Controlled Freezing of Twenty-three Container-grown Herbaceous Perennials

Abstract

Abstract Twenty-three containerized herbaceous perennials were acclimated to ambient outdoor temperature and photoperiod until November 19, when they were moved into a greenhouse and held at 3C (37F) until February 15. At that time, plants were exposed for 30 minutes to a range of subzero temperatures, and returned to the greenhouse for forcing. Plants were rated for survival and salability following 6 weeks regrowth at 15C (59F), using a subjective scale of 1 to 5 (1 = worst, 5 = best, 3 and above considered salable). Of the 23 perennials studied, 15 survived after exposure to at least-8C (18F), however, 40% of the species and cultivars surviving at this temperature sustained a level of injury which rendered them unsalable. Most plants were rated salable at treatment temperatures 3 to 6 degrees warmer than their respective lowest survival temperatures (LST's). Phlox ‘Chattahoochie’, Phlox ‘Morris Berd’ and Tiarella ‘Laird of Skye’ remained salable after exposure to —11C (12F). Campanula takesimana, Heuchera americana ‘Dales Strain’, Penstemon fruticosus ‘Purple Haze’, Rosmarinus officinalis ‘Arp’, Tiarella ‘Running Tapestry’, and Tiarella cordifolia ‘Slick Rock’ remained salable after exposure to-8C (18F). Erodium reichardii ‘Roseum’, Hebe macrocarpa ‘Margaret’, and Verbena ‘Homestead Purple’ remained salable after exposure to-5C (23F). Only the unfrozen controls of Houttuynia cordata ‘Chameleon’ and Thelypteris kunthii remained salable. Treatment means for Ceratostigma plumbaginoides were not significantly different and all 6 Hibiscus cultivars (‘Blue River II’, ‘Disco Belle Pink’, ‘Disco Belle Rose Red’, ‘Disco Belle White’, ‘Lord Baltimore’ and ‘Southern Belle’) failed to survive any of the temperature treatments. In general, plants showed a gradual decline in salability with decreasing temperatures.