Perry's Perennial Pages -- 2002 Research Update

Controlled Freezing Studies with Perennials--Abstract

Cheryl Bruce, Graduate Student, University of Vermont
(prepared as part of her M.S. thesis)

Four studies were conducted to compare the effects of various freezing methods on the hardiness of containerized herbaceous perennials. Four to eight weeks after controlled freezing plant hardiness was evaluated based on a visual rating scale, number of flowering stems, shoot height and width and dry weight. In the first study length of the acclimation period at –2C and duration of freezing at subzero temperatures were examined. Dianthus deltoides ‘Vampire’ and Geranium x cantabrigiense ‘Cambridge’ were acclimated for 1, 2 or 7 days at –2C and then frozen for 30 minutes, 2 hours or 24 hours at one of four treatment temperatures (-5, -8, -11, -14C). A control group remained at 3C. Significantly lower regrowth was observed on those plants subjected to 7 days of acclimation at –2C. Regrowth and survival was also significantly lower for those plants held at subzero treatment temperatures for 24 hours.

A second study compared the hardiness of established, one year old plants that had been used in previous freezing studies with new plants that had been transplanted from plugs ten weeks prior to freezing. Established and new plants of Tiarella cordifolia ‘Brandywine’, T. wherryi ‘Martha Oliver’ and Geranium x cantabrigiense ‘St Ola’ were acclimated for 2 days at –2C and then frozen for 2 hours at one of four treatment temperatures (-5, -8, -11, -14C). A control group remained at 3C. Both species of Tiarella displayed significantly more regrowth as new plants than as established plants. However, Geranium plants that had been established were more successful that those that were newly potted.

Thawing rates after periods of controlled freezing were examined in two separate studies. Rapid thawing is the transfer of plants from a subzero treatment temperature (-5, -8, -11, -14C) directly to 3C conditions. Slow thawing is the slow increase of the freezer temperature to the acclimation temperature of –2C at the controlled rate of 2C/h before transfer of plants to 3C conditions. In the first study, Geranium ‘Cambridge’ was acclimated for 2 days at –2C and then frozen for 24 hours at one of four treatment temperatures (-5, -8, -11, -14C). After freezing, plants were either transferred to 3C conditions or raised to –2C. Plants were held at –2C for an additional 24 hours before transfer to 3C conditions. Plants exposed to rapid thawing displayed higher rates of regrowth and survival than those subjected to slow thawing due to the prolonged freezing duration of slow thawing.

In the second thawing rate study, the process was repeated with Geranium ‘St Ola’ and ‘Karmina’ and Tiarella ‘Brandywine’ and ‘Martha Oliver’ except that in the slow thawing treatment plants were held at –2C for 2 hours rather than 24 hours. Tiarella exposed to rapid thawing experienced higher rates of regrowth and survival than those exposed to slow thawing, while the opposite was true for Geranium ‘Karmina’. The difference may be explained by the hardiness of the genus, where prolonged exposure to subzero temperatures is not as damaging as for a genus such as Tiarella. G. ‘St Ola’ had minimal survival in both treatment groups.

The influence of temperature cycling prior to controlled freezing was studied on Dianthus ‘Vampire’ and Geranium ‘Cambridge’. Temperatures cycled between either –3 and +3C or –8 and +8C and plants were held at each temperature for 24 or 72 hours as temperatures were lowered, raised and lowered again before freezing at treatment temperatures (-5, -8, -11, -14C). A total of six cycling processes were studied (-3/+3C 24h, -3/+3C 72h, -3C 24h, -8/+8C 24h, -8/+8C 72h, -8C 24h). The range of temperature fluctuation is the determining factor on plant hardiness. Plants that cycled between –3/+3C for 24 and 72h displayed significantly higher regrowth and survival than all other treatment groups. Plants that cycled between –8/+8C for either 24 or 72h exhibited significantly lower regrowth than all other groups.

Copyright 2002, not for publication without permission from author.

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