A phase change material (PCM) energy storage unit operating in a greenhouse from 29 Oct. through 21 Dec. 1992 cooled it on the average 1.7C in the day and warmed it 2.2C at night due to both sensible and latent heat absorbed, released, and circulated. Tagetes patula `Mighty Marietta' and `Early Queen Sophia' marigolds and Viola × Wittrockiana `Yellow Blotch' and `Blue Blotch' pansies were grown in a PCM and a control (no PCM) greenhouse. Temperatures went below 0C 10 days in the control greenhouse and 4 days in the PCM greenhouse. The lowest temperature of -7.8C killed the marigolds in the control greenhouse. Neither marigolds nor pansies were killed in the PCM greenhouse, which attained a low temperature of -3.3C. On 4 Dec., plants were destructively harvested. Morphologically the marigolds were taller, and had more leaf area and dry matter when grown in the PCM greenhouse as compared to the control, but pansies were taller, and had more leaf area and dry matter when grown in the control greenhouse, as compared to the PCM greenhouse.