We all love to see beautiful vineyards at harvest – green, full vines with abundant clusters of red or green grapes. Harvest is an exciting time as the vines bring forth their bounty and then take a deep sigh of relief. Following harvest, the vines decorate the landscape with vibrant hues of yellow and orange. But sadly, the vines then give up their leaves to become brown, bare twigs in the ground by winter. But are these vines really as lifeless as they appear? Contrary to their appearance, these vines have prepared themselves to survive and thrive during the cold winter months.
To understand the winter dormancy period, we first need to understand the annual growth cycle of the vine.
Budburst – March-April in Northern Hemisphere (September-October in Southern Hemisphere).
As temperatures begin to exceed 50oF, buds begin to swell and burst open to later become new shoots. Some grape varietals experience bud burst earlier or later than other varietals. Frost at this time can significantly damage these buds and the following early shoots. It is interesting to know that buds will form for the current year’s crop, and also for the following year’s crops (remaining dormant until the following year).
Early Shoot and Leaf Growth – March-April in Northern Hemisphere (September-November in Southern Hemisphere. Shoots develop and begin a period of rapid growth, spawning budding leaves and tendrils. Until leaves mature and can support photosynthesis, this growth is fueled by carbohydrates that have been stored in the woody vines and roots following harvest. Green leaves begin to fully develop as shoots mature to reach the flowering stage.
Flowering and Fruit Set – May-June in Northern Hemisphere (November-December in Southern Hemisphere)
Depending on temperatures, about 40-80 days after budbreak, flower clusters will begin to appear on the end of the shoots. Most cultivated grape vines are “hermaphroditic” containing both male stamens (pollen) and female ovaries (seeds). With a few weeks of some warm sunshine, flowers grow and lose their protective “cap”, leading to “fertilization” with the next gentle breeze. Unfertilized flowers will fall to the ground, sadly never to become a grape. This stage of the vine lifecycle is critical to development of a healthy crop yield and can be damaged by cold, harsh and rainy weather.
Véraison and Berry Ripening – July-September in Northern Hemisphere (January-March in Southern Hemisphere)
After fruit set, both white and red grapes are hard green berries that are low in sugar and high in acid. These berries continue to grow in size for the next 40-60 days followed by “véraison.” This is the time that berries begin to soften and change color, becoming red, purple, black, or a more translucent, golden green. Warm, sunny weather is needed at this time for the grapes to ripen and accumulate sugars.
Harvest – September-October in Northern Hemisphere (March-April in Southern Hemisphere)
Grapes have reached their peak ripeness: sugars and pH have increased, while acids (such as malic) begin to decline. Tannins and other grape phenolics have matured, giving rise to enticing aromas and flavors. Grape clusters are removed from the vines to begin their journey into becoming wine.
After harvest, leaves remain on the vine where they continue to perform photosynthesis and produce carbohydrate reserves. Eventually, the vines begin to pull back these carbohydrates from the leaves to later be stored, causing the leaves to turn brilliant shades of yellow, orange, and eventually brown. Be aware that red leaves on the vines are not a typical harvest color and often indicate the presence of vine disease.
Now that vines are no longer spending energy producing fruit, their energy can now be spent in their root system. Many new small roots begin to grow and seek out vital nutrients in the soil in a process known as “root flush.”
Winter Dormancy – December-March in Northern Hemisphere (July-September in Southern Hemisphere)
By winter, vines have stored their nutrient and carbohydrate reserves in the cordons, woody trunk and roots. These reserves are critical so that the vines have energy to produce budburst and new shoots in spring – a time when there are not yet leaves on the vine to produce carbohydrates via photosynthesis. Also important is that vines have gone through a dehydration process so that water does not freeze within the vine and root tissues during the cold winter months. Damaging cold winter frosts are not a typical problem in California growing regions – but can be a serious problem in many colder regions.
However, this cold “winter dormancy” is vital to the growth cycle of vines. Temperatures must become cold enough that the above ground vine growth will halt. This cold resting period is necessary to ensure normal budburst and growth cycle in the spring.
Winter dormancy is also the time for winter pruning to be performed. This pruning sets the stage for the next years growing cycle. Woody canes from the previous year’s growth are cut off and new canes are selected from which the new shoots will grow come springtime. This pruning also stimulates the vines so that when warmer weather and sunshine arrive, the vines awaken to a new growth cycle.
So, next time you pass a dormant field full of bare, woody vines, just know that the vines are preparing themselves to greet the spring sunshine with a burst of new growth and start making grapes for your next great bottle of wine.
Irene Scott, WSET-3 – Wine Education Chair
References:
Wine and Spirit Education Trust, 2016. Understanding wines: Explaining style and quality. wsetglobal.com, London, UK.
Urska (2018, Dec. 5). Grapevines During Winter Dormancy, Evineyardapp.com.
Burgess, Laura (2016, Jan. 29). Below the Surface: Winter in the Vineyard, Vinepair.com.