How does this year compare in degree days?

Before today's snow storm, it really felt like spring was in the air. Some of us start to question when bud break will happen - revealing the green tissue that signals the need for fungicide protection. Maybe we can turn to degree day accumulations to give us some guidance.

I revisited the archives of the Orchard Outlook Newsletter from the past 12 years to find the approximate green tip dates reported for early varieties at the early monitoring site on Middle Dyke Road, Kentville. Jeff Franklin then calculated the degree day (DD) accumulations starting from January 1 up until the reported dates of bud break. Calculations were started on January 1 because buds have historically met their chilling requirement in the fall by the end of December. After buds meet the chilling requirement they can respond to the heat and begin to grow. 

Jeff created Table 1 below that accumulates base 5 DD from January 1 to the approximate bud break for each year. On April 1 he described to me, "As you can see, the average DD requirement for bud break is 81.9 from January 1. I am considering this [81.9] as a constant for predicting green tip in 2022. So far this year [up to April 1], we have accumulated 54 DD which would leave approximately 28 DD to go until bud break. Looking at the 14 day forecast from Weather Network, we will average only 1.5 base 5 DD per day for the next 2 weeks. Using this calculation, we are approximately 18 days away from bud break this year (according to my very simple model). I also use the table below to estimate the accuracy of my simple model, which would be +/- 4 days." Based on this logic, bud break could happen on April 18 and possibly four days before or after.

Table 1: Heating degree day accumulations for plant (above 5°C) development from January 1st to the approximate date of green tip for the past 12 seasons. Provided by Jeff Franklin (AAFC).

But Jeff says to please note, "This is based on only 12 years of generalized data, and only looks at a single predictor variable which is base 5 degree days. If you look closely at the table, there are two years that deviate considerably from the 81 degree day prediction; 2016 and 2020. I have not figured out why these years were different but it is possible that this year could be similar to those and behave similarly."

If you are also interested in the degree day accumulations from March 1, Jeff included Figure 1 below. As you can see, this season has been above-average since March 1.

Figure 1: Heating degree day accumulations for plant (above 5°C) and insect (above 10°C) development from March 1st to April 1st for the past 17 seasons. Provided by Jeff Franklin (AAFC).

• Approximately 54% more plant development heat units compared to the 5-year average, and 54% more compared to the 10-year average.
• Approximately 43% less plant development heat units compared to 2021, and 163% more compared with 2019.
• Approximately 32% more insect development heat units compared to the 5-year average, and 10% more compared to the 10-year average.

Jeff also commented that pussy willows and poplar species had bud break in mid-March and lilac buds have broken recently. These are signs of an early bud break.