The speed at which many of our crop plants move through their lifecycle can be tracked by the concept of Growing Degree Days (GDD).
Montana Ag Extension's pdf on Growing Degree Days is one of the best discussions of the topic we've found making it well worth the read.
A brief overview - a crop has a temperature base value which is the temperature below which its development stops. Subtracting the known temperature base value from the calculated GDD value informs the thermal time a plant has experienced. Knowing the thermal time allows the creation of a crop thermal calendar that more accurately predicts the progression of a crop through its lifecycle stages. Be consistent with either Fahrenheit or Celsius, don't mix! Power companies use the same concept for Heating Degree Days and Cooling Degree Days for estimating energy demands.
GDD is a way to compare how the season is progressing year to year. For example, my Michigan Amber winter wheat will be ready for harvesting after the same number of GDD's, not necessarily the same date in July.
As I was looking over photos from last May, I saw that my winter wheat was forming a significant number of heads on 26-May-2017. Comparing that with the progress of this year's grains, it had me wondering if our Spring might really be "late". I was looking for a quick answer, so I went to this Growing Degree Day tracker. The Michigan State University site begins their data collection in late February; perhaps that start date was driven by the snow cover? Growing Degree Days (GDD) is a measure commonly used to make predictions in the progress of plant and insect growth, you might also see it called Heat Days. In addition to the MSU site, North Dakota Agriculture Weather Network has well written guides for crops of the Plains that I find useful.
In my Internet investigation I looked at threshold temperatures, the temperature point needed for plant development to occur. Essentially, a threshold temperature is determined for each type of plant, for example, winter wheat has a threshold temperature of 32° F. I collected various threshold temperatures in degrees F (don't mix with °C!):
Whenever the average temperature is greater than the threshold temperature, plants get Growing Degree Day "points". So, if today's average temperature was 52° F, your plants earn points worth 20 Degree Days (52°- 32° = 20). Each day, the "points" accumulate and the plants progress along their developmental curve. For example, wheat needs to accumulate 3000 Degree Days for grain maturity as shown in this table from Montana State University GDD Extension Service guide:
So to answer my initial question is Spring behind this year, I need to compare 2018 to 2017. As you can see in the graph below, we accumulated very little GDD exposure until 10-April in 2018 (blue dots), while that same "Spring Point" happened around 22-March in 2017 (red dots).
The right hand axis shows the "gap" I've calculated between 2018 and 2017 in terms of days, shown as the gray dots on the graph. As of today, it looks like we're running 12 days behind last year which means plant growth is delayed.
PS Here is a 'National' site: