Bud Break, Flowering & Fruit Set – Nature’s Gift to Wine Lovers

Bud break is one of the most exciting times in vineyards around the world. This crucial phase in the vine’s lifecycle kicks off the growing season that culminates in grapes to harvest in the fall. Bud break marks the end of winter dormancy, as vines are waking up to a new spring season. Vineyards magnificently transform from brown and barren to vibrant and bursting with life. Bud break is always a welcome sign of new beginnings. A reminder that no matter how bleak the winter, life goes on and better days have arrived. A cue from nature I’m sure we can all get behind as we prepare to bust out of our own dormancy quarantined at home.

Taking a closer look at this vital stage in the vine’s lifecycle will make you believe that truly great wines are made in the vineyard. Therefore, gaining a greater understanding of viticulture allows you to better understand why and how the wine in your glass came to be. The world leaves its mark on a wine just as the world changes us. Perhaps this is what has kept people enthralled with wine for thousands of years. So, let’s examine this season’s happenings in the vineyard, starting with bud break followed by flowering and fruit set.

What Is Bud Break?

Technically bud break is defined as “the appearance of green tissue through the bud scales or the emergence of a new shoot from a bud during the spring”. Viticulturists use numerous systems to specifically identify when bud break occurs. One of the most commonly used methods for defining bud break is the modified Eichhorn-Lorenz (E-L) system. However, various cultivars (i.e. Riesling, Chardonnay, Cabernet Sauvignon, etc.) demonstrate bud break differently. Some emerge as hair-covered cones with no green tissue, while others emerge with clearly visible green tips. Consequently, the modified E-L system now uniformly recognizes bud break in grapevines as the moment when leaf tips are visibly emerging from the bud.

When Does Bud Break Occur?

Generally, bud break occurs from early March to late April in the Northern Hemisphere and from September to October in the Southern Hemisphere. Though warmer temperatures in late winter and early spring seem to be kicking off bud break earlier each year.

Air temperature is the main factor influencing when bud break occurs. Grapevines have a temperature threshold which they will not grow below. The base air temperature for grapevines has traditionally been 10°C (50°F). This means that average daily temperatures above this threshold will induce bud break and shoot growth in grapevines.

A Brief Look at Dormancy

In order to better understand why air temperature has the greatest influence on bud break, let’s briefly look at what is going on with the vine during dormancy.

• Dormancy occurs when grapevines have transitioned their green materials that are susceptible to frost damage into structures resistant to cold temperatures.
• Cold hardiness refers to the winter survival mechanisms of grapevines. The shorter days and lower temperatures of the late fall genetically trigger these survival mechanisms. Grapevines transition from active growth with green, photosynthesizing leaves to dormant, as the canes progress from green to brown periderm (a thickened protective, corky layer). Certain cultivars are more cold hardy than others.
• Endodormancy occurs when buds become more resistant to freezing damage as periderm advances from the base of the cane to the tip. In this early stage of dormancy buds are resistant to further development and gain greater freezing resistance.
• Supercooling is how a dormant grapevine bud survives below freezing temperatures. Extracellular water is able to freeze at warmer temperatures to prevent ice from forming within the bud cells. The supercooling temperature is the same as the temperature at which 50% of dormant buds die.
• Extraordinarily, grapevines and other perennial plants are able to keep track of the amount of time they spend at a specific temperature range ( 0°- 10°C or 32°- 50°F) in order to ensure they break bud after the conclusion of winter. The chilling requirement is the number of hours that grapevines need to experience at these temperatures.
• Once the chilling requirement has been met, vines transition into a later stage of dormancy called ecodormancy. During ecodormancy, buds will stop resisting temperature changes. Now, vines are only dormant because of cold temperatures and bud break will occur when warmer temperatures arrive.

How Does Dormancy Influence The Timing Of Bud Break?

In regions with especially cold, harsh winters, the vines’ chilling requirement is usually lower. This means that dormant buds are less advanced into the ecodormancy stage and less prone to burst as temperatures begin to rise. The chilling requirement increases in regions with warm or moderate winters. Vines tend to reach the chilling requirement by early winter. In this case, vines are advanced to a state where bud break is rapid. Therefore, warm weather in late winter or early spring can induce early bud break, consequently increasing the risk of frost damage in the spring. This process is closely tied to fall, winter, and spring weather patterns. Thus, seasonal temperature variations due to climate change in recent years have heavily influenced the timing of bud break.

