Everybody gets excited about bud break, veraison, and the actual harvesting of grapes, but few pay much attention to the lag phase that has been likened to the time a train takes going through a tunnel — there’s forward motion going on, but it takes place unseen until the tunnel journey is complete.

“There’s not a lot of active growth during this time and you don’t see a lot of activity, but a lot of other things are happening within the berry itself,” says Larry Bettiga, a University of California Cooperative Extension Viticulture Farm Advisor in Salinas.

“At the beginning of the whole maturation process, there’s a lot of things going on from a fruit chemistry point of view as berries kick-start things like color development and softening,” he says.

“Actual growth happens earlier in Phase One (lots of cell division) and then later in Phase Three, so there’s not a lot of visible changes going on in this, Phase Two. It’s a period of low-to-no growth where berries are kind of green and hard. It’s kind of a chemical switching period where the berries are getting signals to begin their next phase of development. It’s not really until the end of this phase that you get into variation and the state of Phase Three when a lot of activity happens.

 “The lag phase can be very short — or more prolonged — depending on the variety of grape being grown, anywhere from a week or 10 days to 30 days or more for late season varieties.”

Because there are so many growing zones in the state, it’s hard to predict when the lag phase will kick in. It’s always a function of where the grapes are being grown — hotter areas, shorter season varieties, have a more rapid development during this phase.

“In warmer areas with typical, consistent weather, it happens relatively quickly. In the coastal areas with cooler conditions and more variable weather, it stretches out timewise.”

There’s lots going on

While it may look like not much is happening to the outside, there’s lots going on inside the berry as seeds or embryos start to mature. Chemical signals go off that start the softening and color development phases.

There’s never an idle time for growers in this phased process, but many use this lag phase to try and make yield estimates in wine grapes, calculated guesstimates of what their potential growth might represent at harvest time.

“Growers may use this hiatus in visible activity to go out and take crop weights and use them to try and project what their yield might be,” Bettiga says.

“Lots of growers have contracts where they’re promising to produce X amount per ton per acre and this estimate tells them if they’re on target or not, like perhaps they may reduce crop load when there’s excessive product and no tank capacity or consider buying more to fulfill a contracted need. We do pretty well when we’ve got an average crop size, but we seem to miss the highs and lows and lag phase estimates can help there.”

This is also a time for close inspection of vines for pest or disease management. “From that perspective, those berries are pretty susceptible in the early stages, especially for things like powdery mildew,” he says. “They become less susceptible to infection as they get more into the ripening stage."

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