Cold Hardiness of Three Woody Plants

To examine injuries caused by freezing temperatures, three woody plants were placed under temperatures ranging from 0 to –20C. Control plants were placed at 0 or –2 C, depending on the field sampling period. Freezing tests were done three times during the fall: Sept., Oct. and Nov., 1993. Spiraea × bumalda `Flamingmount', Spiraea callosa `Alba', and Spiraea × bumalda `Crispa' were tested. After freezing tests were complete, all plants were stored at –2C for the remainder of winter. In May, plants were repotted into containers. Effects of freezing temperatures on plant growth were recorded at the end of the summer. Results indicated that the most sensitive species to cold temperatures is Spiraea × bumalda `Crispa'. Moreover, the response of plants to the September freezing test was too variable to give a valid statistical analysis. Regression analysis was used as a tool to determine the temperature at which there is a 25% reduction in growth of the stem and the root dry matter, respectively. Results obtained in October are as follows: Spiraea × bumalda `Crispa', –6 and –7.6C; Spiraea × bumalda `Flamingmount', –10 and –8.7C; and Spiraea callosa `Alba' –10.7 and –11.5C. Results obtained in November are as follows: Spiraea × bumalda `Crispa', –7.1 and –8C; Spiraea × bumalda `Flamingmount', –12.2 and –12.3C; and Spiraea callosa `Alba', –8.5 and –8.7C. The reduction in cold hardiness observed for Spiraea callosa `Alba' is caused by warmer conditions (20C) in which plants were placed 2 days before the freezing test.