Jeffrey J. Jackson
University of Georgia
With the arrival of chilly fall nights, I've been getting calls from ornamental pond owners who are wondering what to do for their frogs and turtles during winter.
In the October issue of Pondscapes Magazine, Mike Mullen of Kansas City, Missouri, wrote about his problems in bringing frogs through winter. He noticed that the frogs in his ponds did not hibernate successfully. When spring arrived he found dead frogs in the pond.
Mike had placed a bubbler in the pond to add oxygen, but it didn't seem to help. He placed a container of sand on the bottom for the frogs to hibernate but the frogs didn't burrow into it. They just sat on the bottom and chilled into immobility.
Last year it appeared that Freddie, his favorite bullfrog, would suffer a similar fate. Mike's solution was to bring Freddie into an indoor pond for the winter. Bringing the frog inside is one solution, but it means extra work because, among other things, frogs need live food.
Is there an easier plan for getting your frogs through the winter? There is.
For some insights into why frogs in backyard pools die in winter, let's examine what wild frogs do when winter closes in. Bullfrogs, greenfrogs, leopard frogs and some other frogs squirm down deep through the layers of dead leaves and detritus. There they sit. When the water gets too cold for them to move they are helpless. When spring comes and their world warms up, they swim up to the top, get a breath of air, and phase back into active life.
The winter hibernating season is hazardous to a frog's health. Some die. But then, every day is hazardous if you're a frog. To live a year is to be exceptionally lucky. The problem for most frogs in ornamental ponds is not a lack of oxygen or a potful of mud. A ``clean'' pond bottom is the problem. A clean pond is inhospitable to a frog, and especially inhospitable in winter. That's because the water temperature can't stratify, that is, the bottom layers aren't much warmer than the top the way they are in a ``wild'' pond.
Here's an experiment I did one winter to gather data on this froginwinter problem. I had two identical plastic ponds, about a foot deep, in the front yard. I cleaned and emptied one and filled it with fresh water. Into the other I dumped a bushel of dead leaves. Then one cold winter day I checked the temperature of the ponds with a metal probe type thermometer. I broke a little hole in the ice of each pond. The ice was about an inch thick on the clean pond, and just a skim, about an eighth of an inch in the leafy pond. The clean pond was just slightly above 32°F from top to bottom.
The temperature at the top of the leafy pond was the same, just above 32°F but as I pushed the probe deeper I recorded increasing temperatures. At the bottom, the temperature was a bit over 40°F.
The temperature of the earth keeps frost at bay, both on land and in water. The layering of the leaves slows down water mixing and keeps the temperature warm enough for hibernation. A deeper pond is better for hibernating. In the Piedmont and coastal plain, a foot deep might be enough. In the mountains and colder areas further north two or three feet is better. If you're a neat type and want clean, transparent water right to the bottom, this does not bode well for your frogs. But if you let leaves and detritus accumulate, you will provide a better winter habitat for your frogs. Improved habitat doesn't mean all your frogs will survive the winter, but it definitely improves the odds.
To help turtles hibernate provide the same dead leaf habitat. What will this supply of dead leaves do for the fish? Nothing good. The decomposition of the leaves under the ice will use oxygen and that is bad for fish.
No one system is good for everything. So manage your fish ponds and frog ponds differently.