The Problem with Early Sap Flow


Once the sap starts flowing, bud break will not be far behind.

Problem is, once it’s been cold enough for long enough during the winter, the vines are primed to wake up – which means a stretch of unseasonably warm weather in late winter can cause an early bud break. Much like this winter, the cold December and warm January, has led to sap flow in early February. Sap flow we shouldn’t see on the North Coast until at least the end of the month as the soil warms up and the days get longer.IMG_2390

What’s wrong with an early bud break? Big deal if it’s two weeks sooner than later, right? But earlier in the year, even a week or two, and we are much more likely to have hard frosts. Early bud break means the vines wake from dormancy, lose their natural resistance to cold, and send forth fragile green shoots when really bad frosts are common. As the days get longer and the nights shorter, it’s much harder for conditions to create the temperature inversion of a radiative frost.

Yet another consequence of our winter drought: the warmer days and dry conditions have warmed the soil prematurely, encouraging the microbial life to be more active and stimulate the vine roots. An entire cascade of hormonal events is taking place inside the grapevine as it wakes itself up weeks earlier than it should. Water and sap start flowing. Carbohydrates are mobilized as cells come out of their dormancy and start dividing. The buds swell as the shoots within ready themselves to unfurl.

Each shoot emerges not only with leaves, but with tiny, perfectly formed clusters between them. These clusters were formed during the previous spring when last year’s crop was blooming. Time-lapse crop development is an intriguing adaptation of prescience – how did the vine know to have fewer clusters this year? Well, it took a measure of its health last year and started dividing cells that would come to fruition in two years’ time.

Early sap flow

credit Debby Zygielbaum

The clusters that emerge from the swollen buds are highly susceptible to frost – and while a grapevine might be able to recover by growing a new shoot, the precious clusters only emerge with the first, not the second nor third. The ensuing shoots after frost are fail safes to make sure the vine has something to sustain itself with, not grow fruit for reproduction. Another smart trick of survival – if the year is lean, the energy is spent on sustaining the vine and ripening what little fruit has made it through the difficult spring.

As clever as the grapevine’s adaptations are, however, it still isn’t enough as we begin to feel the first effects of climate change – which this warm, dry winter is no doubt a likely symptom, even if magnified by California’s natural cycles of boom or bust precipitation. I’m bracing myself for an early bud break, cold spring and a long, dry summer. And praying that I have a decent crop to harvest come fall – and I know I’m not the only one.

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Debby Zygielbaum is the Vineyard Manager (or, as she’s fond of saying, ‘Dirt Farmer’), at Robert Sinskey Vineyards in Napa and a frequent contributor on Nopalize. She can be found @walkthevine on Twitter and Instagram.