Temperate deciduous fruit trees have poor and delayed bud break when they are grown in warm areas. The delay is due to a lack of the chilling which is required to break bud endodormancy. Bud endodormancy can be overcome in some species by treatments such as H₂CN₂, heat, and bud scale removal. We tested the effects of chilling, H₂CN₂, heat, and removing scales on bud break of floral and vegetative buds of 'Tifblue' rabbiteye blueberry (Vaccinium ashei Reade).

Hydrogen cyanamide was effective in promoting floral bud break of 'Tifblue' only on whole plants, at chilling exposures between 300 to 500 hours. However, vegetative bud break was increased by H₂CN₂ at a wider range of ~hilling exposures than floral buds in both whole plants and cut shoots. Optimum vegetative bud break was induced by H₂CN₂ at 125 and 250 mM for whole plants and cut shoots, respectively. Hydrogen cyanamide was highly phytotoxic to floral buds compared to vegetative buds. However, floral buds of whole plants became tolerant to H₂CN₂ as chilling increased. Injury to vegetative buds was significant only at 500 mM H₂CN₂.

The chilling requirement for 'Tifblue' floral buds of whole plants was 500 hours. In contrast, vegetative buds did not have a significant relationship with chilling exposure in either cut shoots or whole plants.

Heat treatment was effective in promoting floral bud break of cut shoots only at 190 chilling hours at 30 minutes heat exposure. Heat (47°C) for I hr was effective in promoting vegetative bud break, but the effectiveness varied with chilling level and depended on time of heat exposure. Bud scale removal did not promote floral bud break, but increased vegetative bud break, although not significant compared to control.

Finally, we discovered that vegetative buds remained dormant even after they had received more than adequate chilling. However, both H₂CN₂ treatment and floral bud removal resulted in increased vegetative bud break, although the effect of H₂CN₂ was less than floral bud removal. This suggests that vegetative buds were inhibited by floral buds and that H₂CN₂ could partially overcome this paradormant effect.