The Effects of Frost on Apple Trees
Frost forms on the surface of plants when temperatures drop below 32 degrees Fahrenheit. The effect of frost on apple trees (Malus domestica) depends on how low the temperature gets and the length of exposure, as well as the stage of flower development. Early-season or late-season frosts can cause varying degrees of apple blossom frost damage.
Apple Tree Frost Tolerance
Fruit trees go dormant in the winter, so they are most vulnerable to frosts in late fall/early winter – before dormancy, or in early spring – when the trees begin new growth again. According to the Penn State Extension, fruit trees need this dormancy period, but to protect your trees from damage, you should time your late and early season pruning in order to make your trees less vulnerable to cold damage.
When frosty weather is in the forecast, try to limit damage by covering trees with floating row covers or old bed sheets. Spraying them with water to create an insular layer of ice is also an option, but there is no guarantee that this will prevent apple tree frost damage. Apples from damaged blossoms taste the same, but may have thick, brown bands around the fruit called frost rings and then may soften more quickly.
Silver and Green Tip Stages
The silver stage is the first budding stage after dormancy, when the apple tree buds appear silver. According to the Utah State University Extension's fruit freeze damage chart, exposure to frost at 15 degrees Fahrenheit for 30 minutes causes 10 percent bud kill, and exposure to 2 degrees Fahrenheit for the same amount of time causes 90 percent bud kill. Once the emerging buds mature from silver to green, the apple tree advances to the green tip stage. Exposure to frost at 18 degrees Fahrenheit for 30 minutes causes 10 percent bud kill, while exposure to 10 degrees Fahrenheit for the same amount of time causes 90 percent bud kill.
Half-Inch Green, Tight Cluster Stages
When the buds transform into leaves, the apple tree enters the half-inch green and tight cluster stages. The half-inch green stage begins when the leaves start projecting out from the bud. Then, when the face of the leaves open to the sun and reveal a tight cluster of green flower buds, the tree moves into the tight cluster stage. During the half-inch green stage, it takes 30 minutes of exposure to 23 degrees Fahrenheit for 10 percent bud kill, and 90 percent bud kill when temperatures dip to 15 degrees Fahrenheit for the same amount of time. During the tight cluster stage, it takes 30 minutes of exposure to 27 degrees Fahrenheit for 10 percent bud kill, and 90 percent bud kill when temperatures dip to 21 degrees Fahrenheit for the same amount of time.
First Pink Stage
When the flower buds, still huddled together in a small bunch, break open and reveal a tight cluster of dark pink petals, the apple tree moves on to the stage known as first pink. As the Michigan State University Extension highlights, fruit tree buds become more vulnerable to cold temperatures as they swell. In the first pink stage, 10 percent bud kill occurs after 30 minutes exposure to 28 degrees Fahrenheit, and 90 percent bud kill after exposure to 25 degrees Fahrenheit for the same amount of time.
Full Pink Through Post-Bloom Stages
The amount of apple blossom frost damage to apple trees in the full pink, first bloom, full bloom and post-bloom stages is the same. Recognize the full pink stage, also known as open cluster, when the buds separate from each other and the petals turn a lighter shade of pink as they open ever so slightly. The first open flower blossom of the bud cluster marks the beginning of the first bloom, also known as the king bloom stage. As multiple flowers open in one cluster the tree reaches full bloom and once the flowers begin to mature, it’s the post-bloom period.
During these four stages, 10 percent bud kill occurs after 30 minutes of exposure to 28 degrees Fahrenheit, and the tree suffers 90 percent bud kill after exposure to 25 degrees Fahrenheit for the same amount of time. In terms of harvesting, if apples on your tree do freeze, Washington State University recommends you do not handle the apples when they are frozen. Instead, wait for them to thaw. Then, examine them for frost damage and avoid long-term storage of any apples that have suffered irreversible frost damage.