Winter care for pond plants and garden fish

Why autumn is the real turning point

Most pond problems that appear in April can be traced back to something we did, or forgot, in November. As water temperatures slide from around 15°C down towards 5°C, the whole pond changes gear. Fish metabolism slows, plants stop growing, and the nitrifying bacteria in your filter quieten down to a whisper. Your job now is not to keep everything running at summer pace, but to steer the pond gently into dormancy without shocks.

Start watching a floating thermometer rather than the calendar. It is the single most useful piece of kit for the next five months, and it tells you far more about what your fish and plants need than the date does. A pond in Cornwall and a pond in Aberdeenshire behave very differently in the same week of December.

Keep the water moving: pumps, filters and pipework

Cold water holds more oxygen than warm water, so a still pond is not automatically a struggling pond. But a completely stagnant one, sealed under ice, is a different matter. Aim for gentle circulation rather than the summer torrent.

• Do not switch the filter off entirely. The bacterial colony inside it is your pond's kidney, and it will keep working slowly through winter. Restarting a neglected filter in spring means weeks of poor water quality.
• Raise the pump onto a couple of bricks so it draws from mid-depth rather than the very bottom. That water is slightly warmer, and the pump is less likely to clog with sinking leaves.
• Lag exposed pipework with foam pipe insulation. Water left in a pipe that freezes can split it, and joints and non-return valves are the usual casualties.
• Consider swapping to an air pump and airstone at shallow depth in the coldest weeks. It keeps a hole open in the ice and oxygenates the water without stirring the whole column and chilling it further.
• If you must shut a pump down, drain it, clean it and store it somewhere frost-free. A pump left full of water in a shed can crack.

Adjusting fish feeding as the water cools

This is where more fish are lost than anywhere else, usually through kindness. Fish cannot digest rich food in cold water, and food that sits in the gut can rot.

• Below about 10°C, switch to a wheatgerm-based, lower-protein food. It is easier to digest and produces less waste.
• Below roughly 8°C, stop feeding altogether. Goldfish, koi, tench and orfe will live happily on their reserves until spring.
• Offer food in the middle of the day, at the warmest hour, in small amounts. Remove anything uneaten after ten minutes.
• Let the fish tell you. If they ignore the food and drift away, they have answered your question. Never feed during a cold snap or when ice is forming.
• Leave them undisturbed in the deepest part of the pond, ideally 60cm or more, where the water stays most stable.

When ice does form, never smash it. The shock waves can injure fish and damage the pond liner, and the sound carries alarmingly well through water. Float a pond heater, or stand a pan of hot water on the surface to melt a hole. Keep that hole open, because trapped gases under ice are what actually kill fish, not the cold itself.

Protecting plant crowns and marginals

Hardy water lilies look after themselves: their crowns sink to the bottom and sit out the winter in the mud. If a lily is still in a basket in shallow water, simply move the basket to the deepest part of the pond for the winter.

Marginal plants are more vulnerable, particularly those in shallow shelves where the crown can be caught by repeated freeze-thaw. Cut yellowing foliage back to about 5cm above the crown so it does not rot down into the growing point, and take care not to leave hollow stems that funnel water straight into the heart of the plant.

• Mulch around crowns with bark chippings or gravel where they sit above the waterline, but keep the crown itself clear of soggy material.
• Lift tender and half-hardy plants such as arum lilies, cannas and some ornamental grasses, and overwinter them frost-free.
• Leave oxygenators alone. Most sink to the bottom naturally. Thin out any excess, since decaying weed uses up oxygen.
• Compost floating plants like water hyacinth and water lettuce before they collapse into mush.
• Leave seedheads on reeds, rushes and waterside grasses. They give structure, shelter insects and feed the birds. Cut back only what has gone slimy.

Leaves, debris and water quality

Falling leaves are the quiet enemy of the winter pond. A thick layer on the bottom rots slowly, consuming oxygen and releasing nutrients that will feed algae next spring. Stretch a net over the pond before leaf fall if you can, and scoop regularly with a long-handled net. Empty any skimmer basket far more often than you think necessary.

Stop feeding your plants now. Fertiliser encourages soft growth that frost will kill, and unused nutrients simply feed blanketweed. Skip water changes unless something has gone badly wrong; the pond is better left settled and undisturbed. Once the water is consistently cold, most of the work is done — you are simply keeping watch.

Your winter jobs at a glance

• Fit a floating thermometer and check it weekly.
• Raise the pump, lag the pipes, keep the filter running.
• Net the pond and clear leaves before they sink.
• Switch to wheatgerm food, then stop when the water drops below 8°C.
• Move tender plants and lily baskets to deeper water or frost-free shelter.
• Keep a hole open in the ice — and never break it with force.

Do these few things well and your pond will come through the winter quietly, ready to wake up with far less work in spring.