As I write this, the temperature is 29 degrees and it is snowing like crazy outside. Yet at this time yesterday it was 57 and we were in the middle of a thunderstorm! This has been the pattern over the last few weeks. And we found out today that this February was the warmest on record!
So the questions continue: “What is this doing to the fruit trees?” Well it’s confusing to us for sure, and I’m guessing that the trees are mixed up a little too. In looking at the apple buds day after day, I can see that they have broken out of the dormant stage. The buds on some varieties are in fact quite swollen. If we remain cold for a while the growth should come to a standstill. But with the forecast reaching the upper 50’s for the weekend again, bud development will inch forward again. So how do we know where we should be in a normal weather year? And how do we compare that to where we are now? The answer lies in calculating “degree days”.
In an earlier blog I talked about chilling hours, and how important they are to the fruit growing process. You may recall that calculating chilling hours tells us how far along in the dormancy stage the trees are. But we also calculate “degree days”, and that tells us how the trees are progressing in the growing season. Figuring out degree days is done using a couple different models. For simplicity’s sake we will use the easiest way which is done by adding the low temperature of the day to the high and dividing by two.
Apple trees grow at 42 degrees and warmer. If the average temperature for the day is above 42, you begin counting degree days. So if the low for the day is 35, and the high is 55, the average is 45 degrees. That gives you 3 degree days for that day (3 degrees above 42). Each day is calculated separately and added to the total.
With years of data in the books we can estimate the number of degree days at which fruit buds are in different stages of growth. For instance, McIntosh buds first start showing green at around 127 degree days.
So what does all of this mean? Well as of today we have accumulated around 57 degree days. While that is quite far ahead of normal for March 1, we won’t be adding any more for a few days here with the cold weather. But the forecast will add some more this weekend and we will keep adding up the numbers as the days go by. Hopefully we will just creep along over the next weeks and not really blow up our temperatures like we did in 2012. This early warmth will probably move our season ahead of normal some. How much? I guess we will know when we get there! But at this point a week or two earlier than normal is probably a reasonable estimate. But remember, this is Michigan. It could be still snowing in April!
Have a fruitful week!