After near 20 years of tracking all this you'd think the weather and heat accumulations would start to pattern out where you could say when and what might happen with something close to a bullseye. Not the case and just hitting the target alone seems to be a good goal.
This year we've had a few things that are a bit anomalous. The first was a late spring hard frost that hit in late April taking temperatures to -5c or more. The buds were swelling already some at wolly stage, especially on the south facing ridges, and that cold was enough to kill those buds that were emerging.
We've also seen average temperatures at the vineyard for May and June with no hot spells and quite a bit of rain. In years gone by this means flowering in the last week of June, but we are seeing lots of flowering as of June 15/16 so we are about 1 week early with good vine growth.
So why in an average year of temperatures are we seeing earlier flowering? ...Its not the average, but the daily high-low that is making the difference and here is how...
Our average daily temperature in June is about 16c. So if we have a day with a high of 26c and a night time low of 6c our average is 16c (26+6=32/2=16).
On the other hand a daily high of 22c and night low of 10c also provides an average of 16c...but here is the difference. Grape vines shut down or significantly slow thier growth at temperatures below 8c so when that occurs the vines growth is held back. This 8c is approximate and changes with grape varieties but for our hybrids this is usually.
So if night time temperatures stay above this 8c (+/-) the cellular growth continues even though its night time and the growth stages advance faster.
In most years our night time temperatures in June are lower than 8c which then limits the growth, but this year we've been having more rain and with that we see lower day time temperatures... but because of the cloud cover, weve had higher night time temperatures - keeping the temperatures often above 8c and hence the growth and earlier flowering.
Pretty cool for this to happen and to be ahead in vine development but rainy weather is not good for flowering time so now we are hoping for sunny days in the next few weeks.
