HORTICULTURE
Assessing Frost Damage to
Fruit Buds of Stone Fruit Trees.As fruit trees
begin growth in the spring the buds begin to swell and they lose the ability to withstand
cold temperatures. As the buds, develop warmer and warmer temperatures (still below
freezing) can damage them. The killing temperature is often called the critical
temperature, the temperature that buds can withstand for a half-hour. I have posted a table of critical bud temperatures. In general there is a range of
temperatures over which damage occurs with more buds damaged at lower temperatures until
all the fruit buds are killed. Often the freeze will only damage some of the flowers such
as the most developed ones or flowers in the bottom of the tree.
This file contains pictures of freeze damaged flowers for growers and home fruit growers who wish to determine frost damage after a freeze. People want to know if a frost has damaged fruit immediately after the freeze. It is best to wait several hours (until the afternoon) to let frozen tissues thaw. Dead and damaged tissues will turn black or brown.
At or near the bloom stage where freezing temperatures of 28 F will result in 10% loss and 24 F in 90 % loss. In that temperature range we can expect to see 50% or more loss in tree fruit plantings. Fruit 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 stone fruits (peaches and plums), the loss of 50% of the flower is not significant. We may only want 25% of the flowers to become fruit. For small-fruited fruits such as cherries, blueberries and grapes many small fruit are needed for good yields. Due to the small size of the fruit, the number of fruit needs to be large to have a full crop. Crop losses due to freezing temperatures are almost always significant in cherries.
Swollen Bud
Early in the
season, flower buds are often cut open cross the bud to inspect the pistil. If this is
black the flower has been killed and the fruit will not form. Cherry buds contain several
individual flowers in each bud. The most of the flowers in the cherry buds on the left
look OK. The flower bud in the lower right has 3 flowers in it and the individual flower
on the lower left has been killed by the cold.
The
tart cherry flower buds on the right are from a frost a week later. This frost
occurred in the morning about 4 hours before this picture was taken. I have cut two
of the buds to reveal the flowers inside. The center flower bud has 4 flowers and the
flower on the lower left was damaged or killed by the frost. The bud to the left has 3
flowers and 2 are dead. Normally flowers killed in the bud are easily distinguished as the
bud continues to grow and the dead flowers stop growing.
Prebloom
A cut through this cluster of
sweet cherry buds show that they are all alive.
As the flower develops more you can tear it open to see
if the pistil has been killed.
The
entire pistil of the sweet cherry on the left was killed. By the next day damage was easy
to see.
The flower on the
right shows partial damage to the flower. The style of this pistil was killed by a frost
earlier in the season. There is no place for the pollen to land and fertilize this
flower so this will never be a cherry. This kind of damage is very common in
cherries. People wonder why they have no cherries because 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 but since there is no way to pollinate the
flower and fertilize the ovule there will be no fruit and the flower will fall after
bloom.
Both the pistil and the style are green in this sweet cherry.
As the flower opens you can look into the flower and see the pistil to see if it is discolored
Since peaches have large fruit and only a small number of flowers are needed
for a full crop. The loss of a large number of flowers does not mean the crop is severely
impacted. The peach blossom on the left is alive while the one on the right was killed. An
important factor in peach yields is the number of flowers on the tree i.e. is bloom heavy
or light.
Post Bloom
After
flowering, small stone fruit 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 smaller than the cup
and not touching the walls. But when the fruit fills the shuck, the shuck provides no
protection at all.
The tart cherry flower cluster above shows little damage to the pistils inside the shuck.
In some cases the style was burned by frost but this will not hurt if the ovule,
which becomes the seed, has already been fertilized.
Green Fruit
After shuck split, the fruit are exposed and very vulnerable to frost. Often
temperatures just below freezing can cause extensive damage.
The sweet cherry fruit above were frozen and have a "watersoaked" appearance
after they thaw.
The one green fruit on the left was not killed, but it is likely that it will have frost
marks, a russetting on the skin of the fruit where the surface cells were damaged.
