In the experiments referred to below, two-year-old plants of Hydrangea macrophylla subsp. macrophylla 'Alma' were grown in Denmark in 3.5-litre pots of sphagnum peat.
The pots were buried in the soil in October and by January the plants had become cold-hardened. They were then "dehardened" in greenhouses at day/night temperatures of 20/15 degsC, 15/10 degsC, or 10/5 degsC. The residual cold-hardiness of their stems was estimated by chilling pieces of stem down to -16 degsC, then thawing them and monitoring any leakage of ions. Flower buds were also chilled at temperatures as low as -16 degsC and their cold-hardiness was estimated by examining them for damage after they had thawed.
Buds apparently de-hardened quite quickly when warmed, but many of them subsequently died or were injured if returned to a temperature of -10 degsC or less. Stems also dehardened most quickly at 20/15 degsC,but they could usually be "rehardened" if returned to a freezing temperature.
Evidently, the flower buds of this hydrangea would be more susceptible than the stems to frost injury if a warm period was followed by another cold period in winter.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Frost dehardening and rehardening of Hydrangea macrophylla stems and buds by Pagter and Williams (2011). HortScience 46 (8): 1121-1126. Members of the International Society of Horticultural Science can view HortScience from the website www.ishs.org