The vine's annual cycle has some very interesting stages that farmers pay particular attention to. If you roll back the time to post-harvest, the vines enter a dormant period until budbreak when the shoots first emerge in the spring. One lesser-known fact is that the fruitfulness of a grapevine is determined early the previous summer as the shoots are lengthening. As the green shoot tissue matures, it lays down the "primordia" of the next year's clusters. The weather and vine canopy shading issues can greatly effect the relative success of fruitfulness.
In the photo, you can see the new flower clusters before they bloom. I always thought it was interesting that they look like little grapes before they bloom. As I walked the vineyard, it was apparent that we must have had ideal conditions LAST year to get so many clusters on the vine -- at least with Cabernet Sauvignon in the Coombsville area of Napa Valley.
This would be the third measure of the upcoming vintage, the first being at pruning when the number of buds is determined. Next comes budbreak and the relative health and number of buds that push selectively. With these healthy cluster counts, now we have the biggie: bloom. How many berries will actually set? If it's too hot, too cold, too rainy, etc., the berries will just blow off which is known as "shatter." A little shatter is good, by lowering the berry counts and opening up the clusters. A lot of shatter can be disastrous with even just a few berries on the cluster and some serious questions about what is worth harvesting.
One interesting thing is the fact that bloom roughly coincides with next year's primordia being formed on those same shoots and similar issues effect the success of both processes. Excessive shading generally lowers the success of both so it's not hard to imagine a bad feedback loop being created by inattention at critical times. Some of the science in the last 20-30 years has really helped us grow better grapes and is one reason that quality has really improved overall in the wine business.
If grape growing sounds like a high wire act, I would concur. The funny thing is, sometimes the vines really put out under some amazingly difficult conditions. Sometimes they remind me of tough old weeds but it never lasts long. There's always another threat to a sustainable grape growing venture around every corner.