We all know that Spring can bring some very unpredictable weather, which for fruit growers means the possibility of freeze damage to their fruit trees.
As the trees begin growth in the spring the buds begin to swell and they lose the ability to withstand cold temperatures.
The killing temperature is often called the critical temperature and is defined as the temperature that buds can withstand for a half-hour.
At or near the bloom stage, freezing temperatures of 280 F for 30 minutes will result in approximately 10% bud or blossom loss. If temperatures fall to 240 F, one might expect losses up to 90 %. In that temperatures range (280-240F) we can expect to see 50% or more loss in tree fruit blossoms. Flowers and buds on higher sites or in the tops of trees will be less damaged than those at lower sites. The percent of flowers killed in a frost may or may not relate directly to lost yield later in the season.
With large fruited fruits (apples, peaches, plums and pears) the loss of 50% of the flower is not significant since we may only want 25% of the flowers to become fruit.
For small-fruited fruits such as cherries, blueberries and grapes, many flowers and buds are needed for good yields.
Therefore, crop losses due to freezing temperatures are usually significant in cherries.
How do you determine damage in stone fruit that are in the pre-bloom stage?
In stone fruit, apricots, cherries, peaches, and plums, the flower contains a single pistil (the female part of the flower that will become the fruit) which is exposed inside the flower. Early in the season, flower buds can be cut open to inspect the pistil.
If flower inside the bud is black the flower has been killed and the fruit will not form.
Normally flowers killed in the bud are easily distinguished as the bud continues to grow and the dead flowers stop growing.
This is why some people wonder why they have no cherries even though the trees bloomed. If the pistil is killed in the bud the flower will stop growing. If the style is killed and the pistil remains alive the flower will continue to grow.
However, since there is no way to pollinate the flower, because the style has been eliminated, fertilization of the ovule will not take place and no fruit will be produced. Apples and Pears
Apples and Pears are different than stone fruit. In apples, the fruit buds are really small shoots with flowers and leaves.
In apples the flower in the center of the flower cluster is the oldest and has the potential to be the largest fruit. This central flower is called the “King Bloom” and is the most desirable of the flowers in the cluster. The King Bloom is also the most advanced, and therefore, the most likely to be killed in a frost.
Several weeks after a killing frost it is not unusual to see the side blooms larger than the king bloom, this means that the king bloom was killed earlier in the spring.
Another difference between apples and stone fruit is that the pistil is buried in the floral cup at the base of the flower and not exposed as in stone fruit.
This means that it is often necessary to tear the flower apart to see if the center of the flower is brown or black. When checking apples for frost damage, be sure to check the king and side blooms separately.