AIR-TEMPERATURE, HUMIDITY AND ROOTING VOLUME AFFECTING JURY TO RHODODENDRON AND OTHER PERENNIALS

Intensive husbandry of woody plants grown in containers may increase t he opportunities for cold induced damage especially where rooting volu mes are restricted. Research reported here considered the ranges of ai r temperature, rates of temperature change and the impact of humidity on cold induced damage to leaves and the inter-relation with rooting v olume using species of Acer, Ceanothus, Euphorbia, Magnolia and Rhodod endron. Damage was expressed as visual assessments and as increasing i on leakage from the tissues. Exposing Rhododendron leaves to 2 degrees C for 24 h prior to freezing at -6 degrees C for 5 h was less damagin g than the same treatment after exposure to 25 or 16 degrees C. Damage resulting from treatments with -8 and -10 degrees C was similar for l eaves previously exposed to 25, 16 or 2 degrees C. The extent of injur y was also influenced by the environment during freezing. During short periods of freezing temperatures below -4 degrees C, tissues submerge d in water, and then frozen, generally showed less injury than those s urrounded by air. At lower subzero temperatures and longer periods of exposure, however, the greatest injury formed in tissues placed in wat er. When exposed to -6 degrees C, submerged leaves were damaged after 5 h whereas leaves placed in air withstood freezing for up to 7 h. Sim ilar results were obtained with studies of the moisture contents of le aves and the ambient humidity during freezing. At -4 degrees C leaves saturated with moisture suffered significantly less damage than those placed in dry atmospheres. Extending the drying period from 1 h to 24 h prior to exposure to freezing, however, reduced tissue injury especi ally at the lowest temperatures. Results obtained by freezing plants g rown in containers of several sizes for Acer, Ceanothus, Euphorbia and Magnolia were pooled. This experiment confirmed that survival increas ed significantly (P<0.05) in larger containers as air temperature was reduced over time, with the possible exception of Acer.