Sunscald on trees: what extension services describe

This page describes what the named extension services say. It does not diagnose a tree, say whether any tree is safe, or tell anyone to treat, wrap or remove one. It is about trunk injury from sun and temperature, not about pests. Where the sources differ, the page says so.

What the sources call it

The University of Arkansas System Division of Agriculture (fact sheet FSA6152, 2019) says sunscald is a fairly common physiological problem, found most often on young, thin-barked ornamental and fruit trees, and that it has been reported in the literature for over 100 years. It describes two seasonal forms with one cause, death of bark tissue from damaging temperatures. Winter sunscald, also called frost cracking, comes from rapid temperature swings. Summer sunscald comes from lethal temperatures during sun exposure, and the fact sheet says it is the more common type in Arkansas.

Utah State University Extension Forestry (Utah Forest Facts 021, published December 2011) uses the name southwest winter injury. It says the cells lying just under the outer bark, chiefly phloem and cambium, are injured by winter temperature swings between day and night. It reports research finding the south side of a tree up to 77°F warmer than the north side on a cold winter day. Cells that wake up in that warmth lose some cold hardiness, and are hurt when the temperature drops below freezing at night. The sheet says this injury, as defined, does not happen in Utah in summer, but that some trees show sunscald-like injury there after summer pruning opens the crown to sun.

The South Dakota Department of Agriculture and South Dakota State University forest health update of December 2-9, 2015 (John Ball) calls sunscald a frost canker. It describes death and discoloration of bark reaching into the cambial zone, with injured outer bark sloughing off to show dead tissue. It says these cankers are more common on the southwest side but can occur on any side, and that bark losing its cold hardiness in late winter is hurt by unseasonable cold.

What it looks like

Arkansas says early symptoms are hard to read: discolored bark like a canker, some small vertical cracks, and in spring there may be bleeding from the damaged area. Late-stage injury is easier to recognise as a long vertical fissure with bark coming off, and the margins of the fissure may show wound closure. Utah lists discolored bark, bark cracking, sunken areas from lack of growth and sloughing of surface bark that exposes dead tissue.

Which trees and which side

Arkansas names maple, sweetgum, linden, ash, honeylocust, eastern white pine and apple, and says the problem is common on the lower part of young trunks and on the south or southwest side. Utah says it generally affects young trees of certain species rather than thick-barked trees, and lists apple, ash, aspen, birch, cherry, cottonwood and poplar, crabapple, honeylocust, linden, maple, mountain-ash, oak, peach, tuliptree and willow. It adds that thin, dark bark such as on cherries, peaches and plums can stay susceptible with age, and that evergreens tend to be less susceptible because foliage shades their trunks. South Dakota names thin-barked trees, crabapples, lindens and maples in its region.

Stress and other factors

The sources do not fully agree about drought. Utah says drought-stressed trees are more susceptible than trees that go into dormancy well hydrated. South Dakota says moisture stress in the summer and fall before the injury is the factor most often linked to frost cankers, with drought and transplanting named as key stresses. Arkansas says that although water-stressed trees have been suggested to be more prone, this has never been confirmed by research.

What the sources list for protection

Arkansas says the common strategy is to cover the lower trunk with light-coloured materials such as corrugated plastic guards, kraft paper or breathable white fabric wrap, and says dark wrap intensifies the temperature problem. It says such materials stay on for several years, until bark thickens or the canopy shades the trunk, and that pruning paint is not applied to the wound. Utah also describes white wrap, shading the trunk with a light board, and white interior latex paint used by some orchardists, and says damaged areas should not be sealed with paint or tar. South Dakota is more cautious: it says wrapping might help, but that paper or plastic may not prevent temperature swings and may even speed them, and may trap moisture and pests if left on or girdle the tree. It points to adequate water in the growing season and avoiding trunk wounds as the longer-term means.

For the separate cracking that the same sources link to sunscald, see frost cracks in tree trunks. The other pages in this group are listed on the weather, soil and chemical damage page. For how bark and cambium work, see how a tree works.