Reducing Frost Damage in Tree Fruit

By Kathryn Carter, Tender Fruit and Grape Specialist, OMAFRA and Erika DeBrouwer, Tree Fruit Specialist, OMAFRA

With a few days of unseasonably warm temperatures in March and early April, crop development has been pushed forward and many growers are concerned about potential frosts. The warm temperatures we experienced reminded many of the spring of 2012, where several weeks of warm temperatures resulted in an early bloom and freezing temperatures, which reduced the apple crop by 80%, peaches by 19%, pears by 57%, plums and prunes by 33%.

What is Frost Injury?

Frost damage occurs to the reproductive parts of flowers, when temperatures drop to or below 0°C. When the female parts (style or ovaries) are frozen, (appearing water-soaked and then dark), pollination and seed set cannot occur. If the male parts (stamen and pollen) are frozen, pollination and seed set might still occur if pollen from other flowers survives the cold event. Frost damage can also occur to the skin of the fruit when the outer tissues of the flower are frozen. This often appears as russeting (Figure 1 and 2), and if severe, may cause the disfiguration of the fruit. Early spur leaves can also be damaged by frost, which may affect early season crop nutrition, reaction to plant growth regulators like chemical thinners, and foliar nutrition and pesticide sprays.

When does frost injury occur?

Frost damage occurs mainly in two scenarios:

• When fruit bud development is pushed ahead of normal schedule by mild winter and/or early spring warming trends, or
• When below freezing temperatures occur in late spring after blossoms have opened in a normal sequence. Some crops may be more vulnerable to frost before the blossoms are fully open (e.g. Sweet cherries).

Frost damage should not be confused with winter injury, which happens from extreme cold or fluctuating winter temperatures during the dormant season.

What is the risk of frost in 2021?

Current degree day calculations (Table 1) put this season approximately 1 to 2 weeks ahead of normal for tender fruit, and one week early for apples. Although temperatures have cooled this week, tender fruit bloom has already started in some areas of the province (Niagara-on-the-Lake and Beamsville), and a few days of warmer temperatures in other areas will be pushing fruit trees to bloom as well.  As a result of the early bloom tree fruit are more susceptible to frost damage. The last spring frost can occur in late April through mid-May (Table 2) so crops will be vulnerable until then.

Table 1: Growing Degree Days (base temperature 5ºC) across Tree Growing areas in Ontario¹

	Harrow	London	Goderich	Collingwood	Guelph	Vineland	Oshawa	Ottawa
2021	141	133	132	128	93	83	87	96
2020	68	49	36	36	28	76	30	17
2019	64	39	30	24	17	40	22	4
2018	37	22	18	18	11	34	4	1
2017	119	77	56	48	51	91	34	19
2016	80	57	34	34	34	67	22	11
2015	40	23	25	28	0	44	22	28
2012	175	151	129	127	109	121	106	88

Table 2: Last Frosts across Tree Growing areas in Ontario

	Harrow	London	Goderich	Collingwood	Guelph	Vineland	Oshawa	Ottawa
2020	13-May	13-May	12-May	13-May	13-May	13-May	13-May	12-May
2019	01-Apr	29-Apr	11-May	29-Apr	29-Apr	29-Apr	29-Apr	29-Apr
2018	29-Apr	30-Apr	30-Apr	30-Apr	30-Apr	30-Apr	30-Apr	23-Apr
2017	23-Apr	9-May	8-May	9-May	9-May	9-May	9-May	9-May
2016	15-Apr	14-Apr	8-May	10-May	16-May	15-Apr	9-May	25-Apr
2015	24-Apr	23-May	23-May	23-May	23-May	23-May	23-May	25-Apr
2012	29-Apr	29-Apr	29-Apr	29-Apr	29-Apr	29-Apr	29-Apr	30-Apr

Types of Frosts

Frost occurs when the air temperature between 1.25 and 2.0 m above soil level is at or below 0ºC. When a frost occurs, water within the plants may or may not freeze due to several natural avoidance factors, such as supercooling and concentration of ice nucleating bacteria. Freezing occurs when the extracellular water within the plant changes from liquid ice, which can damage plant tissue. Freezing injury in plants is irreversible and causes physiological changes leading to death or malfunction of plant cells. Damaging plant tissue air temperatures, termed “critical temperatures”, are used by growers to better prepare for potential freezing events. Temperatures normally fall below 0ºC when reductions in heat content within the air near the surface occurs, which can be caused by radiation frost, advection frost or a combination of both processes.

Radiation Frost

This type of frost occurs when the upper soil stored heat is radiated back into the atmosphere at night at an energy loss. Winds are normally low and an inversion develops, causing a drop in ground temperatures (at or below freezing) while temperature increases as height increases above the ground.

Advection Frost

Advection frost occurs when a large system of cold air moves into the region from polar and arctic regions, normally accompanied by winds > 8 km/h. At the surface, temperatures are below freezing and continue to drop with elevation. There is no inversion in advection frosts.

Monitoring Weather for Frost

Monitoring weather conditions for potential frosts is important in preventing frost injury. Publicly available weather data can be found at Environment Canada, The Weather Network and Weather Underground. Keep in mind that temperatures can vary considerably from location to location and even within an orchard. As a result, having weather stations on site can provide valuable information that can help make informed decisions about the impact of weather on crops.

Susceptibility to Frost

When buds and bloom are present, watch the weather conditions and forecast for frost. There is considerable variability in susceptibility to freezing between trees, cultivars, crops and stages of development.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. As a result, some buds are usually killed at higher temperatures, while others are resistant at much lower temperatures. Table 3 shows the average temperatures required to kill 10% and 90% 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. In addition, these critical temperatures are more indicative of damage that may occur following during a radiation frost. As a result, in some situations where advective frosts occur there may see more damage than predicted.

