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Home HortScience Hydrangea Cold-hardiness Tested Under Simulated Climate Warming Conditions
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AARSLEV, DENMARK—Hydrangea (Hydrangea macrophylla) is an extremely popular and commercially successful flowering shrub. Native to Japan, the beautiful ornamental thrives in maritime regions but can also grow and flower well in most temperate regions. Even in milder climates, however, frost injury or winter kill of flower buds and shoots is a common problem for hydrangea.

"The consequences of bud freezing injuries in terms of quality and ornamental value are of horticultural importance for Hydrangea macrophylla," explained Majken Pagter and Michelle Williams, coauthors of a study of hydrangea cold-hardiness. "Flower buds of most hydrangea varieties are formed during the fall and overwinter on dormant stems. Flowering will therefore only occur the next year if terminal and/or lateral flower buds are present and undamaged," they said.

Pagter and Williams, researchers from Denmark's Aarhus University, published their work in an issue of the American Society for Horticultural Science's journal HortScience. "As a result of climate warming, winters are becoming progressively milder, and temperature patterns are becoming increasingly irregular with an increased frequency of warm spells," they wrote. "Warm spells may induce premature dehardening for hydrangea, thus increasing the risk of subsequent freezing injuries."

Pagter and Williams' objective were three-fold: to determine if hydrangea buds are less cold-hardy than stems, to investigate dehardening resistance and loss of cold-hardiness of stems and buds of non-dormant plants under different temperature conditions, and to determine if stems and buds of non-dormant H. macrophylla have an ability to reharden after a period of dehardening.

Plants were acclimated in the field and dehardened in the greenhouse at controlled warm temperatures for various durations. Dehardened plants were rehardened for up to 12 days in an unheated greenhouse (in January) or in the field (in March).

Results showed that buds of Hydrangea macrophylla were slightly less cold-hardy than stems. In both stems and buds, the dehardening resistance and the rate of dehardening were influenced by temperature, but buds appeared to be less resistant to dehardening and dehardened faster than stems. "In stems, dehardening proceeded faster in March than in January, and the capacity of the stems to reharden seemed reduced, indicating that both dehardening and rehardening were influenced by the progression of winter," the study said.

The research results have implications for breeding and introduction of new hydrangeas. Increases in temperature extremes during winters mean that midwinter-hardiness, dehardening resistance, and rehardening capacity are increasingly important considerations in the development and production of hardy, healthy hydrangea varieties.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

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Original Article:

Frost Dehardening and Rehardening of Hydrangea macrophylla Stems and Buds
Majken Pagter and Michelle Williams
HortScience 46:1121–1126. [Abstract][Full Text][PDF]

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