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Flower buds of eight ecotypes representing three native North American azalea species being grown in Burlington, Vermont were compared for cold hardiness by laboratory freezing during the cold acclimation period for three years. Species were Rhododendron calendulaceum, R. prinophyllum, and R. viscosum. There was a high variation in the number of florets killed within an inflorescens in response to freezing temperatures. There was little difference in the cold hardiness of florets of R. Pinophyllum and R. calendulaceum florets, but R. viscosum florets were hardier. Some differences were noted in cold hardiness of florets of ecotypes, but these were not necessarily related to latitude of origin. Cold hardiness showed a relationship with the daily mean temperature of the three days preceding freezing tests.
Florets of eight provenances representing three native North American azalea species [Rhododendron calendulaceum (Michx.) Torr., R. prinophyllum (Small) Millais, and R. viscosum (L.) Torr.] being grown in Burlington, Vt., were compared during three seasons for cold hardiness by laboratory freezing during cold acclimation. There was a large variability in the number of florets killed within an inflorescence in response to freezing temperatures. Cold hardiness of florets of the three species ranked, from most to least hardy, were R. viscosum, R. prinophyllum, and R. calendulaceum. Some differences were noted in cold hardiness of florets of provenances, but these were not necessarily related to latitude or elevation of origin. Cold hardiness of most provenances showed a significant linear relationship with the daily mean temperature of the 3 days preceding freezing tests. Ambient temperatures just before subfreezing test temperatures may affect winter injury more than provenance differences for these species.
Abstract
Rosa hybrida L. ‘Peace’ and ‘Garden Party’ plants covered with mulch in October and November for 2 weeks were less cold-hardy, based on laboratory freezing studies, than unmulched plants. Subsequently, field injury of mulched stems was observed in December and January. Plants covered in mid-December were less cold-hardy than unprotected plants but exhibited less field injury in January than October- and November-mulched plants. Plants covered with Styrofoam cones were not injured as early as perlite-covered plants. There was no significant difference in winter injury when stems were covered with perlite, soil, or leaves. Unprotected plants had less winter stem injury than covered plants during the winter of 1978–79, when the plants were covered with snow. The following winter there was little snow cover, and November- and December-covered plants had significantly less winter stem injury in January than plants with no cover.