As the days become shorter and cooler, it’s time to prepare your pond for winter.
Water holds heat well, and water temperatures may remain above 60 degrees Fahrenheit well into the fall season, giving the fish plenty of opportunity to forage, feed and put on weight. It’s important that pond fish eat well because they need to build up the stores of energy that will keep them going through the winter.
But as the fall season wears on, water temperature will steadily decline, and below 55 degrees F pond fish will be less active and will require much less food, and below 40 degrees F they shouldn’t be fed at all.
So long as water temperature remains above 70 degrees F, pond fish can be fed the same high-protein pellets they’re given during the summer. But between 60 to 70 degrees their diet needs to be changed to a mix of high-protein pellets and wheatgerm pellets, the precise ratio favoring the wheatgerm pellets the colder it is. Wheatgerm pellets are easier to digest than high-protein pellets, and this is important because pond fish will be digesting their food more slowly as the water cools down. Below 60 degrees F, they should be given just wheatgerm pellets because they won’t be able to digest high-protein pellets at all.
It’s a good idea to feed pond fish two or three times a day during the first half of fall, and just once or twice in the second half of the season. Take care to promptly remove any uneaten food. Below 55 degrees F pond fish hardly feed at all, and even wheatgerm pellets should be used sparingly. By the end of the fall season, algae and detritus in the pond will be providing all the nutrition pond fish require, so supplemental feeding will be unnecessary.
Filters and Pumps
Filters will normally be left running throughout the autumn, unless the water temperature drops below 40 degrees F. Because the fish are still active and feeding, it is important that the filter carries on cleaning the water by removing ammonia and solid waste.
If the water gets to 40 degrees F or lower, a filter can do more harm than good. Below this temperature very cold water floats to the surface, eventually forming ice. But the bottom layer of water stays relatively warm, around 39 degrees F, even if the top of the pond freezes solid. That bottom layer is where pond fish will hibernate through the coldest part of the year. Filters prevent the stratification of pond water into a colder upper layer and warmer bottom layer, so unless the pond is artificially heated or expected to remain ice-free throughout the year, the filter should be switched off once water temperature gets to 40 degrees F.
Water features such as fountains, cascades and waterfalls increase evaporation, and in doing so they reduce water temperature. Since the water will be cooling down during the autumn anyway, further lowering of the water temperature is undesirable, so water features should be switched off during this season.
A last water change should be performed in early fall while the water temperature is above 60 degrees F, but if the water is much colder than that the pond should be left alone. As mentioned above, disturbing the thermal stratification of the pond can cause problems for overwintering fish.
Plants, Trees and Algae
Vegetation in and around the pond will be dying back during the fall season. Dead leaves left in the pond will rot and in doing so consume oxygen. Because ice prevents additional oxygen getting into the pond, rotting vegetation in overwintering ponds can deplete the limited supply of oxygen within the water. Autumn is a good time to prevent this by removing as much dead vegetation as possible. This will likely include not just pond plants and marginals but also things like algae and leaves from overhanging trees.
Some plants are annuals and will need to be dug out and thrown onto the compost heap. Take care to remove their roots as well their leaves and stems. Perennial plants and oxygenating plants will need trimming too, in some cases aggressively. Lilies can have all their leaves removed by the end of the autumn, and it’s a good idea to move lily baskets to the deepest part of the pond where the water will be warmest.
Keep a few floating plants and compost the rest. Some species are not frost-tolerant and are best overwintered indoors or in an unheated greenhouse, including water hyacinth and water chestnut. Tropical and subtropical perennials will also need to be overwintered indoors. Tropical lily rhizomes are best kept in damp moss in a shed or basement until spring. Some subtropical plants tolerate frosts if mulched well towards the end of autumn, as is the case with the popular lily-of-the-Nile Zantedeschia aethiopica ‘Crowborough.’ The summer foliage will die, but the roots stay healthy and burst into life next year.
Bringing Fish Indoors
Goldfish varieties with two tails, such as black moors, are too delicate to be left outside over winter. They should be brought indoors in autumn and maintained in an aquarium. Take care not to expose these fish to sudden changes in water temperature or water chemistry. Ideally, fill the aquarium with water from the pond, and jump-start the filter by adding some filter media taken from the pond filter.