Uniformity of growth response of impatiens (Impatiems wallerana Hook. f.) plug seedlings was examined in four identical growth rooms. Differences among growth rooms for dry weight, height, and leaf area of 10- to 24-day-old seedlings were generally not significant. During six experiments over 6 months, an individual growth room was maintained under contant baseline environmental conditions. Differences in growth response over time appear to be related to nutrition and irradiance levels. For three experiments with nearly identical irradiance, temperature, and nutrition levels, dry weight and height growth differences over time were only rarely significant. These results illustrate that rather unsophisticated growth rooms can provide consistent growth response over time among experimental units.