Moderate Root-Zone Cooling Improves Vegetative Growth and Reduces Open-Center Malformation in Standard Chrysanthemum ‘Baekgang’ During Summer

High summer temperatures can reduce growth and flower quality in standard chrysanthemum, while whole-greenhouse cooling requires substantial energy input. This study evaluated whether localized root-zone cooling could improve growth and suppress open-center malformed capitula in standard chrysanthemum ‘Baekgang’ while considering energy use. Rooted cuttings were grown in a commercial hydroponic greenhouse during summer and subjected to a non-cooled control (NC) or root-zone cooling activation thresholds of 28 °C (HT), 25 °C (MT), or 22 °C (LT). Root-zone temperature, vegetative growth, gas exchange, flower quality, open-center incidence, and electricity consumption were measured. Mean root-zone temperatures were 27.8 °C in NC, 26.6 °C in HT, 24.4 °C in MT, and 23.3 °C in LT. Root-zone cooling improved vegetative growth, particularly stem diameter and shoot and root biomass, whereas leaf-level gas-exchange parameters were not consistently affected. Open-center incidence was highest in NC (67%) and was reduced to 33%, 11%, and 33% in HT, MT, and LT, respectively. Electricity consumption was 321, 783, and 1088 kWh bed−1 in HT, MT, and LT, respectively. These results indicate that moderate root-zone cooling, particularly MT, provides a practical balance between flower quality and energy use for summer chrysanthemum production.