Other Factors Influencing The Timing Of Bud Break

While air temperature is the main influence determining when bud break occurs, other factors include the vine species and cultivar, as well as the cane positioning. The order of bud break across different species (i.e. Vitis Vinifera, Vitis Riparia, Vitis Labrusca, Vitis Rupestris, etc.) and cultivars (i.e. Chardonnay, Riesling, Cabernet Sauvignon, Syrah, etc.) tends to be the same regardless of seasonal temperature differences. For example, Chardonnay will always have bud break before Cabernet Sauvignon. However, each variety has a unique set of genes which allow it to respond to climatic conditions differently.

A bud’s position along a cane can also influence when it bursts. If dormant canes are left upright, then the distal buds closer to the tip of the cane and furthest from the base usually burst first. This is called apical dominance. Once the distal buds burst, pruning the cane to only two bud spurs can further delay bud break for the remaining buds.

What Changes Happen In The Vine?

A vine waking up from winter dormancy feels just like we do when our early morning alarm goes off in winter. We both have a hard time climbing out from under the warmth of our covers in the midst of cold temperatures. Gradual temperature increases are like the smell of coffee wafting through the air in the morning, gently enticing the vines out of bed. As the season changes and warmer temperatures become more consistent, water and nutrients stored in the vine’s trunk over winter begin flowing to the limbs of the vine. These liquid reserves slowly make their way to the buds as the threat of freezing temperatures lessens. The buds swell from the liquid and transform from hard nodes on the twiggy canes into soft, fuzzy buds.

Next, the green leaves of the vine begin to emerge from the buds during the first consistent days of spring sunshine. Then the bud breaks and stunning grape leaves explode from the vine seemingly from nothing. The vine uses the last of its stored carbohydrate reserves for this springtime push. Then these gorgeous green new leaves are on a mission to begin photosynthesis immediately to bring new energy to the vine. Within only a matter of a few weeks, the leaves multiply immensely and shoots expand outward as a result of this initial photosynthesis. This process is known as rapid shoot development.  

Can We Predict When Bud Break Will Occur?

Many temperature-based models have been developed to attempt to predict when bud break will happen. Growing Degree Days (GDD) is one such model that is based on the accumulation of average daily temperatures above 10°C (50°F). Daily growing degree day values are added together from the beginning of the season, providing an indication of the energy available for plant growth. You can read more about the formula for calculating GDD here. While GDD can be used to compare the progress of a current season to the long-term average, it is not necessarily a solid formula for predicting when bud break will occur.

The Effects of Climate Change

In recent years, climate change has increased winter temperatures in wine growing regions around the world. According to this study from Cornell, “climate change in the Midwest, North, and Northeast United States is predicted to increase winter temperatures by 2-3°C.” The weather fluctuations from winter temperature increases alter the dormancy cycle of the grapevine. Often times warmer temperatures in late winter or early spring lead to early bud break. Thus, increasing the risk of frost or wind damage when buds prematurely burst thinking that warm spring temperatures are here to stay.

Furthermore, global warming includes extreme cold events as well, which are more likely as warmer temperatures at the poles weaken the trade wind system. As a result, cold air dips down from the poles further than before. This causes sudden shifts in temperature in midwinter or spring that can lead to said frost damage. Below freezing temperatures can destroy delicate buds in just one cold night.

Common Vineyard Practices Around Bud Break

Viticulturists have developed certain techniques for delaying bud break in an effort to avoid the risk of spring frost damage. Pruning in the vineyard is necessary every year because only one-year-old wood produces fruit yielding buds. However, specific techniques like double pruning delay bud break and prevent spring frost damage. In this case, canes are minimally pruned and left long over the winter. Then, the canes are pruned to the desired number of buds after bud break occurs. In certain regions prone to especially cold winters, delaying pruning until after the coldest winter temperatures have passed will help to delay bud break.

Pruning Techniques

Spring pruning practices will vary depending on the vine training method chosen for the vineyard. Here we will look at what can happen with a single guyot trained vine vs. a single cordon trained vine.

