By Kathryn Carter, Fruit Specialist (Tender fruit and Grape), OMAFRA
Despite the relatively mild winter we experienced in 2021/2022, growers are starting to report cold injury in peaches. The extent of cold injury varies between cultivars within sites, and also within different growing regions. With some cultivars in some regions showing significant crop loss, while others will just require less time for hand thinning.
Cold hardiness is the ability of a plant to survive low temperatures. Generally tree fruit in Ontario can withstand several months of below freezing temperatures with little injury by becoming dormant and cold hardy. However, cold hardiness of tree fruit is influenced by many different factors, including: timing/stage of crop development, weather conditions, variety/rootstock and production practices (crop load, harvest time).
Impact of Timing/Stage of Development on Cold Hardiness
Fall cold events
During the fall, short days and freezing temperatures induce dormancy in fruit trees. Warm, mild temperatures in the fall, can result in a delay in acclimation in tree fruit making the trees more vulnerable to sudden drops of temperature in November or December. In Ontario, we saw this occur in some orchards in mid-November 2019 when temperatures dropped from 8° C down to -11 ° C within a few days resulting in cold damage to peach tree shoots. Often shoot damage can be difficult to identify at this time of year, without cutting open the shoots, but in the spring, injury appears as branch tip dieback.
Mid winter cold events
Once fruit trees have achieved endo-dormancy, they have the ability to acclimate to colder temperatures and are resilient to cold provided the temperatures remain below freezing. The maximum cold hardiness of the trees occurs when they have been subjected to cold, subfreezing temperatures for several days or more. Temperatures below the maximum cold hardiness are damaging or fatal to the trees. Maximum cold hardiness varies between species but is around -25° C for peaches, -30° C for apricots/pear, -18° C for tart cherry, -25° C for plum/sweet cherry (Shane, 2019).
However, even while the plant is dormant, its cold hardiness continues to be affected by environmental temperatures. Mark Longstroth from MSU has noted that having one day of temperatures above freezing, loses a lot of the plants cold hardiness, and after 48 hours or more of warm weather (above freezing) the plants are at their minimum cold hardiness (-12° to -17° C for peaches) (Longstroth, 2013).While tree fruit can lose much of their cold hardiness in a day or two, they are slower to reacquire cold hardiness when temperatures drop. Tree fruit only acquire 1 to 4 degrees of enhanced cold hardiness for every day below freezing (Longstroth, 2013). As a result, rapid drops in temperature can be more damaging to fruit buds than a slow decline in temperatures, especially warm temperatures precede the cold snap. In January 2022, we saw a few days of temperatures above 2 C followed by a rapid drop in temperatures on January 16 to -20° C in a matter of days, which may have resulted in some winter injury (Fig 1).
Cold injury that occurs when trees are at their maximum cold hardiness during endo-dormancy is often associated with cold clear nights when temperatures reach -25° C, and damage occurs to fruit buds, or shoots. Often the basal (base) section of shoots exhibit the least amount of damage, while the tip of the shoots are more prone to injury (internal browning). Injury to shoots can vary from tree to tree, with more damage to thin shoots, as compared to thick shoots.
During endo-dormancy, the plant will not grow even under warm, growing conditions, as something inside the plant is inhibiting growth. Fruit trees monitor chilling units to track the passage of the winter. Chilling units are hours of time spent above freezing (0° to 7 °C). The number of hours required for chilling varies for different plant species and cultivars with peaches requiring 700 to 1,500 chilling hours. During warm winters, fruit trees accumulate their chilling hours earlier then in long cold winters, and as a result they start the de-acclimation process sooner. Temperatures below freezing have no effect on chilling hours, but will help to increase cold hardiness in plants provided they are not at their maximum cold hardiness. Once the required amount of chilling hours have been reached (January or February), the trees move from endo-dormancy into eco-dormancy. If warm weather occurs before the plant completes its chilling requirement, no growth occurs.
Once chilling hours have been met, the eco-dormancy phase starts and fruit trees start the gradual process of de-acclimation. During eco-dormancy peach trees are only dormant because of the cold temperatures. As the temperatures increase during eco-dormancy the fruit trees gradually lose their ability to readjust to colder temperatures and they are more vulnerable to cold temperatures. Throughout eco-dormancy there is here is a slow progression of development towards bud swell and green tissue. Although it may not be visible to the naked eye, fruit trees actually begin growing long before the buds start swelling.
