Freezing temperatures, changing winds and snowfall have been factors testing the resiliency of apple buds across Ontario in 2021. Apples have been staged from ½ inch green to early pink as of April 23, displaying variances in frost susceptibility and potential kill percentage (Table 1). The weather will continue to challenge apple growers as frost can persist until mid-May. Thankfully growth staging has slowed across the province even with higher temperatures during the day, but this may not last long.
Table 1. Critical Spring Temperatures in °C for Apples
½ inch Green
For more information on frost mitigation and assessing damage, the following resources are available:
Last frost dates give us an idea of how late in the season a frost can occur, but some of the most detrimental frost crop losses occur over sequential days/nights of freezing temperatures or extremely low temperatures, and is also dependent on how far along the apple buds are.
Figure 1 shows the amount of days below freezing in Ontario in 2021, a 5-year average, 2015 and 2012 for the month of January through April. This figure demonstrates that although 2012 had comparable days below freezing in January, February, and April, it had a low amount of days below freezing in March, yet an 80% apple crop loss occurred. In 2015 a crop loss of roughly 25% occurred, where a higher amount of days below freezing occurred in March. This shows a piece of the puzzle in respect to what can be related to frost crop loss and prediction. The 2021 season seems to follow the 5-year average until April, where the number of days below freezing is minimal.
Figure 2 illustrates minimum temperatures in 2021, the 5-year average, 2015 and 2012 within Ontario. 2015 consisted of constantly low temperatures throughout January, February and March, which could err to the decrease in crop production (25% loss). A comparable January, March and April, with the exemption of having warmer temperatures in February, 2012 showed regular minimum temperatures. 2021 is shows consistent minimum temperatures in comparison with the 5-year average.
Staging, Temperature & Crop Production
As we know, there is never one answer for issues in crop production. Below, Figure 3 combines temperature information in relation to crop production and bud break. Last frost date and bud break follow the vertical Julian Day axis, while the last frost temperature follows the temperature vertical axis. When comparing bud break to last frost date, it seems that a larger gap between the two decreases apple production, which would relate to lower resiliency as apple buds progress in staging (Table 1). 2012 had an early bud break date (Julian Day 79) with a late last frost date (Julian Day 119) and low last frost temperature (-3.5), all of which played a significant role in the major drop in crop production. 2015 had a large gap between bud break (day 109) and last frost date (Julian Day 136) as well, with a last frost temperature of -0.76˚C. Crop production in 2015 was decreased to 63%. In 2021, we are on trend with 2017 in relation to bud break timing and last frost date (thus far) where there was a decrease in crop production, representing only 74% of the average crop produced.
Looking back at previous years, temperature has become more erratic – which has been commonly associated with climate change. Frost dates continue to occur until mid-May, yet spring warming has started to move earlier within the season, allowing trees to come out of dormancy and advance their growth staging. By keeping track of bud break dates, analyzing historical frost dates and temperatures, along with marketable yields, and contrasting them to the current year, growers will have a better idea of what is to come. Every orchard is different and the more precise your data is, the better you can address issues on your site.