HORTICULTURE
Freezes, Critical Temperatures and Fruit Bud Damage
Just
before Easter in 2007, Michigan suffered a severe spring freeze event. The freeze
that caused the damage was an Advective Freeze or wind freeze, not a radiation
freeze. Most of the freezes that cause problems in the spring in Michigan are
radiation freezes. These freezes occur after the passage of a cold front preceding a
mass of cool dry air. Usually there is a stormy period as the cold front moves
through followed by clearing and light winds. In a radiation freeze, the temperature
falls during calm, clear nights and then rises again after dawn when the sun comes up and
warms the earth. The damage tables we use for estimating damage and when we can
protect against freeze damage are for radiation frosts. These tables predict damage
on exposure to low temperatures for a short time, not the exposure to cold, wet, windy
conditions for many hours. These charts were developed by controlled freezing in a
freezer where the buds are collected and the freezer programmed to slowly lower the
temperature to a set point and then hold it at that temperature for 30 minutes and drop to
the next lower temperature. Buds are removed at each temperature and checked for
damage. These conditions mimic a classic radiation freeze with calm clear conditions
and temperatures that rise after dawn. These numbers are posted at several sites on the
Internet. I have an extensive website that has a lot of spring freeze
information at http://web1.msue.msu.edu/vanburen/frost.htm
One of the files there is a one-page sheet handout where I have collected published critical temperatures from other extension bulletins so that they are on a single sheet. Many of these numbers come from Washington State University Bulletins. Greg Lang has a WebPage with this information also and by clicking on the Bud Stage Development for each fruit you can see a picture of the original WSU Bulletin photos of bud development http://www.hrt.msu.edu/faculty/Langg/Fruit_Bud_Hardiness.html
The original Washington State Bulletins can be found at
http://treefruit.yakima.wsu.edu/weatherbuds/weatherinfomain-3.htm
These critical bud stage tables are for temperatures of relatively short duration no three days of snow, wind as cold arctic air moves through the region. The freeze Michigan and the entire Eastern United States Suffered was a classic Advective Freeze where cold air moves into a region. When you have a lot of cold air moving, everything quickly becomes the same temperature as the air and the cold can penetrate deeply into the tissues. I had hoped for conditions similar to the cold snow storm we had in 2005 when temps remained near freezing not to have temps in lower 20's for 6, 8 and 12 hours on successive days. It seems that the conditions were more similar to the wind freeze that affected Northwest Michigan’s cherry crop in 2002. Then cold winds killed swollen cherry buds at temperatures that were above the temperatures in the tables we use to estimate damage from recorded lows. There are other variables that can cause differences between the observed effect of cold and the predicted critical temperatures. Stan Howell has done work showing that wet buds are killed at warmer temperatures than the predicted temperatures in the WSU bulletins determined in a freezer.
The bottom line is that the published values are for specific conditions and if the conditions are very different the tables can be wrong. As I walked orchards and vineyards, I saw plenty of damage to exposed leaves that were brown, black or dark green and watersoaked and were obviously damaged from the prolonged cold. Many times when I cut open these obviously damaged buds the flowers inside looked fine and I wonder how that could be since the one next to it was dead. Warmer temperatures will make it easier to determine damage as this tissue dries out when we get above freezing for a couple of days this week.
Finally, The Easter Freeze of 2007 caused extensive damage to leaf buds in Blueberries
especially the variey Bluecrop. This damage resulted in reduced shoot growth because
the growing point for the new shoots was killed so the number of fruit was larger than the
number of leaves could support.
