For organisms that don’t regulate their temperature (such as plants and insects), the rate of development is largely controlled by the temperature of their environment. As we know, fruit trees and vines need a certain amount of “winter chilling” that they use to measure when winter is over. After the chilling period, plant development depends on temperature. Fruit plants in Indiana are primed and ready to grow – right now they are just waiting for suitable temperatures. Waiting, and waiting and waiting. Typically very little plant development occurs below 50F so we measure heat accumulation above 50F (Growing Degree Days) to predict the rate of plant development.
As we see from Figure 1, this year (solid black line) in Lafayette we have accumulated very few GDD, and those derived from a few warm days at the end of February. We have not accumulated any GDD since March 1, but with a couple of warm days upon us now this is poised to changed pretty quickly. So far, 2018 is shaping up to be one of the latest springs this decade, but temperatures are still running ahead of the long term trend.
When we talk about crop development, we generally talk about developmental stage of the crop and about Growing Degree Days, rather than calendar date. An example of this is Figure 2. When we realize that early crop development is driven by temperature, then we see how different temperatures can be from one year to the next (Fig. 1), then we realize why we talk about bud stages rather than calendar date. Another stark example of this is in Figure 3. On the left are photos taken in 2017 and on the right are photos taken this year – similar calendar date but very different stage of crop development.
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- Figure 1. Accumulation of Growing Degree Days (base 50F) in Lafayette, IN.
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- Figure 2. Diagram of apple bud stages. From “Managing pests in home fruit plantings”. Purdue Extension publication ID146
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- Figure 3. A comparison of apple development in 2017 and 2018.
