Q. I’m planning on building a pond this summer, but I need to keep the project costs down because we have a new baby and other more pressing expenses. I’m going to dig the hole myself. How deep does the pond have to be for the koi I plan on keeping to make it through the winter? I’m concerned with having to get rid of the dirt removed from the hole, and I know that the deeper the pond, the more dirt, and also the larger the liner I will have to buy.
A. Keeping koi outdoors all winter in the northern United States is problematic. The fish are sensitive to the cold temperatures, the decline in dissolved oxygen in the water, and the buildup of toxic gases (sulfur dioxide among other things) that occurs as winter progresses.
A few mild winters can produce a false sense of security, then the fish are wiped out by a more severe winter. Then, too, small koi seem to survive winters better than larger koi. This is due to the fact that the former can survive in lower oxygen concentrations. So, new pondkeepers (who tend to start with smaller fish) seem to have good overwintering success to start, only to end in disaster as their fish reach a larger size.
In New Hampshire I would recommend at least 6 feet of depth across most of the pond. A wide pond floor is a must so there is plenty of room down there. Water at that depth will stay a few degrees warmer than water a few feet up. And that difference in temperature can make all the difference in whether the fish survive the winter.
I also recommend several thermostatically controlled deicers to keep at least minimum gas exchange going at the surface. The previous writer’s concern (see above letter) about fish suffocating is quite real in this context.
You can improve the gas exchange in the pond after the ice cover reaches its normal thickness (which might be 18 inches or more in your area). Once the ice thickness stabilizes, draw out about a half-inch of water from the pond. This creates an air boundary between the ice and the water, allowing the full water surface to exchange gas. The deicers will keep a wider area ice free for the rest of the winter.
Regarding the “extra” dirt, there are basically two choices. First, find a way to use it on site. Create some topography. Make hills. Use it to fill low spots in your garden. Build a waterfall. Or, second, have it trucked away. There are many construction sites looking for clean fill dirt.