#36: Dormancy to Bud Break
April 2024.
Let’s take a break from the History of Wine and chat a bit about what’s been happening in our local vineyards.
The Innate Intelligence of the Vine
By this time of year there have been any number of developments in our vineyards since our last harvest, changes made both by man and by nature. We have pruned the vineyards to cut back the unwanted growth and isolated those perfectly positioned buds that will grow into shoots. Those shoots will eventually give birth to our grapes which we will harvest and make into wine. Since I’ve written already about pruning (March of 2021) I’ll not repeat that information, but think of pruning as making absolute order out of seeming chaos.
Most grape farmers will wait to prune until the approach of spring as early pruning will set the stage for an earlier bud break which can be dangerous. If that new growth of delicate green tissue is exposed to an unexpected spring frost, the moisture inside the tissue can freeze and expand, destroying that bud for the season. (Most folks can relate to prematurely planting tomatoes only to head back to the nursery a week or so later to replace the dead plants….)
The latest research has proven that grapevines were first domesticated by humans around 11,000 years ago. Since that time, both today’s humans and our grapevines have had to evolve in order to survive. The vines have developed an intuitive intelligence over the millennia to create a rest vs growth timetable which will vary depending on variety and climate. We may marvel at the vigor and beauty of the vines during the gorgeous spring and summer months, but it is our fall and winter periods that are all-important to the production of quality grapes.
To shield themselves from winter’s severity, grapevines begin their preparation for the winter well before those cold months arrive. As the summer days get shorter, grapevines inherently know to begin lowering their metabolism. As the evening fall temperatures drop below 50F, the buds will harden and become dormant, the vines will shut down any growth and they begin to store water and nutrients in their trunks. At that point the grapevine begins registering only the number of days and hours the vine will experience ideal winter temperatures below 50F but above 32F. Once the vines have accumulated that prescribed amount of rest time, the vine is triggered to “awaken” out of dormancy and the buds will begin swelling.
Once the vines awaken (March to April) those growing buds begin their growth cycle, swelling up and producing leaves which we call “bud break” or “bud burst”. This is a delicate time as temperatures can still drop precipitously and that new green tissue could be exposed to frost. Within a month, the new growth will produce pencil-thin shoots that will grow into long canes.
In April or May what appears to be a cluster of tiny oval flowers will begin on the young shoots, usually opposite the third to sixth leaf. Wine grapes are “perfect plants” (hermaphrodites) so each of those little flowers has both male and female parts to self-pollinate, with each of those flowers having the potential to develop into a wine grape. Rain, excessive heat, cold or wind can play havoc with the success of pollination and the un-pollinated flowers simply drop to the ground. As the warmer temperatures stabilize, the successfully pollinated flowers shed their petals and tiny green berries develop.
We do not want the self-pollination process to be 100% successful and Mother Nature steps in to control that success rate. If every flower created a berry, the hundreds of berries would make it much too crowded as the clusters developed. We consider a perfect success rate to be about 30-40%. We call that period “shatter” and, fingers crossed, our shatter degree will produce a perfect cluster of grapes. Whatever that success is (or is not) we now have “fruit set” which will give us a good idea of how promising our upcoming crop load is going to be. We then begin counting the 110 to 120 days until harvest!
As you can see, there are many events that take place from dormancy through the awakening period, eventual leading to flowering and set. Mother Nature, at any of those critical moments, can play havoc on our potential crop.
Why is Sonoma County world-renowned for growing quality wine grapes?
That is a great question that folks often ask. Luther Burbank called Sonoma County “God’s Chosen Spot on Earth” and we are very fortunate to live in this blessed area. Our fall and winter months usually deliver the perfect temperatures (average between 32F and 50F) for our vines to successfully store their needed energy during dormancy. Our rainfall here is usually fairly consistent from year to year and, as our winter heads into spring and temperatures go up, the vines begin to use that stored energy to trigger the vines to awaken for bud burst. Then, as spring heads to summer, bud burst leads to flowering, then to pollination, set and growth and eventually to maturity.
The more I study the physiology of the grapevine, the more I am in awe of the intelligence of the vine. During those cold winter months, our grapevines may look almost dead, but they are not. They are simply dormant, working very hard accumulating the necessary days and hours of ideal temperatures in preparation for their buds to begin to awaken.
Can Wine Grapes be Grown Anywhere?
Only about 2% of the earth’s surface has a climate that is perfect enough to consistently make world-class wine. The temperatures of the spring and summer months are certainly key, but the temperatures during the fall and winter months are equally important. Areas such as Minnesota have winters that remain consistently below 32F and make it virtually impossible for the vines to properly store their needed energy. On the other hand, areas where the winter temperatures consistently remain above 50F (Palm Springs for instance) those temperatures are not low enough for the vines to rest. Neither area, because of temperatures needed during dormancy, can produce great wine grapes. Of course, rainfall is absolutely crucial (nothing will grow without water) but that’s for another article.
Once again, I am simply out of space. Next time you are driving by a local vineyard, stop and take a close look at those marvelous grapevines and see what they are up to!
“Great wine requires a mad man to grow the vine, a wise man to watch over it, a lucid poet to make it, and a lover to drink it.” Salvador Dali