Nine Hydrangea macrophylla (Thunb.) Ser. and one H. serrata (Thunb. ex J.A. Murr.) Ser. cultivars were evaluated for midwinter cold hardiness, acclimation, and deacclimation to identify cultivars with increased cold tolerance. Hydrangea macrophylla ‘Endless Summer’, ‘Mariesii Variegata’, and ‘Veitchii’ acclimated later than all other cultivars. ‘Générale Vicomtesse de Vibraye’ acclimated first, and was cold hardy to −6C (21F) by September 28, 2000. The greatest cold hardiness in all cultivars occurred on January 5, 2001. Maximum cold tolerance in all cultivars was within a 6C (11F) range with ‘Endless Summer’ being the least cold hardy [−18C (0F)], while ‘Dooley’, ‘Générale Vicomtesse de Vibraye’, ‘Mme. Emile Mouillère’, and H. serrata ‘Bluebird’ possessed the greatest cold hardiness [−24C (−11F)] on January 5. Deacclimation in all cultivars began after the January 5 collection date as indicated by the February 1, 2001, data. On March 1,2001, ‘Ayesha’ and ‘Mariesii Variegata’ survived only 4C (39F) while all other cultivars survived at least −6C (21F).

remontant flowering, deacclimation, acclimation, Hydrangea paniculata, Hydrangea arborescens
Hydrangea macrophylla ‘All Summer Beauty’, ‘Ayesha’, ‘Dooley’, ‘Endless Summer’, ‘Générale Vicomtesse de Vibraye’, ‘Mme. Emile Mouillère’, ‘Mariesii Variegata’, ‘Nikko Blue’, ‘Veitchii’, H. serrata ‘Bluebird’
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Author notes

This paper is a portion of a thesis submitted in partial fulfillment of a Master of Science degree.

2Former Graduate Research Assistant. Current Address: Dept. of Horticultural Science, Box 7609, N.C. State University, Raleigh, NC 27695.

3Professors.

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2002