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January 2016


                 Cropworx has made an investment in your future!  The last several years I’ve been concerned about the longevity of the instruments we use to develop our pest models (The information used to determine your spray timings).  The network of stations we have in place use a Biophenometer developed in 1985 to record the needed data.  This instrument is no longer sold or serviced. 


                 Over the last several months we’ve set up a weather station at each of the locations where we had a biophenometer.  This new equipment is up and running and accessible through a cell modem.  Our plan is to continue operating the Biophenometers for a few more years to compare the data (timings) between the two systems.


                 This action will add a greater measure of dependability towards keeping your pest control timing on track!


Once again we will be submitting samples of dormant peach buds from Cedaredge and Palisade to CSU.  These samples will be delivered to Dr. Caspari, CSU’s viticulturist at the Orchard Mesa research station.  Dr Caspari will then expose the samples to increasingly lower temperatures in a freeze chamber.  This process gives us a look at the hardiness of these buds at a given point in the season and the temperature thresholds at which they may be killed.  Thanks Horst for your continued help!


                 The information on chilling and bud hardiness has been gathered over the years from a number of sources.  While the science behind the info may be solid, the interpretation is a slippery slope.  The exact degree of hardiness is a moving target tied to high and low temps both before and after any given date. Cold hardiness is the ability of a plant to withstand low temperatures. It  is a vague and often misleading term because low temperature injury can vary depending on when the low temperatures occur (early vs. mid-winter or late-winter), how fast the temperature drops, what the temperatures were during the previous few days, and how long the low temperatures are sustained. For this reason every cold event is fairly unique and a plant may be affected differently by each event. 


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 21ºF.


Stage 2 freezing temperatures trigger metabolic processes that result in a lower temperature hardiness level.  This increase in hardiness is very             rapid and trees may be as much as 10˚ hardier the day after a frost than they were the previous day. This occurs by early-November (soon after defoliation) and brings a hardiness level of – 4 ºto – 9ºF.


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 – 17ºF for T50 of peach and –29ºF for cherry.  This hardiness is lost up to – 4ºF when thawing occurs, provided temps remain below 39ºF.  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 5ºF in October, decreases to near – 5ºF 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  3º to 4ºF per day for peaches and 5ºF per day for cherries if the temperature is below 28º to 30º F (Proebsting & Mills).  They can lose it at a rate of 1º to 2º per hour if the temperature rises above freezing.  Peach flower buds respond to temperatures above 28ºF to deharden to their minimum hardiness level.


-Peaches can develop the ability to deep super-cool (cooling below the freezing point of a liquid without solidifying) and withstand temps below 10ºF 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 5ºF before budswell.   The T50 usually remains below 10ºF until shortly before the buds show pink.   As spring progresses the buds rehydrate which raises their hardiness to 28ºF 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 its 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 than 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), but 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.   


                During the winter, the temperature required to kill peach flower buds may vary by as much as 10˚F. During bloom, the frost tolerance of an open blossom may vary by about 5˚F depending on the temperature conditions for several days before a frost. This is why it is so difficult to predict the temperature that may kill trees or flower buds during the winter and even during bloom.   


                Does pruning affect cold hardiness? The following is an excerpt from an article by Dr. Rich Marini at Penn State.  Although there is quite a bit of anecdotal evidence indicating that pruning early in the winter can reduce the cold hardiness of woody plants, there have been few controlled experiments to determine how hardiness is affected by time of pruning, how long the pruning effect may last, and if pruning severity is involved. Some of the best research on time of pruning of peach trees was done in South Carolina (Nesmith and Dowler, 1976) and Georgia (Prince and Hutton, 1972) in an attempt to identify practices that contributed to peach tree short life (PTSL). PTSL is a complex disorder of peach trees in the southeastern U.S. causing excessive mortality of trees less than 10 years old. Conditions contributing to PTSL seem to include nematodes and other soil pathogens, rootstock selection, and fall pruning. A series of experiments conducted during the 1960’s and 1970’s showed that fall-pruning of trees on sites with a history of PTSL were susceptible to late winter cold injury that killed the trees. Trees on sites with no history of PTSL were less affected by fall pruning. Pruning studies with other woody species, such as grapes (Wolpert and Howell, 1984), crape myrtle and cypress (Hayns, et al., 1991) showed that pruning in the fall reduced the cold hardiness of the plants during the winter. A couple of interesting facts have emerged from these studies.


