Protecting your blueberry bushes from freezing is especially important during late frosts in the spring, after the buds or flowers have emerged, because if these die from freezing, the blueberries won't grow from them. Plant your blueberry bushes beneath larger tree canopies, which will radiate heat down toward the smaller plants beneath it and offer a few degrees of temperature variation. The outdoor temperature itself is one factor, but you should also keep in mind that the wind chill factor can also harm your blueberry plants, causing an otherwise above-freezing temperature to dip drastically into the freezing range.
Run water sprinklers around your blueberry plants during the night when the temperatures are expected to drop to around 20 to 30 degrees. Do this when a late spring frost is forecast, when the flowers have bloomed on the bushes. Watering the plants will keep them on the cusp of 32 degrees and protect the flowers from the colder surrounding air.
Keep the soil bare and moist around the blueberry bushes. Moist, bare soil will draw the heat from the soil subsurface and radiate the heat upward to increase the temperature up to 4 degrees, 5 feet above the surface.
Place buckets filled with hot water in your blueberry patch at intervals around the bushes. This will radiate heat and increase the temperature in the immediate area. If frost is expected during the night, wait until sunset to place the water buckets around your blueberry bushes.
Cover your blueberry bushes with the polypropylene fabric to protect them from frost and potentially damaging cold winds, which can drastically decrease the ambient temperature. This fabric is available at most garden retailers in different sizes and some are equipped with a drawstring to allow you to secure the cover to the base of the bush.