The March of Spring – 2021 Update

Growing Degree Days

The objective of this article is to provide an update on the progression of the growing season, and how the recent warm weather may impact bloom and other early season activities in the orchard.  Plants develop and grow in response to their environment. Air temperature is the single most important environmental factor contributing to plant development. Because of this, and because data on air temperatures is readily available, apple tree bud development can be estimated using heat units — expressed as growing degree days (GDD).  The GDD concept assumes that each plant species has a specific base or threshold temperature below which growth does not occur. For fruit trees, 5˚C is widely used as the base temperature. The amount of heat accumulated during a particular day can be obtained by simply subtracting the plant’s base temperature from the mean temperature this is referred to as growing degree-day accumulation.

GDD calculations have been made over several years at the Simcoe Research Station. As of April 14, 2021, the GDD accumulation has been estimated to predict how advanced the growing season is compared to other years. This will help inform growers how this spring compares with other years, and to plan according.


As of April 14, 2021, 131 GDD have accumulated at the Simcoe Research Station, Simcoe (Figure 1). This is approximately 52% of the total GDD required for apples to reach full bloom. With the unusually warm weather this spring, the 2021 season is currently well advanced of the past five years. This is concerning news because the irreversible advancement of flower development predisposes trees to a greater risk of frost injury. Fortunately, the 2021 season is not as far advanced as 2012 – the year when we had major frost and crop loses throughout most of the province. There appears to be some good news, however; the predicted short-term trend is for cooler weather to prevail for the latter part of April. This will slow bud develop and push bloom later.

Using forecasted temperature and a growing degree day model that I’ve developed, as of April 14 (the deadline for this article) full bloom for apples should occur very close to May 14th in Simcoe (Table 1). It is worth noting that this prediction is only as good as the daily temperature data it relies on– and it is generally difficult to accurately estimate data five weeks in advance.

Table 1. Dates of full bloom for apples, Simcoe Research Station, Simcoe1

May 14 (Predicted)
May 25
May 25
May 20
May 15
May 17
May 13
May 21
May 16
April 26
1 Not cultivar specific

In summary, based on the observation using cumulative GDD, the 2020 spring growing season is shaping up to be about one week ahead of ‘normal’ by the predicted date of full bloom, well ahead of the past 3 years, but similar to 2017 (Figure 1).

Figure 1. Heat accumulation from Jan 1 – April 30 at the Simcoe Research Station, as expressed as cumulative growing degree days (base 5oC) for the 2012, 2016 – 2021 growing seasons. The data from April 14 – 30, 2021 (dashed line) has been estimated based predicted daily minimum and maximum temperatures.

Weather Resources

The following are select weather resources you may wish to investigate:

Growing Degree Resource:

Online Weather Data and Forecasting

Dr. John A. Cline
Dr. John A. Cline

Pomologist and Tree Fruit Physiologist, University of Guelph