Field Evaluation of Heat Tolerance in Rose

Roses generally experience a decrease in flower productivity and size in response to summer temperatures greater than 30 °C, which is common throughout the southern states of the U.S. In 2011, the August mean and mean maximum temperatures in College Station, TX, were about 40 °C and 32 °C, respectively. Under these extreme heat stress conditions, large rose germplasm collections in two field trials in Texas were evaluated for flowering intensity which takes into account both flower number and flower size. Of the 600 accessions evaluated, 20 (3%) showed excellent flower intensity and 86 (14%) showed good to very good flower intensity during August. Among those that showed excellent flower intensity during the heat stress were ‘Baby Jane Clare’, ‘Lady Ann’, ‘Ring of Fire’, ‘Little Emma’ and various breeding selections. Cultivars such as ‘Homerun’, ‘Knockout’ , ‘Robi’, ‘Alburquerque Enchantment’, ‘Julie Link’, ‘Orange Parfait’, ‘Woodstock’, ‘Saint Mary’, and ‘Green Ice’  showed good to very good flower intensity during the summer heat. This preliminary evaluation indicates that developing rose cultivars that flower well in the summer months is possible.