How does late-season cold weather affect the life cycle of grapevines?

How does late-season cold weather affect the life cycle of grapevines?

cold vineyard Flashback: May 1995 "Paul, how did the recent rains we read about in Northern California affect the vines and the 1995 harvest?"
– E.McG., Ventura, CA

In many places it had a profound effect. However, in order to understand that effect, you should understand how the vine's cycle works. The yearly cycle begins early in spring, usually the first week of March. This year it came in February, before the last rainstorm. Because of this early warmth, the sap stored in the vine's root system rises through the vine trunk and upward into its branches, called "canes." On each cane are many tiny nodules, called "buds," from which new growth is generated.

By controlling the number of buds on each cane through pruning, the viticulturist can control the number of grape clusters generated. This will affect the final yield of the grapes. To grow great Chardonnay grapes, it is generally thought that a yield of 2.5 to 3 tons per acre is desirable. Only by eliminating grape clusters is this possible. If left alone, the vineyard would produce 7 to 9 tons per acre of very ordinary, flavorless grapes from which you would make ordinary, flavorless wine. The root system can only provide so many nutrients to the grape clusters. Generally speaking, each cluster will get its share as long as there aren't too many other clusters on the vine in competition with it.

The sap rises, and the pressure increases. This forces the buds to swell until their protective covers split and the first tiny leaves and floral clusters emerge. This is called "bud break." The "shoot" of new growth containing the first leaf, the floral cluster, and additional tiny leaves then grows at a very fast pace, as much as six inches per day, forming new canes. Each cane, with its supporting leaves, will bear 1 to 2 bunches of grapes.

This is the most critical cycle for the vine. At this point the vine "thinks" it's time to produce a flower and the resulting fruit. That's when the rains came. The vines froze up and dropped many of those clusters. Then, just a couple weeks later, it warmed up and new clusters began to emerge. The biggest problem in this situation is that you have a plant with one set of flowers, which will eventually turn into grape clusters, that are two to three weeks ahead of a new set of flowers, which will also turn into grape clusters. Which ones do you pick? It's impossible to tell the early ones from the late ones six months later when it's time to harvest. If you test the earlier grapes, and they're ready, the others on the same vine are not. They will be too acidic. If you wait for the later crop, the earlier ones will be over-ripe and lose their acid.

The solution is to go through the entire vineyard right after the new crop appears and cut all the new growth off the vine. That way there's no guessing which grapes were early and which were late. Unfortunately, as you can imagine, the cost of this procedure is enormous and, until the wines are released, we won't know which winemakers bit the bullet and spent the money and which didn't.

Suffice to say that we will have our work cut out for us when evaluating the wines from the 1995 harvest. This isn't to say that there won't be any good or even great wines. There certainly will be. It will just take a bit more work to find them.