Blueberry floral bud, flower, and fruit sensitivity to freeze has been extensively researched from which university extension systems have published guides on floral growth development. Based on this research, cold sensitivities have been suggested based on the phenological stage. There are nine stages of flower bud development from tight bud to petal fall, at which point the small green fruit is sensitive to 0 °C (32 °F). Bud swell (stage 2) indicates the plant has deacclimated, and at bud break (stage 3), the flowers are sensitive at −7 °C (19 °F). Tight cluster (stage 4) is sensitive to −7 to −5 °C (20 to 23 °F). As the flower develops toward bloom, three stages are noted—early pink (stage 5), pink (stage 6), and late pink (stage 7)—which can tolerate −4.4 to −2.8 °C (23 to 27 °F). Floral full bloom is when the corollas are fully expanded (stage 8), which can tolerate −2.2 (28 °F). Once the petals fall (stage 9), freeze damage will occur at 0 °C (32 °F). Floral bloom for SHB is not strictly regulated by chill hours and even with sufficient chill, SHB will bloom unevenly. This is observed by the variation of bloom progression within a bush, causing growers to estimate the cold sensitivity of the crop. The appropriate response is dependent on the growers’ desire to salvage the crop.

Tight bud (stage 1), bud swell (stage 2), and bud break (stage 3)
Tight bud and bud swell can tolerate cold temperatures of −12 to −9 °C (10–15 °F). Bud break is more sensitive to cold, where at −7 °C (20 °F), damage can occur.


Tight cluster (stage 4) and early pink bud (stage 5), which are cold tolerant to −5 to −4 °C (23–25 °F) and −4.4 to −2.8 °C (24–27 °F), respectively.


Pink buds (stage 6) and late pink bud (stage 7) stages,  which are cold tolerant to −5 to −4 °C (23–25 °F) and −4.4 to −2.8 °C (24–27 °F), respectively.


Full bloom (stage 8) and petal fall (stage 9), which are cold tolerant to −2.2 °C (28 °F) and 0 °C (32 °F), respectively.

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