Critical Spring Temperatures for Frost Damage

Early spring is the time of the year when apple trees are beginning to wake up from their winter sleep, and is also the time for colder night temperatures that we are sure to see. It's always a good idea to revisit the critical temperatures that can cause frost or freeze damage to fruit trees, specifically apple trees.  We are currently at silver tip to green tip here in northern Illinois with a forecast of night temperatures in the low 20's later in the week. Fortunately, our trees haven't progressed to far this spring, but we could see green tip next week.   This spring has marked another unprecedented weather pattern that raised our temperatures in late March and and is dropping our temperatures the first few days of April.  The early warm temperatures accelerated bud swell, and the lower temperatures that are anticipated present the threat of frost or freeze damage to new green tip leaf tissue.  It seems that each spring since about 2012, we have been on the verge of critical temperatures for frost or freeze damage with our fruit trees. 

Dark brown centers and signed appearance of the petals
indicate that both king and side blooms were killed in a
freeze the morning this picture was taken.  (Photo credit:
Mark Longstroth, MSU Extension)

As the trees develop in the spring and buds start to swell, they lose the ability to withstand the cold winter temperatures that they could withstand in dormancy during the cold winter months. The young, actively growing tissue can then be damaged or even killed. Swollen fruit buds can better withstand temperatures in the teens without any damage. As the buds open, temperatures in the low 20s can cause harm, but sometimes leave other buds undamaged.  As growth moves from green tip to 1/4” green to 1/2” green to tight cluster to pink in apple trees, temperatures in the upper 20s can cause considerable harm to an early blooming tree. Near bloom, the range between slight and severe damage can be very small. Freezing temperatures of 28 degrees F. will result in about a 10 percent loss and 24 F will result in a 90 percent loss, as indicated by the charts down below.

The dark brown center of this apple flower  indicates it was
killed by a freeze. (Photo credit: Mark Longstroth, MSU Extension)

In a radiation freeze with clear, calm conditions, fruit at higher elevations or in the tops of trees will be less damaged than those at lower elevations, since colder air is more dense than warmer air and sinks to ground level, pushing the warmer air up. The percent of flowers killed in a frost may or may not relate directly to lost yield later in the season. With large-fruited fruits such as apples, peaches, plums and pears, the loss of 50 percent of the flower may not be devastating since we may only want a small percentage of the flowers to become fruit, meaning that fruit thinning may be totally unnecessary.  So the stage of bud and bloom development determines how susceptible any given fruit crop is when freezes occur.  For more information on what those critical temperatures are that can cause freeze damage to trees during development, I have added two charts on the Critical Temperatures For Frost Damage on Fruit Trees from Utah State University below that you can download by clicking on either chart below.

(Click on the photos to download the chart in PDF format.)

 

Given the weather patterns we have experienced so far this spring a spring frost could still be possible.  Once the fruit has set, then the critical temperatures that can damage the fruit become lower.   We will need to constantly assess the stage of development our trees are at over the next weeks and their susceptibility to possible freeze injury.

If we continue in a spell of colder weather, apple trees will continue to develop more slowly, but once they begin showing tight cluster, pink and bloom, the critical temperature rises from the low 20’s to the high 20s, to levels just below freezing at bloom time, which is the most critical time to get frost damage.   

 

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Reference in this blog to any specific commercial product, process, or service, or the use of any trade, firm, or corporation name is for general informational purposes only and does not constitute an endorsement, recommendation, or certification of any kind by Royal Oak Farm, Inc.   People using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

Main Pests, Tree Growth Stages, and Spray Guide

If you have a Home Orchard or just a few apple trees in your back yard, and if you have not thought about how to handle those pest problems you had last season yet, now is the time to actively examine a spray protocol  for your fruit trees!  And start right away, before it is too late and the pests have a chance to establish themselves in your trees!  The question everyone needs to ask before spraying is “Do I want to spray or not?”   Well, unfortunately, in northern Illinois, we have four main pests that we will almost always have to spray for.  This is pretty much true for any area east of the Rocky Mountains.

The decision to spray or not depends on how much fruit loss you are willing to take. That is your threshold.  If you can accept some fruit loss, then the need to spray diminishes greatly.  But if you only have a few trees and some fruit damage may mean losing half your fruit, then spraying becomes more important.  Let’s meet our top four pests in northern Illinois,  southern Wisconsin, and in most of the states east of the Rocky Mountains. 

 The Four Main Pests

 

The four main pests that we face in apple orchards here in our climate zone of 5/5A are apple scab, plum curculio, codling moth and apple maggot, in that order. What isn't listed here is fire blight, whihc is a devastating disease that must be sprayed for because it can wipe out entire trees in a matter of days.  For more information on these and other potential apple tree pests, visit our web site Growing Guide page.  But how do we know when to spray for them if it is a last resort to protecting our fruit? 
 

All tree fruit have several distinct growth stages as the fruit matures.  Knowing and identifying those growth stages is  very important for the home grower because recommendations and spray timing for spray applications are linked to these specific growth stages.  The chart below shows the common growth stages for apple trees.  

Tree Growth Stages (Phenology)

 

Since the average home grower does not have access to their own weather stations or degree day calculators, the fruit tree development stages play an ever greater  role in pest management for the average grower.  Most spray schedules (protocol) follow the tree development stages to aid in the timing of sprays so they are most effective.  It is important to note that many diseases and some insects can only be controlled by spraying before they can be seen like apple scab.  Spraying less frequently or at the wrong time will typically result in poor results.  And, spraying more frequently will not necessarily give greater control.

The tree developmental stages or tree phenology gives us a guide as to when to spray, but what do we spray if we have to spray?  If we consult some of the various spray guides available to the home grower, we will find that most of the spray guides provide us with the tree’s development stages (phenology) and the insects or diseases that frequently  occur during each of theses stages.  So the tree phenology serves as timing guide when the application of a particular spray is recommended in order to control specific insects or diseases at the right time.   The following spray guide for apple trees will give us the time to spray based on tree phenology, the pest to spray for and the product recommendation for that pest(s).

Apple Spray Guide

This particular spray guide is included in “Managing Pests in Home Fruit Plantings” from the Purdue University Extensions Publication web site or our web site as a free download.  It goes into detail as to the various products available for spraying that include both conventional and organic alternatives.  The publication includes apples,  pears, peach, cherry, grape, strawberry and raspberry guides as well as the phenology charts for each fruit type.  For recommendations on spray products for the Home Orchard visit our spray product recommendations.

Reference in this publication to any specific commercial product, process, or service, or the use

of any trade, firm, or corporation name is for general informational purposes only and does not

constitute an endorsement or certification of any kind by Royal Oak Farm.

People using spray products assume responsibility for their use

in accordance with current label directions of the manufacturer.