Protecting Fruit Trees from Winter Freeze Injury
The tree fruits grown in Illinois, particularly the northern part of the state, are winter hardy but can still be injured by severe winter temperatures, said a University of Illinois Extension horticulture educator.
The extreme cold winter temperatures and high fluctuations between day and night temperatures may cause injury to fruit trees.
"The flower buds, young shoots, tree trunks and roots of fruit trees can be killed by freezing temperatures. The plant tissues are injured more when there is exceedingly fast drop on night temperatures during winter and the situation becomes worse when it is accompanied by strong winds. The damage is even more severe on frozen areas that thaw very fast."
There are different types of winter freeze injuries that may occur underground and on the above-ground parts of fruit trees in home gardens. The most commonly observed winter injuries on fruit trees occur on crown or collar, crotches, trunks as sunscald, young shoots, twigs, and roots.
The crown or collar injury may occur on the trunk near the ground surface and may extend a few inches below the soil surface killing the bark of the tree leading to reduction in surface area for movement of sap back to the roots. The crotch injury may occur at the point where the branch joins the trunk, and this may lead to development of canker on the affected areas that may require the branch to be pruned.
`The winter scald or sunscald may develop on the south or southwest sides of the tree trunk and lower branches due to rapid drop in temperatures on cold, sunny days in mid-winter. The side of the trunk on the sunny side thaws while the other side is still frozen leading to cracking of the bark that may expose the woody part of the stem. The split area may develop into a wound that may turn into a canker that can kill the tree.
"The young shoots and twigs from the growth that occurred late in the season are not winter hardy and can be killed by severe winter temperatures," he said. "The injury may also occur to leaf and flower buds. Apple leaf and flower buds are more resistant to winter injury than are other tree fruits.
"In northern Illinois, more damage may occur on peaches and other tree fruits that are sensitive to extremely colder winter temperatures leading to death of leaf and flower buds. The bud or shoot death can be minimized by not fertilizing fruit trees with high nitrogen fertilizers in late summer or early fall, and reducing or stopping irrigation in early fall. It can also be minimized by growing tree fruit varieties that are winter hardy and adapted to the area."
The roots can also be killed by cold winter temperatures, particularly the roots that are closer to soil surface. The symptoms of severe cases of root injury may be manifested on shoots and is mostly observed in midsummer.
"The fruit trees grafted on dwarfing rootstocks that are shallow rooted need to be mulched in winter to protect roots from winter injury," Ogutu noted.
Sunscald can be managed by wrapping the tree trunks and lower branches on the southwest facing side with burlap, aluminum foil, craft paper or special tree wraps. These are referred to as trunk guards. The trunk guards can be used in younger trees that have been in the garden for the last two or more years. The trunk guards should be of light color so that they can reflect sunlight during winter thereby reducing the temperature on the bark. The trunk guards must be removed in early spring.
"White latex paint has also been used to protect fruit trees from sunscald for more than one winter," he said. "White latex paint used for interior painting is much better than other types. Do not use oil based paint as they can injure the tree."
Trees to be treated in this manner need to have been planted at least two years ago. They should be painted in late fall so that the paint can stay longer on the bark. Paint the trunks on sunny, warm days so that the paint can dry the same day. In order to avoid tree injury, do not paint when air temperatures are below 50°F. Apply the paint using a brush or other materials such as a sponge in order to get a thick coat for better protection.
"The whole trunk can be painted although the southwest, west, or south parts of trunk may need more protection," Ogutu said. "Parts of the trunk that need to be protected by the paint should be at least 18 inches above the ground and may extend into parts of the trunk above the lower limbs into the 10 to 12 inches from the base of each of the lower branches."