How to Tell if Your Fruit Blossoms Froze

 

You can see the yellow style that extends up from the pistol in the center of these plum flowers.

Every time the temperatures dip down to freezing, you worry about loosing some or all of your fruit crop. Apricots, plums, and peaches cause the most concern because they have such early bloom times.  In New Harmony, plums can be in full bloom, apricots dropping petals, and peaches just starting to bloom at the beginning of April.  With an Average Last Frost Date in mid May, each cold front threatens the crop. So what are the critical temperatures?  Let's look at some facts.

Dormant fruit trees can tolerate very cold winter temperatures. As we move into the warmer weather in March, April and May trees and small fruits lose their winter hardiness as the tree breaks dormancy. As buds begin to swell, the ability to tolerate cold temperature changes with each growth stage. 

This range of damage from cold varies with the growth stage of the buds, fruit species, age of the tree, and even the cultivar. 

On this photo you can see the blackened pistil of an apricot flower.

 Early in development, at first green and green tip stages, temperatures need to drop into the teens or lower to cause significant bud damage. 

Temperatures in the upper 20s can cause considerable harm to an early blooming species or variety and leave other fruit crops unaffected or with only slight damage. 

The stage of bud development determines how susceptible any given fruit crop is when freezes occur.

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 F in a 90 percent loss. In a freeze with clear, calm conditions, fruit on higher sites or in the tops of trees will be less damaged than those at lower sites. 

Heathly peach blossoms

Stone Fruits:  Apricots, Peaches, & Cherries

After a freeze, people often want to know if their fruit took a hit. 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.

The buds of stone fruit trees are either flowers or leaf buds, and not a mixture of both. In cherries and plums where there is more than one flower, all the flowers in a bud are about the same age.

Stone fruits have a very simple flower structure and are the easiest to detect damage. The flower contains a single pistil (the female part of the flower that will become the fruit)  If the pistil is brown or black after a freeze, that flower will not develop into a fruit because the pistil has been killed. Early in the season, when the buds have swollen but not opened, the flower buds can be cut open across the bud to inspect the pistil. If this is black, the flower has been killed and the fruit will not form. A healthy green pistil means the bud is alive. 

Peach flowers are harder to assess because of the fuzz on the pistil. Having a heavy bloom is helpful because only a small number of flowers are needed to get a crop.  If you have a heavy bloom and lose flowers, the crop may not be severely affected.

Crop losses due to freezing temperatures are almost always significant in cherries.

After flowering, small stone fruits are in the shuck. The shuck is formed by the floral cup. The shuck provides a little protection from the cold when the fruit is small and not touching the sides of the cup. When the fruit fills the shuck, the shuck provides no protection at all.

Apple blossoms notice the large king bud in the center.

Apples and pears

Apples and pears are very different than stone fruit.  In apples, the fruit buds are really small shoots with both flowers and leaves.  In apples, the flower in the center of the flower cluster 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.

Since the king bloom is also the most advanced flower in the cluster, it is most likely to be killed in a frost. Another difference between apples and stone fruit is that the pistil is buried inside the base of the flower and not exposed above it 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. 

So basically if a fruit tree is blooming that is when it is most susceptible to damage.  But always take a closer look at the flowers and actually examine them for damage. This information comes from various Extension Offices.