Studies were conducted with Physocarpus, Weigela, Hibiscus, Euonymus, Forsythia, Spiraea, Lonicera, and Taxus to evaluate the effects of warming temperatures on shoot dehardening. Container-grown plants were stored pot-in-pot, allowing shoots to receive natural outdoor conditions until early March. Control plants remained at 0C (32F), while treatment plants were placed in a temperature-controlled chamber at 21C (70F) and given up to 8 days of warming. Controlled-temperature freezing was used to evaluate plant hardiness. Hardiness levels of Weigela, Spiraea, and Forsythia rapidly decreased after 1 day of warming and again after the 7th day. Hibiscus gradually decreased in hardiness until the 7th day. The influence of polyhouse storage, in which plants were stored pot-in-pot, on the dehardening of Weigela, Hibiscus, and Euonymus was compared to outdoor storage, where plants were stored pot-in-pot. The warming effects of the polyhouse decreased the cold hardiness of the species studied. Results of the warming effects will be presented.
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