Since the beginning of the year 2021, it is raining almost every day in Singapore but temperatures are pleasant, around 26 C.
Meanwhile it is snowing in some part of France, as we can see on these beautiful pictures from Domaine Dalmeran in Les Baux de Provence.
A bit of snow is actually beneficial to grapevines as it eliminates some pests, provide water and nitrogen and protect the soil from overly cold temperatures (permanent damage would occur below -10 C.)
Winter dormancy has an important role in a grapevine growth cycle, as it helps vines tolerate cold temperatures, and ensure bud burst and normal spring growth. Two types of dormancy occur during the winter: endodormancy, and ecodormancy.
After the leaf falls at the end of the growing season, endodormancy is triggered by shorter days (photoperiod signal) and cooler temperatures.
The plant growth regulators that play a key role in the regulation of the metabolic changes are called abscisic acid (ABA).
Although it may seem that nothing is happening in the vineyards, grapevines undergo a number of processes in order to prepare for lower temperatures and become cold hardy.
Grapevines respire in order to maintain basic metabolic functions and survive based on carbohydrates reserves stored during the previous growing season, mostly as starch, in permanent wood structures, such as roots, trunks, and cordons.
Grapevines decrease the level of water in various tissues with the process of dehydration through movement of water to intercellular spaces, and accumulation of sugars and protein complexes that bind water and serve as cryoprotectants. In that way, cell content won’t form damaging ice crystals and freeze in cold temperatures.
In order for the grapevine to transit from endodormancy to ecodormancy, it needs to be exposed to cold temperatures for a certain period of time, about 150-200 chilling hours depending on grape varieties, in order for the bud break and normal spring growth to occur during the next season.
Note that chilling units are hours of time spent above freezing. Temperatures below freezing have no effect on the chilling requirement, but increase cold hardiness.
The second phase of dormancy is called ecodormancy, a phase in which the plant is actually ready to grow, but environmental conditions, low temperatures, short day length and lack of moisture, block the bud development.
Once the warmer temperatures occur, the plant will start to grow and can’t adjust back to colder temperatures. Since vines reach the ecodormancy phase in most temperate locations before the end of winter, it can happen that due to unusually warm weather grapevine budburst prematurely. This can be devastating for vines, especially when a cold front arrives, leaving no choice for grape growers but to find a way to overcome frost events.