With autumn slipping into winter the pond enters a state of suspended animation as the water cools and the metabolism of fish and plant life slows right down. At this time of year, a few last jobs remain to be done to prepare the pond for the colder months ahead and to ensure that its inhabitants will over-winter successfully and have the best possible chance of a good start when things warm up again the following spring.
Now is a good time for a final clean up, to remove any leaves which may have found their way under the net cover, or any dead plant foliage which was missed during autumn pruning. Leaving excess organic matter in the pond over the winter can harm the water quality, which is why some pond-keepers recommend using this opportunity to do a partial water change and remove some of the sludge from the base of the pond. This material is made up of a mixture of decaying plant matter, fish waste and other material which has sunk to the bottom and its gradual decomposition will add significant quantities of nitrates to the water, over time.
By the onset of winter, any frost-sensitive pond and bog plants should already have been protected or removed as appropriate and hardy water lilies lowered into the deeper reaches of the pond, the non-hardy varieties being covered and stored somewhere cool, but frost-free. It is also the time to transplant lilies, if desired – cutting off the leaves and stalks to leave the buds and shortening the rhizomes by around a third.
Having been winding down the feeding of fish and moving to a low protein diet as autumn progressed, once the water temperature reaches 10–12 degrees C, a good quality wheatgerm-based food – available in stick or pellet form – should be used until winter finally grips. At around 7 degrees C, fish naturally stop eating and drift into a state of semi-hibernation. At this time, the fish tend to retreat to the deeper portions of the pond, where during winter, the water is warmer – and the deeper the pond, the more noticeable this temperature effect. To avoid disturbing the warm layer that they are languishing in, it is a good idea to take steps to reduce the pond re-circulation which will tend to mix in colder surface water. Some pond-keepers choose to switch off their pumps, often removing them for routine maintenance, while others favour decreasing their flow rate and relocating them away from the deeper reaches.
In the same way as the fish slow down for winter, at around 10 degrees C and below, the biological activity in the filter is also much reduced – so switching off the pump is not quite so drastic a step as it might seem. However, before doing so, it is as well to check the manufacturer’s instructions, since some kinds of pumps must be removed and re-greased if they are not to be run for any length of time. With no real need of filtration throughout the winter, this is an ideal chance to disconnect and drain the bio-filter, strip it down and clean it thoroughly, storing it – along with the pond’s UV clarifier – in a safe, dry place until it is needed again.
In bad winters, ponds in some areas of the country may be prone to be iced over for prolonged periods. This is not ideal for any fish they contain, since an enveloping cover of ice stops the natural exchange of gases at the surface, trapping carbon dioxide and others in, while keeping oxygen out. There are various solutions, from small electric heaters to floating something on the surface, the idea being that its gentle movement breaks up the ice as it forms, stopping a complete layer forming. Opinions vary as to whether a bobbing football really works – but the plastic ducks some pond-keepers use certainly make the pond look cheery!
A final point to consider is adding a net over the pond if it does not already have one – at this time of year to keep out cats or herons rather than leaves. In their semi-torpor and made more conspicuous by the lack of vegetation in the pond, fish make easy targets for passing predators, so it can sometimes be a good idea to give them a bit of added protection.
Winter in the pond is something of a dead time, when plants, fish and other creatures have done their growing and breeding for one year and now must simply survive the cold to start all over again in the next. While this enforced dormancy may offer little spectacle to captivate the water-gardener, it is never-the-less an essential part of the cycle and with a little bit of care and attention as the days draw in, we can be sure that the pond and its inhabitants are up to the challenge.