Answer: Blueberry flower buds and flowers become more sensitive to cold as they develop. Swollen but closed flower buds tolerate 15 -20 F. At tight cluster or early pink bud (individual flowers are visible but still tight in bud), injury will occur between 18 and 23F. Once flowers have separated from one another but the corollas (petals) are still closed, 22?25 F may be lethal. By the time the corolla is half their full length, they are damaged at 25 to 26 F. Fully open flowers are killed at 27F. The most sensitive stage is just after the petal fall, when 29F may cause damage.
You can protect your plants by covering them with floating row cover (poly spun fabric) or by simply draping old sheets or lightweight blankets over each plant or by using the insulated frost blankets available at your garden center. These will give about a ten degree difference between the outside air and the air around your plants. Best wishes with your blueberries!
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