Treatment wetlands are solar powered ecosystems, resulting in annually cyclic temperatures. This paper reports data and models for temperatures and energy flows for subsurface flow wetlands. The water temperature seasonal cycle follows the air temperature during unfrozen conditions, with small hysteresis. Winter under-ice water temperatures are approximately 2 degrees C. The energy balance is dominated by radiation to and from the wetland, and evaporative losses. Sensible heat flows, conduction and convection are of smaller magnitude. Lateral energy losses were measured to be small. Vertical gains and losses were also small, but of importance in winter conditions. A simple model for ice formation shows that ice formation may be held to an acceptable minimum by addition of mulch or by early snow accumulation.