Cold hardiness is the ability of trees to tolerate low temperatures. Cold hardiness may depend on different variables like how fast the temperature drops, what temperatures were before the temperature drop, and how long the severe cold lasts. Trees may be cold hardy in one location, and not in another, depending on site-specific variables. Cold hardiness of flower buds is genus and variety dependent.
Researchers at the WSU Irrigated agriculture research and extension center (IAREC) are developing a more accurate way to measure cold hardiness in apple and sweet cherry buds, to better help PNW growers protect their trees during frosts. The information on new apple and cherry variety cold hardiness will create a model for AgWeatherNet, to provide farmers with knowledge to help them make frost protection decisions. AgWeatherNet will send growers Cold hardiness warnings if critical injury temperatures for apple and cherry buds are reached. There are different variables that make trees more vulnerable to frost damage.
Resources for Cold hardiness:
- Critical Temperatures and Bud Stages Charts, webpage, Chelan-Douglas County Extension
- Researchers create better cold hardiness measure, WSU News, September, 2012.
- Testing cold hardiness in tree fruit, WSU provided story, Growing produce, February, 2013.
- Critical temperatures and bud stage charts, WSU Chelan-Douglas county extension webpage.
- How the cold affects fruit buds, L. E. Long, Oregon State University, Good Fruit Grower, December, 2013.
The term winter injury encompasses many symptoms of tree damage including: cambium injury on stone fruit, crown injury, winter sunscald (south west injury), trunk splitting, and shoot dieback. This is according to Jon Clements, Extension tree fruit specialist at the University of Massachusetts, describing some forms that fruit tree injuries can take in the Fruit Growers News article Cold injury to fruit trees is a big concern.
Buds and blooms, young shoots, and roots can also be injured during the winter, affecting crop load and fruit finish. Tree tissues are injured more often when there is a fast drop in temperature. Trees can exhibit poor growth, damaged/dead tissue, or die from injuries sustained from freezing.
As mentioned earlier, painting white latex paint on trunks of fruit trees protects the bark from splitting and cracking when exposed to freezing temperatures followed by thawing temperatures (sunscald of tree trunks). The paint reflects sunlight during the day, and keeps the tree warmer at night. See Fruit Growers News article, Painting tree trunks.
Also see Southwest crop injury: An After-effect of a cold winter, C. Herrick, Growing produce, May, 2014.
Frost damage to fruit blossoms is a concern for growers early in the growing season during bloom, when frost occurs, and for fruit late in the season when temperatures get cold before harvest. Fruit tree tissues will freeze at certain temperatures, depending on the stage of tree development. When fruit trees come out of dormancy and buds start to become active, they are susceptible to freeze damage. The amount of damage incurred depends on the stage of development, the cultivar, and the length of time the freezing event lasts. See Frost injury to apple in the quick identification cards on the Cullage Assessment & Education website.
Apple fruitlets can be injured when freezing occurs during bloom time. When fruit develop and mature, freeze damage blemishes appear as russeted areas partly encircling the fruit or in irregular spots.
“Apple varieties are prone to freezing injury when temperatures reach 27.3° F to 29.4° F depending on variety. Ice crystals form between cells in an apple. Injury occurs when forces are exerted on the cells by ice crystals, and from chemical reactions in cells where excess water has been removed.”( excerpt from: C. Pierson et al., Market diseases of Apples, Pears, and Quinces Frost and Freezing Injury, C. Pierson, et. al. Agriculture Handbook No. 376, ARS-USDA, 1971.)
As mentioned above, buds and blooms, young shoots, and roots can be injured during freezing conditions, affecting crop load and fruit finish, and possibly killing trees. Trees can exhibit poor growth, damaged/dead tissue, or die from injuries sustained from freezing one or more years after the injuries.
Over-tree sprinkler and misting systems, evaporative cooling systems, and solid-set canopy delivery systems may be used for frost and freeze protection in orchards. Overhead sprinklers form clear ice, in which an endothermic reaction takes place, and the warmth of the plant is trapped inside. When water freezes, it releases heat to the surrounding environment. Growers also use the overhead sprinkling to slow down bud development during warm windows in late winter. For more information see the article: Protecting your fruit from frost and freeze, D. Sigler, Fruit Growers News, January, 2013.
Moist soil rather than dry soil has a greater ability to get and store heat energy during the day when it is warm, and release heat at night, helping to maintain air temperatures. Cold air is heavier than warm air, and settles to the lowest areas near the ground, so mowing tall grasses and cover crops is helpful in preventing freezing damage on trees. For more information see these articles: Mist cooling for freeze protection, R. Lehnert, Good Fruit Grower, February, 2013; and Critical temperatures for frost damage on fruit trees, M. Murray, (WSU data), Utah State University Fact Sheet, February, 2011.
Other management practices used to protect fruit tree crops from frost and freezing conditions include:
- Wind machines which are tall vertical fans (35 ft.), used for frost protection. They pull warmer air down to the orchard and mix it with the cold air near the orchard floor. These large fans can protect approximately 10 acres of fairly flat orchard. For more information see the article: Basics of frost and freeze protection for horticultural crops, K. B. Perry, HortTechnology, 8(1),1998.
- Windbreaks near orchard blocks must be placed in positions that will allow cold air to flow out of a block, and frost protection to occur. Vegetation, bins, buildings, etc. will slow the movement of cold air out of an orchard.
- Helicopters are sometimes used to move air for frost protection, but this option is very expensive.
- Heaters are also used to warm orchards during frost conditions, but are not used as much today because of the cost of fuel, and they are not efficient at heating orchards. They are banned in some areas due to air quality issues. Propane heaters and diesel furnaces are still used in some areas for frost protection though. For more information see the article: Frost protection strategies, R. Lehnert, Good Fruit Grower, February, 2013.
- Cryoprotectant chemicals generally have not proven successful at preventing freeze damage for blossoms, buds, or fruitlets when sprayed on foliage.
- Growth regulators applied in the Fall, such as ethephon, have been shown to increase winter bud hardiness in peach.