Blueberries: Frost Damage | Berry Diagnostic Tool

Flowers

Frost during bloom may cause serious damage to developing flower buds and blooms. Once flower buds open, cold hardiness for the most part is lost. Temperatures of 28 F or lower may result in frost damage.

Cluster of blueberry blossoms. White blossoms have tan-orange tint, especially on most exposed blossoms. Flowers look wilted. — Mild frost damage on blueberry blossom cluster.

Fruit

Frost injury causes fruit to have a dark, shrunken appearance that readily distinguishes them from unaffected fruit. Severely damaged fruit appear hollow when cut in half. Frost or freeze injured fruit may drop to the ground prematurely before harvest. View pictures of mature frost damaged fruit.

Immature, green fruit is also susceptible to frost damage. Injury may cause water-soaked appearance on developing berries.

Two small, flat, green blueberry fruits with stem cap attached, viewed from the blossom end. Flower on right is opaque and green, fruit is same color as stem cap. Fruit on right is semi-translucent, dark, and appears water-soaked. The stem cap is waxy and pale, similar to the healthy fruit. — Blueberry fruit in early stages of development. Fruit on left is healthy. Fruit on right has frost damage.

Leaves and shoots

Frost injury can cause shoot and tip dieback. These tissues are more vulnerable to the gray mold botrytis.

Blueberry leaves with purple-brown dead tissue on outer edges of newest growth. — Leaf dieback in blueberry is most prominent on the youngest tissue.

More information:

Spring Freeze Injury (Michigan State)
Using Sprinklers to Protect Blueberries from Spring Freezes (Michigan State)
Using Your Sprinkler System to Protect Blueberries from Freezes (Michigan State)
Blueberry Freeze Damage and Protection Measures (North Carolina State)

Use these resources if you need additional help with diagnosis and to find solutions to your problem.