving water

  • An electrical stoppage could result in frozen pipes and pumps.
  • An ice dam can direct the water out of the pond which can cause major damage to equipment.

My waterfall uses a fairly expensive, above ground pump. Since I don’t spend much time outside in winter I decided to bring my equipment inside so there is no chance of damaging it.

An in-pond pump is less likely to be damaged.

Pumps,  Filters and UV Systems

If you decide not to run the pump in winter, simply bring all equipment inside to keep it warm.

If you run the pump, filtration and UV systems are not needed since they don’t do much when it is cold. Pressurized filters should be removed, drained, and stored dry. Gravity Filters can be left in place or removed. Bog filters should be left in place but you may have to bypass them if they tend to freeze solid.

UV systems should be disconnected and stored warm and dry. The quartz sleeves, O-rings and gaskets will last longer that way.

Natural Ponds

Natural ponds do not use pumps or filters as explained in Ponds and Pond Filters. Except for dealing with the equipment, all of the other sections of this post also apply to these kind of ponds.

If you are interested in Natural Ponds consider joining our Facebook Group called Building Natural Ponds. It is a great place to get questions answered.

Fall Cleanup

Fall is a good time to do some general cleanup. Keep the amount of organic matter in the pond to a minimum so that you control algae growth next season and keep toxic gas levels low during winter.

  • Remove fallen leaves before they start to decompose. This is especially important in a river or waterfall system where the leaves can accumulate, forming a dam that causes water to overflow the sides.
  • Remove sludge from the bottom of the pond.
  • Cut back dying vegetation. Things like water lilies can be cut back in fall. Other plants like cattails are more easily cut back once the ice forms thick enough for you to walk on it.
  • Remove floating plants like water lettuce and water hyacinths.
  • Move non-hardy plants indoors.
  • If you bring equipment inside give it a good cleaning, to make the spring job of putting it back easier.

Things You Should Not Do in Fall

Lots of pond advice is given for people who like to use a lot of chemicals and equipment to maintain their ponds. The ideas are heavily promoted by companies selling these products but many of the recommendations are not necessary, and in fact can harm the natural balance in your pond.

Add Cold Water Bacteria

Bacteria have a preferred temperature in which they like to live. Some like it hot, others like it cold. As the temperature in your pond drops some will start to die off or go into a state similar to hibernation, and others will start to become active. Your pond has lots of bacteria and they will adapt to changes in temperature.

Buying special bacteria makes no sense. For more on this topic see Beneficial Pond Bacteria – A Waste of Money.

Empty the Water and Clean the Liner

This is another piece of advice that is nonsense. The slime you see on your liner is where all of those beneficial bacteria live. Just leave them alone. My pond is 10 years old and I have never cleaned the liner. The water is algae-free all summer long.

Start Feeding Cold Water Fish Food

Some companies sell special food which should be fed in fall and winter. This ‘cold water food’ has lower levels of protein. Fish don’t need special food, this is just a way to sell more products. If you feed less they will also get less protein. The best thing you can do is stop feeding the fish.

As temperatures drop, fish need less food. If you keep feeding them the extra food just ends up adding extra nutrients to the water, which increases nutrient levels and toxic gas levels.

In a healthy pond, you don’t need to feed goldfish at all, even in summer.

Keep Ice From Forming

Animals in the pond, including fish, die because the ice on the water prevents gas exchange with the air. The simple solution to fix this problem is to keep the ice from freezing and there are several options you can consider.

If the ice freezes over for a week and then thaws again, don’t worry about it – you don’t have a problem. Once the surface is frozen for more than two weeks, fish may start to die. This depends on many factors like the amount of sludge on the bottom, the depth and volume of water, and the amount and type of fish.

If ice does cover the pond, Do Not use a hammer or other device to crack the ice. The vibrations can be deadly to animals in the water. If you must make a hole in the ice, use a container of boiling water.

Keep Water Moving

If you have a pumping system, the best way to keep the surface from freezing over is to keep it moving. You will notice that the pond can freeze over quite a bit, but the area around the waterfall stays open. As long as the water is moving, it won’t freeze until the temperature gets really low. Higher flow rates work better than low flows.

Air Pumps

An air pump can be added to the pond in fall. The pump creates bubbles in the water, which keep the water moving, and prevents it from freezing. At lower temperatures you will need more air to keep the surface from freezing.

I use a small aquarium pump (see the above picture) which works quite well here in zone 5 provided we don’t get several weeks of really cold weather. When that happens, ice will form a dome over the bubbles sealing off air exchange.

The bubbling air will also help a bit with air exchange getting rid of toxic gases.

Since the pump is outside of the water, there is no fear of it freezing.

Pond Heater / De-icer

Several companies make special pond heaters also called de-icers, or deicers. Just drop them in the water in late fall, and they will keep a hole in the ice. Some devices combine heating with bubbling air.

The operational cost of a de-icer is quite low since they warm the water just enough to keep it from freezing and use very little electricity. They also don’t need to be turned on until temperatures get low enough to cause the formation of ice. A good quality de-icer will have a thermostat to control when it comes on, further reducing operation cost.

Pond Plants

To find out how to handle non-hardy pond plants have a look at, Overwintering Pond Plants – Part 2 .

Building Natural Ponds

If you are interested in natural ponds why not join our special Facebook group Building Natural Ponds. Please join the group at

Building natural ponds face book group
Building natural ponds face book group

Build a Better Pond

Here are some other posts that will help build a better natural pond.

Overwintering Pond Plants – part 1

Water Lilies for Ponds

Beneficial Pond Bacteria – A Waste of Money

Pond Pumps & Pond Filters

Selecting the Best Pond Liner

10 Ways to Control Duckweed in Your Pond

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

4 thoughts on “Winterizing Ponds and Water Features”

  1. That is very true you cannot put antifreeze in your pond and expect living animals to stay alive. Too many other ways to keep it from Freezing over.

  2. Thank you!! I have a first year natural pond from runoff water. Very healthy, frogs, minnows, etc. Your information is very helpful to me as I get the pond ready for winter.


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