As trees begin growth in the spring, buds begin to swell and lose the ability to withstand cold temperatures. There is a range of temperatures over which damage occurs with more and more buds and flowers damaged at lower and lower temperatures until all the fruit buds are killed. Often the freeze will only damage some of the flowers, typically beginning with the most developed ones or those lower on the tree.
After a freeze, people often want to know how bad the damage was. It takes several hours for the symptoms to develop. As frozen tissues thaw, they will turn brown or black if they were damaged or killed by the cold, revealing the extent of the damage.
At or near the bloom stage, the critical temperature is the same for almost all fruits and flowers. Freezing temperatures of 28 degrees Fahrenheit will result in about a 10 percent loss and 24 degrees will result in a 90 percent loss. The percent of flowers killed in a frost may or may not relate directly to lost yield later in the season. With large fruits such as apples, peaches, plums and pears, the loss of 50 percent of the flower is not devastating since we may only want a small percentage of the flowers to become fruit. For small fruits such as cherries and grapes, many small fruits are needed for good yields and a full crop. Crop losses due to freezing temperatures are almost always significant in these.
Stone fruit (apricots, cherries, peaches and plums)
The stone fruit flower contains a single pistil (the female part of the flower that will become the fruit). This can be exposed by tearing the cup formed by the petals of the flower. If the pistil is brown or black after a freeze, that flower will not develop into a fruit because the pistil has been killed. A healthy green pistil means the blossom was not damaged and still has fruiting potential. This evaluation is best done no earlier than noon on the day following the freeze. Symptoms are more evident a day or two later.
Pome fruit (Apples and pears)
Apples and pears are very different than stone fruit. In these, the fruit buds are actually small shoots with both flowers and leaves. In the flower cluster, the center flower is the oldest and most developed and will be the first flower to bloom. This central flower is called the king bloom and is the most desirable of the flowers in the cluster. The king bloom has the potential to be the largest fruit. Depending on the developmental stage of the tree when frost occurs, the king bloom may be killed but another of the radial blossoms may not be. While fruits produced from a radial blossom are typically smaller, only one of the blossoms in the cluster needs to set fruit. Evaluation of individual blossoms is the same as described for stone fruit, but king blossoms should be inspected separate from radial blossoms.
Images showing developmental resistance to frost damage can be seen at: https://extension.usu.edu/productionhort/files-ou/CriticalTemperaturesFruitTrees.pdf
A video showing damaged and healthy pistils within the flower can be seen at: https://www.youtube.com/watch?v=cYzohw3Y558