After one of the warmest winters on record, we have entered a roller coaster of high and low temperatures this spring that is impacting plant development. Based on weather data collected by the Ontario Tender fruit Growers Association we are approximately one week ahead of normal. However, with the warm temperatures that are forecasted to occur this week, it may push development further ahead.
Although there is little risk of frost in the current forecast. It is important to monitor fruit tree development to assess the risk of frost injury. As most tender fruit growers know, susceptibility to frost varies between orchards, cultivars, crops and stages of development. As we are starting to move towards bloom (Figure 1) fruit buds become less resistant to freeze injury. Depending on the stage of development, freezing temperatures can result in partial/total crop loss.
Figure 1 Apricots starting to bloom in Niagara on the Lake, April 4, 2024 (K. Wiens)
Critical Temperatures for Fruit Development
A table is available summarizing the critical temperatures for tree fruit. The table indicates the average temperatures required to kill 10 percent (LT 10) and 90 percent (LT 90) of buds.
Keep in mind that cold hardiness is complex and is affected by a variety of conditions. Prolonged cool weather tends to increase bud hardiness during the early stages of bud development. The duration of the cold event also impacts the level of damage.
Types of frost events
Options for mitigating frost injury on tree fruit are impacted by the type of frost event.
Advective frosts are a widespread weather event resulting in strong, cold winds that sweep into a region in the day or night bring cold temperatures. Unfortunately, protective measures like wind machines are ineffective against advective frosts.
Radiation frosts occur when a dry, cold air mass settles over an area with minimal cloud cover and wind at night. Wind machines and other active strategies in managing frost are effective in helping safeguard crops from radiation frosts.
Mitigating frost
Passive strategies
Passive methods used well in advance of the actual freeze danger are probably the most economical and effective way of mitigating frost. Some strategies are common sense and already widely used. Following are some examples of passive methods which can be used.
Strategic site selection: Selecting an orchard site with optimal air drainage can help avoid cold air settling in low lying areas.
Cultivar optimization: Planting late blooming cultivars, or crops that are more tolerant of cold in low lying areas to minimize susceptibility to frost. The trend towards an earlier bloom in recent years, has resulted in increased research on the development of late blooming cultivars.
Strategic pruning: Delaying pruning susceptible crops (peaches) until after bloom, to allow growers to accommodate the amount of fruit left based on frost injury.
Delaying bud break chemically: Currently there aren’t any plant growth regulators registered for frost protection in tree fruit. Researchers in the US have shown that a fall application of ethylene (a plant growth regulator) can delay bloom in peaches the following spring, resulting in reduced frost risk. However, the rates required to delay bloom also resulted in injury to the trees (gummosis).
Active strategies for managing frost
Active frost protection takes place just before and during the occurrence of the frost after a warning has been issued in the weather forecast. They are usually only effective under radiative frost conditions when winds are light or calm, and are most suitable in low-lying, frost prone areas. Advective freezes usually cannot be prevented by active strategies.
Wind machines: These machines blend warmer air from above the inversion layer with cooler air near the vines, potentially raising temperatures by 1-4 °C. This method can be effective when there are large temperature differences between air layers near the surface and those up higher. Wind machines should not be used when wind speeds are above 13 km/h as there is the potential risk to damage the wind machines. Additionally, temperature inversions are minor at higher wind speeds. The use of wind machines can result in noise complaints from neighbours. Information is available on best management practices when using wind machines.
Figure 2 Wind machine in orchard
Sprinkler systems: Overhead sprinklers create an insulating layer of ice around buds and shoots, harnessing the heat released when freezing occurs to protect the buds. Protection from freezing temperatures as low as -6°C have been reported for low growing berry and vine crops, when 1.5 to 2.5 mm per hour of water was applied.
Plant-based products: Researchers in Washington state and other areas have shown that the application of cellulose nanocrystals (CNC) to fruit buds forms a thermal insulation layer, resulting in buds that are more resistant to freezing temperatures. Some research suggests that cold hardiness was improved by 2–4 °C with CNC treatment. Research continues on the use of cellulose nanocrystals on tree fruit and grapes.
Micro-organisms and proteins: Researchers in BC are currently developing a frost protection tool for plants made of naturally occurring micro-organisms and protective proteins that directly inhibits molecular ice nucleation and arrests ice crystal growth.
Conclusion
As fruit growers continue to deal with the challenges of late winter freezes and early spring frosts, it is important to have strategies in place for mitigating frost injury. Including a combination of active and passive strategies for your orchard can help prepare for the risks of late winter and early spring frosts which can have negative impacts on yields and tree health.
Resources
Critical Spring Temperatures for Tree Fruit Bud Development Stages – Fruit & Nuts (msu.edu)
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