As you can see, the single guyot trained vine has one cane left after winter pruning. Growers in cooler climates often use cane pruning to limit the risk of frost damage. Eliminating most of the hardened wood on the vine other than the trunk achieves this goal. However, this method requires more skill as only one cane from the previous year’s growth must be correctly selected for the next year’s production. Here there are 8 buds that have emerged on the cane and 2 buds that have emerged on the renewal spur on the right. Before shoots emerge from these buds, the cane must be trained down horizontally. As the shoots which will eventually bear fruit emerge, the vines may be pruned further so there is only one shoot per bud.

The single cordon trained vine below illustrates how double pruning to delay bud break can be implemented. In this case, winter pruning left the canes long. Bud break will typically occur on the buds towards the tips of the canes and further away from the cordon. Known as apical dominance, this inhibits the development of medial and basal buds. The canes are then spur pruned down to 2-3 nodes to effectively delay bud break further. Spur pruning can also be done late in the winter before the apical buds burst.  

Additional Techniques Used To Combat Frost

Following bud break, various techniques can be implemented to protect the new buds from frost damage when temperature drops are expected. Many vineyards will utilize giant fans and wind machines to circulate the air, pushing the warmer air down towards the ground where the cold air normally settles. Some wineries, like Jordan winery in Healdsburg, use sprinkler systems to help protect the new buds. By running the sprinkler system at night when the temperature drops, a protective shell of ice forms around the buds, keeping them at 32°F.

In regions like Burgundy that are especially prone to cold temperature drops, growers will light fires either in large drums or with paraffin candles called ‘bougies.’ Set up down the center of vineyard rows, these fires keep the vines warm and provide protection against damaging frosts.

Next Comes Flowering & Fruit Set

Flowering, or bloom, is the next step in the vine’s life cycle after bud break. Depending on temperatures and rainfall, flowering generally occurs 40 – 80 days after bud break. Average daily temperatures need to remain between 59°-68°F for flowering to occur. Now is the time growers pray for good weather and hope that no heavy winds, hail, or rain show up.

Grape vines are hermaphroditic. They have both male and female parts and can quite literally get down with their bad selves. After a month or more of green, vegetative growth, tiny green spheres called “calyptras” or “caps” for short emerge. All of the pollen-carrying parts of the flower are within these mini green balls. When the vine is ready, the caps pop open and reveal the pistil and pollen-carrying stamen. These basically look like a bunch of off-white tiny strings or hairs. Over the course of a few weeks, the pollen from the stamen transfers over to the pistil. Eventually, once pollination and fertilization have happened, the green caps and pollen fall to the ground. Tiny, green pea-sized berries emerge, and these pea-sized berries eventually grow into the grapes we know and love.

Fruit set describes this metamorphosis from flower to berry. This process continues as these tiny berries grow into the clusters we know and love. However, not all flowers become fruit. Some flowers might not pollinate due to wind, rain, hail, or an unexpected late frost. Flowers that are damaged or don’t pollinate are known as shatter or coulure. Shatter often results in lower yields.

Vineyard Practices During Flowering & Fruit Set

As flowering commences, fans and heaters warm vineyards again to protect flowers from damage during unseasonably cold temperatures. The carbon and nitrogen levels in the vineyard soils also influence fruit set. Planting nitrogen-rich cover crops in vineyard rows improves soil health in nitrogen-deficient soils. These cover crops may include fava beans, wild mustard, peas, clover, and more.

Leaf pulling and shoot thinning are also another crucial activity at this time. These practices allow for increased air movement and controlled light penetration within the vine’s canopy. The air flow helps to keep pests away while the varied light prevents the grapes from sunburning.

We are shown each year that wine is truly a gift from nature as vineyards transition from bud break to flowering and fruit set. Let the vineyards remind us that beautiful things come from hard work, patience, and a little luck.

Sources

Penn State Extension Wine & Grapes – Bud Break

Penn State Extension Wine & Grapes – Spur Pruning

Research News from Cornell’s Viticulture & Enology Program

Fruit Evaluation Project from NDSU-Carrington REC

Vine Pair – Bud Break

Vine Pair – Flowering