Frost damage to Hemerocallis esculenta in a mire: relationship between flower bud height and air temperature profile during calm, clear nights

As air temperature near the ground changes with height, the present study investigates the effect of those changes and their role on the relationship between frost damage and flower bud height of Hemerocallis esculenta Koidzumi during scape elongation in the Sarobetsu Mire, Hokkaido, Japan. Meteorological observations near the ground and flower bud height of H. esculenta were measured from April to August in 2001, 2002, and 2003. In the present study, the intensity and duration of low temperature were combined as one variable instead of being treated as separate variables, as is usually done in laboratory experiments. In addition, the raised minimum temperature phenomenon was observed for the first time in a mire. The frost damage event occurred during predawn on 15 June 2001, 5 June 2002, and 5 June 2003. The weather during those nights was calm and clear, and the wind was not strong enough to prevent frost damage. The lower height limit of frost damage, observed in 2003, was found to be around 0.10–0.15 m, as buds below this height were protected by their own leaves. The upper height limit of frost damage was 0.34 and 0.43 m for 2001 and 2002, respectively. Frost damage of H. esculenta depended on the air temperature profile, which was affected mainly by vegetation height during scape elongation during calm, clear nights in the Sarobetsu Mire. Other factors that contributed to frost damage were also intensity–duration of low temperatures, which was treated as one variable in the present study, and the raised minimum temperature phenomenon.Key words: air temperature profile, bud height, lethal temperature, raised minimum temperature phenomenon, scape elongation.