As autumn fades into winter, it is important to keep the water surface clear of fallen leaves and other debris. If possible, cover the surface with a fine mesh net to catch any leaves, leaving it in place until nearby trees are bare. If leaves are left on the surface of the water, they will gradually sink to the bottom and decay, causing a build-up of toxic methane gas which can damage wild life and will eventually silt up your pond.
Stop feeding fish once the water temperature drops below 5 degrees centigrade as they hibernate over the winter, and the food will just sink to the bottom and add to the decay. Cut down and tidy up any perennial planting that has collapsed, removing any perennial weeds still remaining. If your soil is heavy, it would also be a good time to turn it over with a fork to allow the frosts to work on it during the colder weather. You could also prune back excessive growth of submerged plants such as Elodea and Myriophyllum. Gunnera leaves should be folded down over the crown of the plant and tied or pegged down to offer some protection from the cold, using extra straw kept in place with frost protection fleece if necessary. In colder areas move tender water lilies into a bucket of water and store in a frost-free place over the winter, ensuring they don’t dry out. Other tender aquatics should be lifted. Remove any dead or dying foliage and put them into a container with moist soil or sand to prevent them from rotting when light levels are low and store under glass in a frost-free place until spring. Use a thick mulch of clean straw to help protect the roots of bog garden and moisture loving plants.
In really severe weather consider covering a third of the surface area with planks, making sure they are not in contact with the water. Cover these with polythene sheeting or weed suppressant membrane, followed by a layer of straw, bracken or fir tree branches. Add a second layer of plastic above to help trap some warmer air. If the ice becomes more than a couple of inches thick, cut a hole in it to allow oxygen to get to the water. Never hit the ice with a hammer as it causes a shock wave in the water which can easily kill fish.
If you have a pond which is more than 75cm deep, your pump should be fine providing you run it constantly so that the water doesn’t have a chance to freeze. This also benefits any wildlife or fish by helping to keep a small area free of ice as well as oxygenating the water. Otherwise remove the pump and clean it thoroughly, replacing any damaged or worn parts, and store in a dry place until the spring.
Ice can cause damage by trapping methane gas below the surface which will seriously harm your fish, and also exert a huge amount of pressure on rigid or concrete ponds as it expands, which can cause them to crack. Floating a ball or a log on the surface can help to keep an area clear of ice. Alternatively you can buy a floating electric pool heater which will keep a small area of open water.