Apples Pears

Frost Warnings and Tools for Frost Protection in Apples and Pears

by: Amanda Green, Tree Fruit Specialist, OMAFRA

Low nightly temperatures are expected for the rest of the week. There are frost warnings for many areas of Southwestern Ontario tonight as the forecast will be down to 0 to -1˚C tonight, 0 to -2˚C on Thursday in many areas of Western and Central Ontario. Temperatures are expected to be the lowest on Friday night with the temperature lows forecasted to be -2˚C from Essex to Middlesex Counties, -3 to -4˚C from Elgin to Waterloo, Niagara and along Lake Ontario, up to Ottawa and -6˚C in the Georgian Bay area. Please make sure to check your local forecast. Critical temperatures for frost damage in apples and pears can be found in Table 1 and 2, respectively. Green tip occurred earlier than normal, the cool temperatures through April have slowed down bud progression but many areas in Ontario may have their crop at a vulnerable stage for these temperatures that are forecasted for this week.  There are fairly effective methods available to protect your crop against frost listed below if temperatures are a few degrees below critical temperatures (Table 1 and 2) and/or an inversion temperature layer is present. The most effective tools for frost protection require the most infrastructure and many acres are not protected as it requires a significant investment. You may be looking at alternative options that may help protect your orchard from frost, like spray products that may provide frost protection or be used as a rescue spray. Descriptions of these products and preliminary trial results will also be covered in this article.

Table 1: Critical Spring Temperatures in ˚C for Apples (adapted from Washington State University Extension Bulletin WSU EB0913)

Silver Tip
Green Tip
1/2 Inch Green
Tight Cluster
First Pink
Full Pink
First Bloom
Full Bloom
Post Bloom
10% kill
90% kill

Table 2: Critical Spring Temperatures in ˚C for Pears (adapted from Washington State University Extension Bulletin WSU EB0978)

Bud Swell
Bud Burst
Tight Cluster
First White
Full White
First Bloom
Full Bloom
Post Bloom
10% kill
90% kill

Most Effective Tools

Wind Machines

Wind machines are very effective at providing protection when there is an inversion layer on still nights by mixing the warmer air on top with the colder air below. Temperature inversions can be up to a 5-10˚ C difference between ground level and 20 m.  For more information on using wind machines please see the factsheet .

Overhead Irrigation

Overhead irrigation (Figure 1) has been an effective frost protection method used on strawberries in Ontario and in apples in Washington State but is not commonly practiced in Ontario for apples. Overhead irrigation works to protect the crop by latent heat releasing when water changes state from liquid to solid. This works similarly to how steam can burn your hand as it condenses into a liquid. You have to be cautious with overhead irrigation as you need to make sure you can deliver enough water for the entire time, until ice begins to melt. The amount of water that needs to applied per hour per acre depends on the wind speed and temperature. For more information on overhead irrigation for frost protection please see this factsheet.

an overhead irrigation sprinkler that can be used for frost protection
Figure 1. Overhead irrigation for frost protection

Last Resorts


Helicopters work similarly as wind machines with mixing the air when there is an inversion layer resulting in warming the air down by the crop. The set back to using helicopters is that they are expensive and cost hundreds of dollars per hour to rent.

Under-the-Tree Irrigation

Another method to protecting your crop with irrigation is under the row irrigation. This would work similarly to overhead irrigation in that as the water changes state from liquid to solid, latent heat is released. It may not be as effective as overhead but there is less risk of freezing your crop if you run out of water. Even having wet soil from irrigating your orchard the day before a frost event may help.

Orchard Heaters or Fires

Orchard heaters or fires can be used to try to raise the temperature in the area around the crop. This can be fairly inefficient as the heat will rise straight up and may have little effect on raising the temperature around the crop.

Spray Products

Promalin can be used in apples as a frost rescue spray by promoting parthenocarpic (fruit with no seeds). It can improve fruit set when flowers are damaged but not completely killed by frost. The downside to parthenocarpic fruit is that they don’t size as well and will have a limited shelf life but some fruit in this condition is better than no fruit at all, especially if you have an on-farm market.

After a frost event, Promalin can be applied from pink to petal fall within 24 hours after a frost event with a single application of 1.2 to 2.3 L of product per hectare. Do not apply to frozen foliage, blossoms or developing fruit. Allow trees to completely thaw prior to application. Do not use a surfactant. If you are unable to apply Promalin within 24 hours of a frost event, apply Promalin as soon as possible as you may still get the frost rescue benefits up to 6 days after the frost event, according to Steve McArtney who presented this data in his presentation at a spring thinning meeting in 2019.

Pristine fungicide (Boscalid and Pyraclostrobin) works by inhibiting the mitochondrial breathing chain at the level of the b/ccomplex for improving the tolerance of plants to low temperatures (Rademacher et al. 2009). It is sold as a fungicide but may help with frost protection. When tested in the field by Dr. David Rosenberger et al. from Cornell, the seed count was higher in trees treated with pristine but there was no yield benefit to using Pristine. For more information on this trial please see:

Zinc and Copper work by inhibiting ice-nucleating bacteria and can be applied the night before. Water melts at 0˚C but does not necessarily freeze at 0˚C (Snyder and de Melo-Abreu, 2005). If water is homogenously pure it won’t freeze until very low temperatures (Snyder and de Melo-Abreu, 2005). With foreign particles like ice-nucleating bacteria, ice can start forming at higher temperatures below zero (Snyder and de Melo-Abreu, 2005). At temperatures above -5˚C, ice-nucleation bacteria cause ice formation on plant surfaces; damage is more likely when the concentration of ice-nucleation bacteria is high (Snyder and de Melo-Abreu, 2005). The theory behind applying zinc and copper is to inhibit the bacteria and will prevent ice from forming at temperatures just below zero (Snyder and de Melo-Abreu, 2005). There is mixed and limited evidence on the effectiveness of using zinc and copper.

