We are often asked about what to do with pond plants and fish during the winter.
In our part of the country (Piedmont North Carolina) we probably could leave the ponds alone as far as the plants are concerned.
The koi are another issue, and water parameters are very important all year long. Sometimes we overlook the koi in the winter and we should not.
During the winter the koi’s immune system begins to slow down. The cold water temperature does not allow the fish to fight off diseases, and this is especially important in the early spring. Springtime when the water temperature is from 50 to 77 degrees is referred to as Aeromonus Alley. Once the water temperature gets up into the 80’s problems usually go away. Below 40 degrees Fehrenheight the bacteria are inactive.
To protect the koi we like to salt the pond in the late fall. This gets rid of most of the pathogens that bother the fish. Some of the chelated invertebrates like Argulus (fish lice) and Lernia (anchorworm) can be eradicated with Dimilin. We want the overall water quality to be superb.
If you have plants in your koi pond, the salt will probably destroy most of them so you need to take another approach. A potassium permanganate treatment would be an option. See the potassium permanganate page for a detailed analysis and how-to by Roddy Conrad.
If you are concerned about plants freezing, then you can place them in the bottom of the pond and raise them in the spring as the water starts to warm. In our area where there is not too much freezing temperature, many plants such as lotus can survive in their container outside the pond. Some years ago, I talked to a man who raised lotus just a little north of Charlotte, and he had some lotus that were sitting on soil which overwintered well. He also set some similar lotus on a cement driveway and they all died. It appears that the warmth from the soil was just enough to keep the containers from freezing.
The lesson here is that things change seasonally, so be alert to environmental issues that might be different in your area.
What to do with the foliage on the plants
Hardy waterlilies will lose their leaves as well many of the other aquatic plants. They will come back on their own in the spring. If you have fish they might become very interested in the new foliage on your waterlilies and keep them nibbled down so the plants will never get a decent start. The solution is to protect the plants with a basket and netting, or even move them to another pond with no fish to become established. If your fish are six inches or so that will not be a problem, however, if the koi are closer to twelve inches in length, they will enjoy your plants.
Lotus foliage will die also, but you do not want to cut the stems below the water line. The lotus stems are hollow, and I am told that water will go down through the stem and kill the tuber. So it is OK to cut off the foliage, but leave it above the waterline.