Horticultural Update: Freezing Temperatures Predicted Over the Weekend

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Ginger Gold apple

Sub-lethal freeze injury to a ‘Ginger Gold’ apple. Developing apples that have been exposed to near-lethal temperatures will sometimes take on a ribbed “pumpkin” appearance, the result of damage to tissues around the vascular bundles. Image courtesy of Rob Crassweller (Penn State University).

Apple growers across the Eastern and Midwestern US are bracing for an unusually late cold front (polar vortex). Low temperatures in the forecast are near or below critical temperatures in many southeastern apple production regions. Nearly all cultivars are in the post bloom window and some varieties have ~18 mm fruit.

In my short career, I have not personally observed freeze damage this late in the season. In an exchange with colleagues who have experience in fruit production in other regions (ME, NY, and PA), an excellent discussion regarding the anticipated freeze event occurred. Here are a few key points from this discussion:

  • The critical temperature for the onset of post-bloom cold injury is 28 °F. Fruit of larger size are likely to withstand 28 °F for a short period (~1-2 hours). Soluble solids in plant cells will lower the freezing temperature to ~28 °F.
  • With fruit, 3-4 hours at 28 °F is the point at which the duration of cold temperature exposure is likely to have an effect. If temperatures get down to 25 °F, there will be severe damage and fruit mortality from even a short duration.
  • Fruit finish and quality will be a serious concern. In addition to potential for frost rings, russet, and seed damage, developing apples that have been exposed to near-lethal temperatures will sometimes take on a ribbed “pumpkin” appearance. This is the result of damage to tissues around the vascular bundles that run from blossom end to the stem.
  • The cold weather that is predicted is not a frost (or a radiation freeze), but an advective freeze, meaning that there is a large cold air mass (polar vortex) moving in with winds. If high winds are observed, it is very unlikely that frost control measures like sprinklers and wind machines will help, because there is cold air throughout the atmosphere. Winds may simply increase the cold from evaporative cooling and cause more damage. High winds are forecast on Saturday morning at some sites, but lower wind speeds are predicted on Sunday (at present). Wind machines will likely be ineffective during this event due to a lack of an inversion layer, and will mix cold air with more cold air.
  • Will Promalin/Perlan work for frost rescue? The label for Promalin only extends to the bloom period. If past the bloom period, you should not use this material for frost rescue. If you still have some blocks that are in bloom, see page 81 of the 2020 Integrated Orchard Management Guide for Commercial Apples in the Southeast.
  • This freeze event may complicate future thinning decisions. If appropriate, I’ll provide a thinning update early next week to coincide with the rescue thinning window for many varieties.

Comments in this discussion were provided by Jim Schupp and Rob Crassweller, Penn State University.