The Grapevine Vegetative Cycle Part I
Over the next few months, we are going to follow the development of the grapevine. Indeed, March is truly the beginning of the growing cycle.
A Recap of the Vegetative Cycle:
• Bleeding of the vines between February and March
• Bud break in April, dependent upon temperature
• Flowering starts in May, 40-80 days after bud break
• Fruit set almost immediately follows
• Veraison (the onset of ripening) takes place around 40–50 days after fruit set
• Maturation and harvest
Water uptake in the vine is through the root system. In actively growing vines, water is lost through transpiration. This loss must be continually replaced. Inside the plant there is a complex mechanism by which water is constantly drawn into the vines. The plant overcomes the surface tension of water in the soil and transports the water from cell to cell, up to the aerial portions of the vine.
Water can also be brought in through osmosis. When the osmotic pressure of the root cells is greater than the soil moisture, moisture then moves into the epidermal cells of the vine roots.
Bleeding is not harmful to the vine and it is a normal process. In fact, it does some good, pushing out air bubbles from the plant’s vascular system. The water pressure also stops some wood diseases that could try to enter inside the wounds after pruning.
The Bud Break
As temperatures begin to warm in the spring, stored starch is converted to sugar and sap begins to move in the vine. Buds begin to swell, then burst (break). The newly emerged shoots grow very rapidly, and will continue to do so for several weeks in the absence of stress. Soon cluster inflorescences become visible, usually opposite the third and fourth leaves on a shoot.
I stopped in a vineyard in Saint Helena this morning. In the middle of March, the dormancy period of the vines is over. Depending on the vine variety and vineyard location, bud break can take place over a two month period throughout the Napa Valley.
With such great viticultural diversity, one might think that Napa Valley is quite large. However, our little region is only 30 miles long by five miles wide at its widest point. Only 9% of Napa Valley is planted to grapes and the region has just one-eighth of the vineyard acreage of Bordeaux.
The 2018 vintage has just begun. Vineyard managers need to be very vigilant regarding the dangers of fungi and frost at this time of the year. Over the next couple of months, we will look at the next stage of the grapevine’s vegetative cycle – the flowering and fruit set.