Perry's Perennial Pages -- 2005 Research Update

Controlled Freezing Studies with Perennials--Abstract

Andrea Luchini, Graduate Student, University of Vermont
(prepared as part of her M.S. thesis, 2005)

Perennial growers overwinter stock and unsold plants in containers and need to know the factors affecting the cold hardiness of these plants. Therefore, three studies were conducted on container-grown herbaceous perennial plants to examine effects of acclimation date, age, and temperature cycling on cold hardiness. In January of 2003 and 2004, plants were exposed to -2, -5, -8, -11, or -14°C in controlled chest freezers. After a two day acclimation at -2°C, temperatures were lowered at a rate of 1.5°C/h and remained at each treatment temperature for 30 minutes. Following freezing, plants were returned to the greenhouse and data was taken in mid-May to mid-June each year when plants began bloom

In the first study, plants were allowed to acclimate in an outdoor setting for different lengths of time in the fall of 2002 and 2003 prior to controlled freezing in Jan. 2003 and 2004. Plants were moved from outside to inside a glass greenhouse maintained at 4± 4°C on 18 Oct., 1 Nov., or 15 Nov. 2002. In 2003, plants remained inside, or were moved in on 15 Oct., or 15 Nov. In 2003, Geranium xcantabrigiense ‘Cambridge’ plants moved inside on 18 Oct. received the highest rating compared to other dates for this cultivar. In 2004, ‘Cambridge’ plants which had remained in the greenhouse had the highest rating and largest dry weights; plants from 15 Oct. had the smallest dry weights. Geranium xcantabrigiense ‘Karmina’ (2004) showed no significance in acclimation date, temperature, or interaction. Most plants of ‘Dilys’ (2003) did not survive below -8°C at any treatment date. Dianthus ‘Vampire’ showed differences in dry weights due to treatment dates and in ratings due to treatment temperatures. Coreopsis ‘Tequila Sunrise’ showed differences due to treatment temperatures for both ratings and dry weights.

Another study examined the effect of age on freeze hardiness. Plant age affected ratings for Dianthus, with established plants rated lower than plugs and divisions. There was interaction between treatment temperature and plant age in dry weight data. Plugs were of equal size from all temperatures; divisions were largest from the -2°C and -5°C treatment temperatures and established plants from the -5°C treatment. Plant age of Geranium significantly affected the plant’s ability to withstand sub-freezing temperatures. Well-established Geranium xcantabrigiense plants best survived freezing temperatures.

A third study examined the cycling of temperatures, five cultivars of perennial plants were subjected to temperature cycles of -3/+3°C for 24 hours at each temperature, followed by controlled freezing. Cycling treatments were 2 cycles, 1 cycle, or no cycles. In 2003, Geranium xcantabrigiense ‘Karmina’ and Iris sibirica ‘Pirate Prince’ showed no practical significance in cycling or temperature treatments. Geranium ‘Dilys’ plants were affected by both treatment temperature and cycles in both Jan. and Mar. 2003. In 2004, ‘Dilys’ was only affected by temperature treatment. Geranium xcantabrigiense ‘Cambridge’ plants were not affected by either factor. Coreopsis ‘Tequila Sunrise’ was not affected by cycling treatment; plants were not hardy below -11°C.

It seems, with the plants studied, acclimation date has little practical effect on cold hardiness. Plants that have been in pots for 2 growing seasons may be slightly less hardy than newer plants. Temperature cycles affect cold hardiness of some cultivars but not others.

Copyright 2005, not for publication without permission from author.

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