-Pruning in November tends to reduce the cold hardiness of woody plants until late February, so the effects of pruning are fairly long-lasting.

-Pruning experiments with peach show that fall-pruned trees had higher levels of the growth regulator indoleacetic acid (IAA). This is the naturally occurring auxin that is mimicked by the synthetic auxins that are used commercially, including  NAA, 2,4,5-TP and 2,4-D. 

Cambial activity (the tissue in the bark responsible for cell division contributing to trunk and stem radial growth) seems to be stimulated by the increased IAA levels following pruning. Peach trees pruned in November exhibited enhanced cambial activity in February. 


                More recently, summer pruning of peach trees in August was shown to delay leaf abscission and cold acclimation, so flower buds on summer pruned trees were less cold hardy than non-pruned trees in the early winter, but not in the mid-winter. I also made the observation a few years ago that peach trees that were pruned in the pink stage had more injury than non-pruned trees when a frost occurred two days after trees had been pruned. Based on observing peach trees that were approaching bloom, I think that in some years pruned trees begin to bloom and leaf out a little earlier than trees that were not yet pruned.    


                Based on everything that has been published we can conclude that woody plants do not attain maximum cold hardiness when they are pruned in the fall. Trees are affected more by heavy pruning than light pruning. There is still much that we don’t know about the practical implications of how pruning affects cold hardiness. We especially don’t know how rapidly pruning causes de-acclimation, or if de-acclimation is similar in the early-, mid-, and late-winter, and we don’t know how long the trees remain de-acclimated. We also don’t know if the de-acclimation following pruning is affected by mid-winter warm spells, which we seem to be experiencing more frequently.


               Are you thinking this winter’s low temperatures have taken care of your pest problems? The following information is from a college textbook,  Destructive and Useful Insects by Metcalf, Flint and Metcalf.  Regarding codling moth: “They remain dormant and are able to withstand low temperatures.  A drop in temperature to – 25°F or below however will kill many larvae.”  I suspect that if winter temps take care of your codling moth problem (and other pests), your harvest worries will also be over!  Suppose we can keep the sprayers in the barn this year since all the pests have frozen to death?  Please reread the preceding paragraph before continuing.  What is the impact on pest populations from low temps?  Each year there’s a certain percentage (high or low) of winter mortality.  Air drainage, snow cover, micro-habitat, genetic characteristics of local populations, and simple raw temps all impact the survival of pests.  Let’s say for discussion’s sake you’re down to a handful of individuals left in your orchard.  Forget what lies behind and focus on the future!    The greatest factor on the coming season’s pest populations is the early spring conditions we encounter.  Favorable development weather early in the season can easily compensate for a small over-wintering population.  Conversely, a cold, wet and extended spring can be as effective as a well-timed early season spray program towards reducing populations.  The growth of most pre-bloom arthropod populations is pretty much determined for the first half of the season by early spring weather patterns.  Mites, aphids, plant bugs and lygus are obvious examples that suffer from cold, wet, rainy and windy springs.  But, bring on a hot, dry and quick spring and watch what happens! 


Check out the following links for a wealth of information on pruning cherries.   Follow the “links” tab as well as the “video” tab.  If you struggle to grow large cherries it may be time to adjust your pruning style!




 “All that is required for evil to prevail is for good men to do nothing." -- Edmund Burke 



“May those that love us, love us.  And those that don’t love us May God turn their hearts.  And if He doesn’t turn their hearts, May He turn their ankles, So we’ll know them by their limping.” – Unknown


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