Winter protection for the pond: protect the pump, filter, UV unit, and plants from frost

Every late winter, the same parts return to the Nodebais shop: a cracked UV lamp sleeve, a burst pump housing, a filter connector that has failed. In almost every case, the culprit is not the cold itself, but water trapped inside equipment that has been switched off. Protecting a pond for winter starts with deciding, equipment by equipment, what keeps running, what gets drained, and what is brought indoors to dry. The plants and fish come next, but forgotten equipment is what costs the most in spring.

Why frost damages equipment

When water freezes, its volume increases by around 9%. In a pipe, pump housing, or the quartz sleeve of a UV lamp, this expansion has nowhere to go: the plastic splits, the glass bursts, and a seal deforms. Conversely, equipment that runs continuously and remains submerged at a sufficient depth is almost unaffected. Moving water freezes less easily, and the bottom of a sufficiently deep pond generally remains frost-free in our regions.

This is due to a property described in every limnology textbook: freshwater reaches its maximum density at around 4 °C. In winter, this relatively “warm” water sinks and remains at the bottom, while the surface cools and then freezes. This bottom layer protects your fish, and it should not be stirred unnecessarily. The practical rule follows from all this: anything that stops must be drained, while anything that keeps running stays in the water and does not disturb the bottom.

Equipment by equipment: what should you do before frost?

In a well-stocked koi pond, we often recommend leaving the filtration running at reduced flow. Filter bacteria work very little at low temperatures, but the filter continues to retain particles and the water is less polluted when it returns in spring. In an ornamental pond with a few goldfish, completely stopping it is a reasonable option, provided an opening is kept in the ice. In both cases, read your model's instructions: some manufacturers require submersible pumps to be stored in a bucket of water protected from frost so that the seals do not dry out.

The UV lamp, winter's first victim

When the water drops below about ten degrees, suspended algae grow very little and the UV lamp has practically nothing left to do. Leaving it running serves no purpose; leaving it full of water while switched off is the worst option. Unplug it, open it or tilt it to drain it completely, dry the inside and store it in a frost-free place. Take the opportunity to clean the quartz sleeve, often clouded with limescale, and note the bulb's installation date: its radiation weakens with operating hours, and manufacturers' technical specifications generally call for regular replacement, often every season.

Keep an opening in the ice

A layer of ice is not dangerous in itself. The problem is that it blocks gas exchange between the water and the air, while the sediment and leaves on the bottom continue to decompose slowly. An opening, even the size of a plate, is enough to let some of the gases escape and maintain a little oxygenation. An ice-prevention float, a small air pump with a diffuser placed about twenty centimetres below the surface, or a thermostat-controlled heater can do the job without intervention.

Never break the ice with a hammer or shovel: the shock waves travel through the water and are poorly tolerated by fish lying dormant at the bottom. If the ice has formed and nothing was planned, place a pan of hot water on the surface and let it melt a hole gradually.

The plants: cut back without drowning the stems

Yellowed water lily and marginal plant leaves will end up at the bottom anyway if you leave them. Cut them back as soon as they wilt; that means less silt in spring. For plants with hollow stems, such as cattails, rushes, or reeds, cut above the water level, never below it: a stem cut underwater fills with water and the rhizome may rot. Leave a few dry clumps in place along the banks; they provide shelter for insects and mark where the plants are.

Hardy water lilies can be lowered in their basket to the deepest area. Tender species, such as tropical water lilies, papyrus, or certain marsh plants bought for the season, should be brought indoors in a water-filled container, in a garage or cellar that does not freeze. A practical tip: label each basket before moving it; by March, it is hard to tell anything apart.

The net: essential in October, a nuisance in January

Taut during leaf fall, a protective net saves you hours of netting and greatly limits the amount of organic matter entering the pond. Once the trees are bare, remove it or retension it well above the water. A slack net that touches the surface gets caught in the ice, accumulates snow, and eventually collapses into the pond. If herons visit regularly, keep it in place but stretch it high, and clear the snow after every snowfall.

Monitor without interfering

Once the pond has been prepared for winter, the best maintenance is to leave it alone. A floating thermometer is enough to guide decisions: switch to an easily digestible food when the water drops below around ten degrees, stop feeding when it remains steadily around 5 °C, and restart filtration gradually when it rises again. Check after every prolonged freeze: is the opening in the ice still there, is the net secure, is the air pump running? Five minutes a week, no more.

Read also

Once the equipment is safely stored: The right steps for your pond or water garden in winter.

READ THE ARTICLE

Frequently asked questions

— Should the pond pump be removed in winter?

Not necessarily. A pump that continues running can remain submerged, raised to mid-depth. A stopped pump should be removed, cleaned and stored away from frost according to the manufacturer's instructions.

— Can the UV lamp be left plugged in all winter?

It is unnecessary, as algae develop very little in cold water. Above all, a lamp that is switched off but still full of water may burst: empty it and store it in a dry place.

— How can you keep a hole in the ice without breaking it?

With a frost-protection float, an air pump with the diffuser placed near the surface, or a thermostatically controlled heater. As a temporary solution, place a pan of hot water on the ice.

— When should the leaf net be removed?

When nearby trees are bare. If it remains in place to protect against herons, it must be stretched well above the water and cleared of snow.

In summary

• Anything that stops is drained: UV lamp, waterfall pump, pressure filter, hoses.
• Anything that keeps running stays submerged, without stirring the bottom layer of water.
• Always make an opening in the ice, and never strike it to break it.
• Plants cut above the water, tender species brought indoors away from frost.
• Net removed or retensioned after the leaves fall.

Last updated: September 29, 2026.