HORTICULTURE HORTICULTURE
Assessing Frost Freeze Damage to Blueberries.
In the winter, blueberries can take pretty cold temperatures. The northern highbush blueberries grown here in Michigan can take a lot of cold when they are dormant. If the temperatures have been cold they can withstand -20 to -30F with little damage. If the temperatures have been above freezing for several days they may be damaged if by temperatures around 0F. As the blueberry plant begins to grow the spring the buds develop and lose their ability to withstand deep cold. As the buds develop warmer and warmer temperatures (still below freezing) can damage them. The killing temperature is often called the critical temperature and is defined as the temperature that buds can withstand for a half-hour. I have posted a table of critical fruit 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 the flowers, those that are most developed ones.
This file contains pictures of frost 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.
As the
buds begin to swell they can tolerate temperatures below 20F. In 2007, a
week of very cold temperatures just before Easter resulted in
lows of 18F just as the plants were starting to grow. At this time there was a wide
range in growth between different varieties and different areas. Many fruit buds had
started to swell and withstood the low of 18 F. In the early varieties, flower buds
had burst. The flower buds that had begun to open were damaged. When the
flower buds are small and closed you need to cut across the bud to exposed the interior of
the bud. By making slices across the bud you expose the different flowers in the bud
as you cut deeper and deeper into the bud. Normally there is a range of temperature
damage with less damage at warmer temperature than at colder tempertures. In severe
frosts almost all the flower buds will be killed. By slicing across many buds and
determining the number of total dead and live flowers you can determine the percent loss
from a freeze.
The petals on some of these flowers were killed by freezing tmeperatures. Even though the pistils of these flowers are OK the petals will not open and bees will not be able to pollentate the flowers.
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 may be less damaged than those at lower sites. The percent of flowers killed in a frost can relate directly to lost yield later in the season. For small-fruited fruits such as cherries, blueberries and grapes many small fruit are needed for good yields. Due to the small size 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.