As buds and blooms begin to swell it is important to watch the weather conditions and forecast for frost. There is considerable variability in susceptibility to freezing between crops, cultivars, and stages of development (Table 1). As flowers begin to swell and expand into blossoms, they become less resistant to freeze injury. Buds that develop slowly tend to be more resistant to cold temperatures, resulting in variability in cold tolerance between buds.
The table below shows the average temperatures required to kill 10 percent and 90 percent of buds. Keep in mind that weather conditions preceding cold nights can affect bud hardiness. Prolonged cool weather tends to increase bud hardiness during the early stages of bud development. With forecasted temperatures of -2° C for tonight, growers should be monitoring weather conditions for their impact on crops and using frost mitigation practices as needed.
Table 1.0 Critical Spring Temperatures for Tree Fruit and Small Fruit Bud Stages
Compiled by Mark Longstroth, District Extension Horticulture Agent, MSU Extension – Temperatures in ˚Celsius
Old standard temperature is the lowest temperature that can be endured for 30 minutes without damage.
This chart also shows the temperature that will kill 10 % and 90 % of normal fruit buds.
These numbers were taken from Washington (WSU), Michigan (MSU) and North Carolina (NCS) Extension Bulletins. Apple – WSU EB0913, Pears – WSU EB0978, Sweet Cherries – WSU EB11-2.2, Peaches – WSU EB0914, Apricots – WSU EB1-4.40, Tart Cherries – MSU Research. Rpt. 220,
Portions of these bulletins are posted at Gregg Lang’s Fruit Bud Hardiness Page at the MSU Horticulture Department
Varietal and rootstock variances on cold hardiness
Research conducted by Dr. Ionnis Minas from Colorado State has shown variability in different cultivars tolerance to cold injury. Some cultivars may be slow to acclimate in the fall, making them more vulnerable to sudden drops in temperature in October or November. Other cultivars are vulnerable to cold temperatures during the mid-winter season, but are more hardy pre-bloom as they have a delayed bloom (ie. Galaxy Donut, New Haven, Glowingstar, Starfire, Blushingstar, O’Henry, PF-24C). Some peach varieties that are more tolerant of cold mid- winter temperatures are prone to blooming early making them more vulnerable to spring frosts (PF-23, Suncrest, PF-19007, Glohaven and Redhaven). Dr. Ionnis Minas has also shown that rootstocks also have a significant impact on the cold hardiness of peach trees. The least cold hardy peach rootstocks include Atlas and Krymsk 1. Guardian and Lovell rootstocks are quick to acclimate in the fall, but also de-acclimate quickly in the spring increasing their susceptibility to spring cold events. Krymsk 86 is late to de-acclimate in the spring, but is slow to acclimate in the fall, making it more vulnerable to cold temperatures in the fall (Minas, 2020).
Dr. Bill Shane, MSU evaluated the internal cold injury on 1 year old wood in peach trees affected by the polar vortex in January 2019 (Shane, 2019). His study found that peach cultivars varied in the amount of cold injury to shoots.
Varieties with little damage to wood included: Rising Star, Desiree, Red Star and PF 11-nectarine. Variteties that had some damage to wood included: PF-11 peach, PF 8 Ball, Redhaven, Coralstar. Varieties with more damage to wood included: HW109, Harrow Diamond, Allstar, Blushingstar, CrestHaven and Fantasia. While Earlystar and PF23 had significant cold injury to the wood.
Assessing Cold injury in peaches
For simple fruit buds (peaches), damage to fruit buds can be assessed by using a razor blade to cut fruit buds in cross section. Cold injured buds will have a discoloured centre indicating that the bud is dead. In same cases, mid-winter cold injury may be obvious in the spring as they fail to swell and develop and simply dry up and fall off the tree. Cold injury tends to be more severe lower on the tree. Fruit trees typically have an excess of flowers, especially peaches and apples that require only a small portion of the blossoms to set to have an adequate crop.
For fruit trees that have buds with multiple flowers and leaves (pear, plum and cherry) the easiest method of determining the viability in early spring is to place the branches in water for a week or so to allow the buds to swell, before cutting to look for individual flowers with discoloured centres.
The impact of cold temperatures on tender fruit trees varies depending on stage of crop development, weather conditions, variety/rootstock and production practices (crop load, harvest time). It is important to monitor weather conditions and check tree fruit buds for cold/frost injury as injury can have a significant impact on pruning and production decisions.
Longstroth, M. 2013. Winter cold hardiness in Michigan fruit crops – MSU Extension
Shane, W. 2019.Cold damage to peaches – Peaches (msu.edu)
Minas, I. 2021. Presentation at OFVC.
Shane, 2019. PowerPoint Presentation (weebly.com)