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 | |||||
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.
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