November 2010 Freeze

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November 2010 Freeze Injured Tender Trees

  Where the buds survived, new shoots and foliage are seen emerging on “brown” conifer branches.
  Some trees, including peach and this chokecherry, expanded their spring foliage, but the loss of water transport capacity resulted in wilting soon afterward, and in some instances, tree death.

The fall of 2010 was long and warm in northern Utah. Temperatures were mild up until November 20—in the 40s at night and no cooler than 45 during the day. Over the next 3 days, temperatures plummeted to the single digits for 3 consecutive nights, followed by mild to average temperatures for the remainder of the fall. Some trees growing on the “edge” of their hardiness zones such as Atlas cedar, giant sequoia, Amur maackia, Kousa dogwood, and others, experienced varying levels of dieback. Peach and sweet cherry growers reported bud injury, dieback, and tree death.

Day length and temperature are two important factors in triggering acclimation from the growing phase to the dormant phase in plants. Among other changes, water moves from within cells, where ice formation would be very damaging, to between cells, where ice masses can safely form. Trees growing in northern Utah that are marginally hardy (i.e., growing a zone 6 tree in zone 5) typically are not fully acclimated until late December. Because of the long period of warm fall days in 2010, the acclimation process to dormancy had barely begun.

Damage to evergreens (mostly conifers) was visible right away this spring, seen as dead, brown needles or scales and in some cases, dead shoot tips. Some trees looked completely dead because all the foliage was brown. Evergreens, because they retain their foliage, need water in winter to replace water lost to transpiration. Normally, the tree is able to access some water from the soil. But the hard freeze that followed the mild temperatures resulted in a situation where the lost water could not be replaced.

Damage to deciduous plants is more difficult to discern, and also is more difficult to blame on the November freeze due to the wide array ofproblems and the length of time it takes to see the damage. Some of what happened includes the following:

  1. Frost cracks are vertical splits in the trunk that occurred due to uneven contraction of the wood during the drastic temperature change. Cracks or fissures may not be visible for several weeks to a month after the tree starts growth in spring, or even when secondary problems have arisen, such as entry by borers or wood-decay fungi.
  2. Branch dieback or death of entire trees occurred due to xylem cavitation, which is when xylem vessels become air-filled and essentially non-functional due to the freeze-thaw event. Branch dieback, where the leaves never emerged, is the most common type of injury we have seen. The roots are not killed in this case, and may produce heavy sprouting.
  3. Bud/twig death occurs when water freezes within plant cells. This damage was widespread on peaches and sweet cherries, resulting in a reduction in yield.
  4. Infection by opportunistic pathogens occurred on tissue not fully hardened off or damaged by freezing. Bacterial blight was prevalent this spring on sweet and ornamental cherry, apricot, viburnum, peach, ornamental pear, and mountain-ash. Bacterial blight is caused by Pseudomonas syringae, a pathogen that infects cold-damaged buds in autumn. P. syringae contain proteins that can cause water to freeze at higher temperatures, contributing to wounding. Other opportunistic pathogens include cytospora canker on spruce, cherry, peach, and poplar, and Thyronectria canker on honeylocust.


In general, maintain even soil moisture (i.e., do not let the tree suffer from dry conditions), and fertilize in spring.

Conifers and Broadleaved Evergreens
Evergreens should be left to recover; do not prune until next year, when it is clearly visible which branches survived and which didn’t.

Deciduous trees

  • Branches that have not leafed out by summer can be pruned out. (When pruning, do not leave stubs, and do not cut through the branch collar.) If more than 50% of the tree was killed, consider tree replacement.
  • Thin out excessive shoots and suckers of fruit trees as well as growth that is shading fruit-bearing shoots to ensure fruit production for next season.
  • Any fruit trees that have a heavy crop should be thinned more than usual to prevent stress on the tree. Thin peach fruit, for example, to about 10 inches between fruitlets. Injury from the November freeze was unavoidable, but you can prevent injury in “normal” years by planting the right tree for the right site, avoiding late-season pruning and fertilization, watering in October when needed, and applying white trunk wrap for the winter.

-Marion Murray, IPM Project Leader, and Michael Kuhns, Professor and Forestry Extension Specialist