Damage by frost and lower lethal temperature in field-grown cut dahlia

Abstract

Dahlia (Dahlia variabilis Desf.) is a widely cultivated ornamental crop valued for its rusticity and aesthetic appeal, but highly sensitive to thermal stress, particularly low temperatures. This study aimed to determine the lower lethal temperature (LLT) for five field-grown cut dahlia cultivars in order to support adaptive management strategies under extreme climatic events. Field experiments were conducted from January to July 2024 in Santa Maria, Jaguari, and Esteio, located in the subtropical climate zone of Rio Grande do Sul, Brazil. Daily minimum and maximum air temperatures were obtained from INMET (Santa Maria and Jaguari) and NASA POWER (Esteio). Damage symptoms, including leaf curling, wilting, necrosis, bud rot, and flower mummification, were visually assessed by independent observers and correlated with minimum temperatures and plant developmental stages. LLT was defined as the lowest temperature recorded on the day prior to the occurrence of irreversible damage in at least 50% of plants. In Santa Maria and Jaguari, minimum temperatures of 0.2 °C and 0.1 °C over two consecutive days caused severe vegetative and reproductive damage. In Esteio, a single-day minimum of 0.5 °C did not result in visible damage, highlighting the role of exposure duration and plant phenological stage in determining cold stress severity. The LLT for the tested cultivars was estimated at approximately 0 °C. Mitigation practices such as low polyethylene tunnels, floating row covers, thermal meshes, sprinkler irrigation, plastic soil mulching, selection of more cold-tolerant cultivars, and adjusted planting schedules are recommended. These findings provide valuable insights into dahlia responses to cold stress and support adaptive management strategies for ornamental crops in frost-prone regions.