This paper evaluates the performance and winter hydrology of two small‐scale rain gardens in a cold climate coastal area in Trondheim, Norway. One rain garden received runoff from a small residential watershed over a 20 month study period while the second rain garden with a shorter study period of 7 months was used as a control. The objective of the study was to investigate the extent to which cold climatic conditions would influence the hydrology and performance of the rain gardens. The hydraulic detention, storm lag time and peak flow reduction were measured and compared seasonally. No significant difference between seasonal lag time could be found, but there was a clear decreasing trend in lag time between rain, rain‐on‐snow and snowmelt. The average peak flow reduction for 44 storms in the study period was 42% compared to 27% for the winter seasons, indicating that the performance of the rain garden is reduced in the cold season (below 0 °C). The average hydraulic detention time for the rain garden was 0·84 ( ± 0·73) with runoff inflow and 1·91 ( ± 3·1) with only precipitation. A strong positive correlation was found between the time since the last wetting event and lag time, and between air temperature and hydraulic detention. This indicates that the time between events and seasonal air temperatures are key parameters in the hydraulic performance of cold climate rain gardens. The rain gardens were not used for snow storage areas, and a volume requirement for this was not evaluated in the study. Copyright © 2007 John Wiley & Sons, Ltd.