Table 3: Critical Spring Temperatures for Tree Fruit and Small Fruit Bud Stages
Compiled by Mark Longstroth, District Extension Horticulture Agent, MSU Extension – Temperatures in ˚Celsius

Pome Fruit
Apples	Silver Tip	Green Tip	½ “ green	Tight Cluster	First Pink	Full Pink	First Bloom	Full Bloom	Post Bloom
Old temp 10% kill 90% kill	-8.9  -9.4  -16.7	-8.9  -7.8 -12.2	-5.6 -5.0 -9.4	-2.8 -2.8 -6.1	-2.8 -2.2 -4.4	-2.2 -2.2 -3.9	-2.2 -2.2 -3.9	-1.7 -2.2 -3.9	-1.7 -2.2 -3.9
Pears	Bud Swell	Bud Burst	Tight cluster	First White	Full White	First Bloom	Full Bloom	Post Bloom
Old temp 10% kill 90% kill	-7.8  -9.4  -17.8	-5.0   -6.7  -14.4	-4.4 -4.4 -9.4	-2.2 -3.9 -7.2	-1.7 -3.3 -5.6	-1.7 -2.8 -5.0	-1.7 -2.2 -4.4	-1.1 -2.2 -4.4
Stone Fruit
Apricots	Bud Swell	Bud Burst	Red Tip	First White	First Bloom	Full Bloom	In the Shuck	Green Fruit
Old temp 10% kill 90% kill	— -9.4 —	-5.0  -6.7 -17.8	—   -5.6  -12.8	-3.9 -4.4 -10.0	— -3.9 -7.2	-2.2 -2.8 -5.6	— -2.8 -4.4	-0.6 -2.2 -3.9
Peaches	Bud Swell	Calyx Green	Calyx Red		First Pink	First Bloom	Full Bloom	Post Bloom
Old temp 10% kill 90% kill	-5.0  -7.8  -17.2	—  -6.1  -15.0	—  -5.0  -12.8		-3.9 -3.9 -9.4	— -3.3 -6.1	-2.8 -2.8 -4.4	-1.1 -2.2 -3.9
European Plums	Bud Swell	Side White	Tip Green	Tight Cluster	First White	First Bloom	Full Bloom	Post Bloom
Old temp 10% kill 90% kill	— -10.0 -17.8	— -8.3  -16.1	—  -6.7  -13.9	— -4.4 -8.9	-5.0 -3.3 -5.6	-2.8 -2.8 -5.0	-2.8 -2.2 -5.0	-1.1 -2.2 -5.0
Sweet Cherries	Bud Swell	Side Green	Green Tip	Tight Cluster	Open Cluster	First White	First Bloom	Full Bloom	Post Bloom
Old temp 10% kill 90% kill	-5.0 -8.3  -15.0	-5.0 -5.6  -12.8	-3.9 -3.9 -10.0	-2.2 -3.3 -8.3	-2.2 -2.8 -6.1	-1.7 -2.8 -4.4	-1.7 -2.2 -3.9	-1.7 -2.2 -3.9	-1.1 -2.2 -3.9
Tart Cherries	Bud Swell	Side Green	Green Tip	Tight Cluster	Open Cluster	First White	First Bloom	Full Bloom
10% kill 90% kill	-9.4  -17.8	-4.4 -12.2	-3.3 -5.6	-3.3 -4.4	-2.2 -4.4	-2.2 -4.4	-2.2 -4.4	-2.2 -4.4

Mitigating Frost Injury

Passive frost prevention strategies include site selection, soil conditions, soil water content, soil texture and ground cover management. There is an excellent summary of passive frost prevention strategies in Frost, Critical Temperatures and Frost Protection (Crasswelller, 2021).

There are a variety of active strategies to mitigate the risk of frost injury in orchards. Wind machines (frost fans) the most common tool for mitigating frost in Ontario orchards. Wind machines (Figure 3) are tall, fixed-in-place, fans that pull warm air down from at least 15 m above ground during strong temperature inversions, blowing it down and out, pushing away and replacing cold air near target crops. This raises air temperatures around cold-sensitive perennial crops such as tender fruit, apples and grapes.  Wind machines are less effective in providing frost protection in advective frosts and can’t be used under high wind conditions.  Wind machines can cover relatively large acreage (1 wind machine for 15 acres). Wind machines range in cost roughly between $150 to $400 per acre with a lifespan of 25 years. Wind machines often require cement pads and fuel tanks, which have been included in the cost. For more information on using wind machines please see the OMAFRA factsheet.

Overhead irrigation is another effective tool in mitigating frost injury. When water from sprinklers turns to ice, the heat released protects the plant from injury. As long as a thin layer of water is present, on the bloom or on the ice, the blossom is protected.  Overhead irrigation requires access to large quantities of water, and water must be continuously applied to prevent freezing, requiring constant monitoring.

Applications of certain protectants, such as zinc and Pristine could provide some frost mitigation, yet limited evidence has been shown in the field. (http://www.omafra.gov.on.ca/english/crops/hort/news/orchnews/2017/on-0417a1.htm)

Resources

Weather Risks: Strategies to Mitigate the Risk of Frost (gov.on.ca)

Frost, Critical Temperatures, and Frost Protection (psu.edu)

Frost Protection Tools (gov.on.ca)

Frost Protection Methods in Michigan-Costs and Considerations

Wind Machines for Minimizing Cold Injury to Horticultural Crops (gov.on.ca)

Frost protection: fundamentals, practice, and economics – Volume 1 (fao.org)