Preliminary Product Spray Trial for Frost Protection

In May, 2017, there was a frost forecasted to occur in apple growing areas in western Ontario further from the Great Lakes. This frost was forecasted within enough days to acquire product and set up a trial on products that may provide protection or rescue from frost. Temperatures did not go low enough to cause significant economic damage but there was enough damage to provide some preliminary research results.  The products that were trialed were:

  • Pristine fungicide (Boscalid and Pyraclostrobin) applied 2 days before frost at 1200 g/ha
  •  Promalin (6-BA and GA4+7), a plant growth regulator applied at 2.3 L/ha the morning after  frost 
  • A combination of Copper and Zinc (CuMax (4.2% Cu)and ZincMax (10.2% Zn and 0.5% B) applied in the afternoon 2 days and 1 day before frost at a rate of 2.5 L of CuMax and 1.2 L of ZincMax / ha

Sites were chosen based on areas that were expecting the lowest temperatures that were at their most vulnerable stage (Table 1). Stages of bud development and temperatures experienced at each site are found in Table 3.  

Table 3. Stage of growth and temperatures at each site

Site 1
Site 2
Stage of Growth
First Pink
May 8th
-Low of -1.5 ˚C
- Low of 0.5 ˚C
May 9th
- Low of -2 ˚C for 15 minutes
-Between -1 ˚C and -2 ˚C for 1 hour
-Low of -2.5 ˚C twice for 30 and 15 minutes
-Between -2 ˚C and -2.5 ˚C twice for 45 and 30 minutes and
- Between -1 and -2 ˚C twice for 1 hour and 1 hour 15 minutes
May 10th
- Low of 3.5 ˚C
-Low of 3 ˚C


Bud Damage

There was some damage that resulted at both sites; Site 1 had 15% damage in the untreated check and Site 2 had 2% damage in the untreated check when evaluated 4 weeks after frost (Figure 2). Both the Zinc + Copper and Promalin treatments significantly reduced the amount of bud damage to 8% and 6% respectively at Site 1. At Site 2, there was slightly more damage with the Zinc + Copper treatment at 4 % damage compared to the untreated check with 2 % damage.

Figure showing the percent bud damage in each treatment at each site. There was significantly less bud damage at site 1 with the zinc + copper treatment and the Promalin treatment
Figure 2. Percent bud damage in each treatment applied at frost, evaluated 4 weeks after frost event

Pre-Harvest Evaluation

In August, the number of fruit per pre-selected branches were counted and fruit was collected to evaluate the number of seeds and the fruit size. At Site 1 there was 2X the amount of fruit per branch with the Pristine and Zinc + Copper treatments than the untreated check and the Promalin treatments (Figure 3). There was no difference in the fruit count at Site 2. The seed count per apple was reduced with the Pristine and Promalin treatments compared to the untreated check at Site 1 by approximately 1 and 2 seeds, respectively (Figure 4). However, the opposite occurred at Site 2 with Pristine and Promalin having, on average, approximately 1 seed per fruit more than the untreated apples. Fruit diameter across the treatment were fairly similar at both sites (Figure 5), there was a slight decrease in fruit diameter with the Promalin treatment at Site1 and with the Pristine treatment at Site 2.

Graph showing the number of apples per branch in each treatment. At site 1, the Pristine and Zinc + Copper treatments had nearly twice as much apples per branch than the untreated check
Figure 3. The number of apples per branch in each treatment applied at frost, evaluated in August before harvest
Graph showing Pristine and Promalin treatments have less seeds per apple at Site 1 compared to the untreated check and more seeds per apple at site 2. The zinc and copper treatment had similar levels of seeds per fruit as the untreated check
Figure 4. Average seed count per apple in each treatment applied at frost, evaluated in August before harvest
Figure 5. Average diameter of fruit in each treatment applied at frost, evaluated in August before harvest


Overall, each treatment did show a benefit in using the product, however one needs to keep in mind that conditions for damage were not very severe (Table 3). Also, this is only one year’s worth of data. Promalin treated fruitlets showed the least percent damage 4 weeks after frost but the number of fruit per branch in August was similar to the untreated check; damaged fruitlets could have already dropped by the time of bud damage evaluation which occurred 4 weeks after the frost event. Pristine had the highest number of fruit per branch but lower seed count per fruit at Site 1.  The Zinc+ Copper treatment had less bud damage, more apples per branch and unaffected average fruit seed count and fruit diameter compared to the untreated check.


Special thanks to the two grower co-operators who allowed a block of apples to be trialed on and applied all of the treatments.

Also thanks to the companies who donated the products to be trialed: Valent BioSciences, BASF and NutriAg


Snyder, R. L., and J. P. de Melo-Abreu, 2005. Chapter 6 :Passive Protection Methods in Frost protection: fundamentals, practice and economics. In Frost Protection: fundamentals, practice and economics, Vol. 1. FAO

Rademacher, W., H. Köhle and V. Ulstad,  2009. Method for improving the tolerance of plants to chilling temperatures and/or frost. U.S. Patent Application No. 12/281,926.

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