Recent temperature extremes have caused concerns for fruit growers, and created much speculation about how the weather is affecting fruit trees. Cold hardiness is an amazing process and understanding what it is and how it works may help explain why it varies so much.

What is cold hardiness?

It is the ability of plant tissues to withstand extremes in cold temperatures. It is a complex physiological process that begins in early fall, and progresses until bud break in the spring.

How does it happen?

Progressively cooler temperatures in the fall signal the plant tissue to move water from inside their cells into the spaces between cells. Some of this water is lost through transpiration, but what remains in the intercellular space eventually freezes into ice crystals. Crystals formed in this space do not damage the cell, but crystals forming inside the cell kills it by destroying the cell membranes.

How fast does it happen?

Accumulation of cold hardiness is a slow process, and as temperatures get colder, tissues increase their cold hardiness. The maximum hardiness is reached in mid-January.

What are some factors that affect cold hardiness?

Cold hardiness is affected by a variety of factors including tree health, temperatures, species/cultivar, plant anatomy, and pruning.

Tree health - Trees that are most vulnerable to injury following a sudden temperature drop are ones that lack adequate vigor, or are too vigorous. Maintaining healthy trees through best management practices can help to promote cold hardiness - so avoid: moisture stress, overcropping, late season over-irrigation and excessive fertilizer applications, pruning in late fall or immediately before temperatures drop, or pruning sensitive trees in winter (ie. apricot, peach and nectarine). Trees that have undergone recent pruning may be more susceptible to midwinter deep freeze damage, so avoid pruning immediately before temperatures drop and delay pruning your most valuable trees until the risk of severe cold is past. These practices may make trees more vulnerable to cold temperatures.

Temperature - Cold hardiness of a plant varies depending on the severity/duration of the cold event, when it occurred, how quickly the temperatures dropped, how long the temperatures were sustained and the temperatures leading up to the cold event. For this reason every cold event is fairly unique and a tree may be affected differently by different cold events.

Each tree has a minimum temperature that they can withstand. Temperatures around -25°C to -28°C make many growers nervous (especially Asian/Japanese plum, and peach/nectarine). Temperatures below -29°C worry all fruit growers. Widespread extreme winter cold damage doesn't occur frequently, and was last reported with the Michigan fruit industries in 1993/94, when temperatures fell below -29°C. In Ontario, historical records show that temperatures fell as low as -29°C in Harrow in February 1918, and -26°C in Vineland station in December, 1980.

When fruit trees are approaching their chilling requirements, the trees remain hardy as long as the temperatures remain fairly cold. In late winter when the chilling requirements for the fruit tree have been satisfied, trees begin to lose their ability to re-acclimate. As a result fruit trees may be more susceptible to winter injury following warm periods in mid and late winter when temperatures rise, and this is often when winter injury occurs. This winter has alternated with warm and cold temperatures, and while trees have not yet achieved their chilling requirement, they likely have lost some of their deep dormancy.

If temperatures have remained consistently low in the days leading up to the freeze the trees should have minimal injury, even in cold temperatures. Trees are better able to re-acclimate if temperatures gradually return to "normal" following warm weather, but sudden drops of temperature of 6° C (20° F) or more can cause winter injury can occur. The worst conditions are when we have several days above freezing followed by a cold snap with temperatures dropping below -17.8 °C.

Species/Cultivar - Cold hardiness varies between species and cultivars. Trees grown on the northern limits of their climatic zones (e.g. peaches and nectarines, plums), are less cold hardy in general than trees in the central area of their climatic zones (e.g. apples and some pears). Cultivars like Loring peach are less cold hardy than cultivars like Redhaven and Harrow Diamond.

Plant anatomy - Different levels of cold hardiness exist for different plant parts. Often flower buds are more susceptible to winter cold injury. For example, peach flower buds begin to freeze and die at -25° C, but the tree itself is not damaged until the temperature drops below 29 ° C.

How much time does it take for plants to re-acclimate after a warm spell?

Plants can re-acclimate to the cold, but it takes longer once they have lost their cold hardiness. While hardiness can be totally lost over a few warm days, the process to re-acclimate is much slower, achieving only a degree or two of hardiness with each cold day.

What are the symptoms of winter injury?

Winter injury symptoms include dead buds, root damage, bark cracking, trunk splitting crotch damage, blackheart injury, flower bud injury, sunscald and root injury. Most of these symptoms will be difficult to diagnose until temperatures rise above freezing and tissues thaw. Some winter injury symptoms do not appear until very hot weather returns in early June - trees with damaged trunks may leaf and flower normally, but collapse when the damaged trunks do not allow moisture to flow upwards to the tree under stress.

What research is being done on cold hardiness in tree fruit?

There is still much to learn about cold hardiness in tree fruit. Researchers are looking at more accurate methods of measuring cold hardiness for apples and sweet cherries (http://news.cahnrs.wsu.edu/2012/09/20/wsu-researchers-create-more-accurate-cold-hardiness-measure-for-apples-sweet-cherries/). The Ontario Tender fruit Marketing Board, KCMS and Brock University have received CAAP funding for a project to evaluate bud hardiness in peaches. This data is being collected and analyzed to see at what temperature lethal freeze events occur on different peach varieties in various regions in Ontario.