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December 2003


It’s December already!  It sounds crazy to wish you a Merry Christmas but it’s not far off…..So…

Merry Christmas!





At what point is it safe to prune peaches in the fall?  That’s a question that seems to come up each year.  What makes it an issue?  Peach buds (all fruit buds) progress through a series of steps each winter that cause them to be more or less sensitive to cold.  When a pruning cut is made, there is an invigoration of the tissues around the wound to heal the area.   This hormone release seems to deharden the tree. The greater the percentage of wood removed the greater the stimulation to the tree.  Big cuts on small trees are the most dangerous.  Weak trees are more susceptible than healthy ones (young or old).  I’ve seen literature that states the effect of pruning lasts anywhere from 5 to 10 days.  One thing is certain, the sensitivity decreases with each subsequent day.  The goal then is to not start pruning until the buds are sufficiently hardy to withstand this dehardening process RELATIVE TO EXPECTED TEMPERATURES!   The greatest danger occurs when temperatures drop quickly, especially if the previous days have been mild.  Keep you eye on the long range forecast! 


Once hardening develops the difference between a few dead buds and a lost crop can be a matter of a few degrees of temperature change.  Proebsting and Mills (1966) determined the percentage of survival of ‘Elberta’ peach flower buds and expressed the deviation from the temperature required to kill 50% of the buds (T50 ).  Only 4C above the T50, 90% of the flower buds survived and only 3C lower temperature killed all the buds.


The winter acclimation process is presumed to develop in three stages:

  • Stage 1 is initiated by shortening daylength.  In peach and cherry this stage is usually reached in late August or early September and is usually limited to a hardiness level of approximately 21F.


  • During stage 2, freezing temperatures trigger metabolic processes that result in a lower temperature hardiness level.  This occurs by early-November (soon after defoliation) and brings a hardiness level of  – 4 to – 9F.


  • Stage 3 is induced by continual freezing (below 27F) within the tissues that draw water out of the cytoplasm resulting in increased osmotic concentration and continued hardening.  How’s that for big words!  In other words….as it stays dang cold, the water is sucked from the bud making it more resistant to really low temps.  This hardiness level can reach – 17F for T50  of peach and       –29F for cherry.  This hardiness is lost up to – 4F when thawing occurs, provided temps remain below 39F.  This level that thawing induces on a non-frozen bud is known as the minimum hardiness level.  This level will remain somewhat constant until rest is completed and temperatures rise.  It typically starts near 5F in October, decreases to near – 5F in December and January, then increases slowly as the bud completes rest and starts to develop on mild days.





Flower buds are slow to gain hardness.  Cherry and peach buds harden at a rate of  3 - 4 F per day for peaches and 5F per day for cherries if the temperature is below 28 – 30 F (Proebsting & Mills).  They can lose it at a rate of 1 to 2 degrees per hour if the temperature rises above freezing.  Peach flower buds respond to temperatures above 28F to deharden to their minimum hardiness level.


Peaches in Central WA develop the ability to deep super-cool (cooling below the freezing point of a liquid without solidifying) and withstand temps below 10F in early October. At this time the buds are more hardy than the woody tissue on which they grow (Probsting).  As spring development occurs the minimum hardiness level rises slowly, rarely going above 5 F before budswell.   The T50 usually remains below 10F until shortly before the buds show pink.   As spring progresses the buds rehydrate which raises their hardiness to 28F at bloom time.  As bud development progresses, the ability to reharden with freezing is lost.


Apricot, cherry, sour cherry, peach, and plum showed no ability to supercool in September or October; all buds super-cooled in November and December.  Plum lost it’s ability to super-cool in January.  Peach, apricot, and cherry lost their ability sometime between February and March.


Sweet cherries develop the capacity for deep super-cooling 1 – 2 weeks earlier that peaches (Andrews & Proebsting).


Most of the acclimation of fruit trees is determined by the genetic make-up of the tree.  Cultural practices can affect it only to a small degree (no pun intended).  Here’s a few thoughts though.  Any practice that extends growth into the fall will decrease hardiness.  Acclimation is a metabolic process that requires photosynthesis.  Maintaining the foliage in good condition not only though the season but into the fall, will produce the maximum photosynthate possible increasing cold resistance.  Pruning immediately prior to low temperatures can increase the risk of injury. 


Use the above info as a “guideline”.  When it comes to killing temperatures and your orchard, it’s never an “exact” science! 




If you’re an apple grower in Delta County and haven’t checked in lately with the Farm Service Agency you should.  There’s a disaster program that is open until the end of January.  It covers loss of quality and tonnage for either the fall of 2001 or 2002.  Give Alan Bull a call!




Something else that’s right around the corner is the Western Colorado Horticultural Societies’ annual convention!  Here’s a few notes regarding this year’s program.


Are you a Hort Society member?  If not, why not?  The hort association’s primary source of income is through membership dues.  If you’re receiving this newsletter odds are that you’re involved in the Western Slope fruit industry and will pick up more than enough “farming tips” at the conference to pay back the cost of dues!


Who says there’s no such a thing as a FREE LUNCH???   


BASF is releasing a new fungicide called Pristine.  It is labeled for stone fruits, grapes, and lots of other crops.  Stop by and see Larry Schild at the BASF hort show booth. He’ll fill you in on the details of Wednesday’s free lunch during the show! 


Have you ever been to the “meet the speakers” social hour? It’s Tuesday evening at 7PM at Two Rivers convention center.  There’s hors d’oeuvres (French for finger food), beverages, and lots of time to visit with our “experts” from out of town.  It’s a great opportunity to see what’s going on in their part of the world.  Here’s a few of the folks that you should come meet:


Anita Azarenko.  Anita is a first generation American from Russian and German parents. Her position at Oregon State University involves teaching, research and extension.  Her emphasis is on woody plant or tree fruit physiology.  On the weekends she “applies” her expertise to the family farm and orchard consisting of apples, cherries, plums, and hazelnuts.  Don’t miss your chance to get to know her!  She has an outgoing personality and loves to share what she knows!



Charlene Jewell, senior post harvest scientist with FMC Technologies in California.  Charlene has 15 years experience in the field of post harvest treatments.  If California’s peach industry is a concern to your farm, plan on attending these talks.


Paul Friday, the breeder of the “Flaming Fury” peach series.


Randy and Annete Bjiorke, the breeders of the “Stellar” peach series.  If you’re interested in “new” peach varieties for your place, don’t miss your chance to talk to these breeders. 


John Caldwell.  A Napa Valley grape grower, nurseryman and vintner specializing in the major Bordeaux varieties.  One of the countries best sources for French wine grape clones. 


At last but not least….Don’t forget the banquet!  A great meal, a great time with friends, and a great show will be provided by the Yampa Valley Boys!  Their pickin, songs and stories will send you home grinning!





“We make a living by what we get, we make a life by what we give.”

Sir Winston Churchill



Hope Your Holiday’s Are Blessed!

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