Critical Temperatures for Tender fruit April 17, 2023

By Kathryn Carter, Fruit Specialist (Tender fruit and Grape), OMAFRA

The 2023 growing season is off to a quick start for tree fruit growers due to the warm temperatures (27° C) we had last week. With some tree fruit crops in full bloom (Apricots, early nectarines, golden plums) in many areas, and overnight temperatures dropping to 1° C and 3 °C this week, there are concerns about the risk of frost.

Apricots in bloom

Frost Susceptibility

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. Some buds are killed at higher temperatures, while others are resistant at much lower temperatures. Table 1 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 1: Critical Spring Temperatures for Tree Fruit

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

https://www.canr.msu.edu/resources/picture-table-critical-spring-temperatures-for-tree-fruit-bud-development-stages

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.

Mitigating Frost Injury

Using wind machines is one of the most common methods of protecting tree fruit from cold injury. Wind machines are most effective when there is a strong temperature inversion; when air temperatures 20 metres above the field are at least 3˚C warmer than at crop level. When using wind machines set start-up air temperatures as close as practical to expected critical air temperatures (2°C–3°C for spring frost). Set the wind machine stop temperature 2°C–3°C higher on wind machines than for start-up temperatures. If you are planning on using wind machines consider letting your neighbours know in advance to avoid noise complaints. For growers that don’t have wind machines there is some interesting research being done on the use of cellulose nanocrystals to insulate crops against frost damage. While these products are not commercially available yet, they may be a valuable to tree fruit and grape growers in the future.

• Frost mitigation factsheet
• Frost mitigation strategies article
• Wind Machines for minimizing cold injury to horticultural crops

Assessing Frost injury

After a frost event, it is important to evaluate fruit trees for frost/cold damage to determine future crop protection efforts and thinning programs. It takes several hours for symptoms of frost damage to develop, and as the frozen tissue warms up it will appear black or brown showing the extent of the damage. It is best to evaluate damage by collecting shoots with flower buds and bring them indoors to warm up. Looking at buds in the field can give you a “feel” for the damage, but it is much more objective and accurate to determine exact levels of damage inside, with a good light and magnification if necessary. For additional information on assessing frost injury see frost bud assessments.