Dormancy: In winter, grapevine buds are isolated from the plant’s vascular system, desiccated, and filled with compounds that resist freezing. Vascular elements (phloem which conducts nutrients) in the canes and trunks are either plugged with callused tissue or emptied of water (xylem which conducts water) to avoid freezing.
Dormant buds will not break until they have reached a minimum time below about 40°F (5°C) which “primes” bud for spring break.
Spring: As daily temperatures surpass 10 °C (50 °F) soils begin to warm which increases root absorptive activity. Capillary action, along with osmotic pressure, draws water containing low concentrations of organic acids, hormones (which control all aspects of growth and metabolism) as well as minerals and sugars up from the root system through the trunk’s xylem vessels to the bud. Due to this re-hydration the buds become less resistant to cold temperature and a vine can begin to “bleed”.
Next the scales are separated as the bud swells revealing brown/white woolly hair.
With each 10°C rise in temperature, temperature-dependent enzymes (which mediate all reactions in a cell) double their rate of activity.
As the sap flow increases new vascular connections are initiated at the buds connection to the canes. Fully re-establishing vascular connections between buds and roots will be completed around bloom.
Inside each bud lies a fully formed and compressed shoot complete with cluster primordial and several nodes of leaf primordia.
As the apical meristem (the tip of the bud) begins to produce the auxin hormone, it initiates new growth and expands the stem like an antenna unfolding leaves and clusters. This shoot will be called the primary shoot.
As soon as the buds are hydrated but particularly after bud break, the young shoots are very vulnerable to frostdamage. Vineyard managers go to great lengths to protect the fragile shoots should temperature drop below freezing.
This can include setting up heaters, sprinklers or fans in the vineyard to keep cold air from settling on the vines.
If the primary bud is damaged, one or two smaller buds at its base can become activated.
These (termed the secondary and tertiary buds) bear fewer leaves and fewer or no clusters and therefore may produce a leafy canopy but a poor crop. And yet in certain vintages this can be a saving grace.
A horizontal cut of the bud, viewed under a microscope, can also give an early indication of how fruitful the harvest is. This is not very frequently done because it takes a certain amount of skill and equipment and more importantly the weather condition through the end of spring will have a significant impact on the vintage.
The whole valley turns green.
During this “grand period of growth” moderate rain is not a problem but we still worry about frosts or conversely extreme heat. This year has been mercifully mild so far but we are heading to a week of 90+F and high winds. Both can be hard on new shoots.
For fun I like to check the Farmers Almanac. Why not? This year they predict the following:
Annual Weather Summary: November 2012 to October 2013
Winter temperatures will be above normal, on average, with the coldest periods in mid- and late December and mid- to late January. Rainfall will be a bit below normal in the Bay Area and above normal elsewhere. The stormiest periods will be in mid-November, mid- to late January, and early March. Mountain snowfall will be near to below normal, with the heaviest snows in mid- to late January.
April and May will be drier than normal, with near-normal temperatures.
Summer will be warmer than normal on the coast but cooler inland, with near-normal rainfall. The hottest periods will be in early and late July.
September and October will be cooler than normal, with above-normal rainfall inland and slightly below-normal rainfall on the coast. Expect hot weather in mid-September.
Temperature and Precipitation November 2012 to October 2013
Next big event: Flowering!