Winter Pruning: The Devil is in the Details

The devil is in the details. Viticulture, even more than winemaking, is an apprenticeship-based practice.  The complexities are only hinted at in the handful of books on the subject.  The science spans many different disciplines, and since the subject is as varied and inscrutable as Mother Nature herself, there is no sign any time soon of us truly understanding any of the myriad aspects of viticulture, which range from the inner truth of the grapevines’ genes, proteins, and metabolic pathways out to the huge forces of pests, weather and climate.  Though we will never understand much of why things are really happening in the vineyards, humans have developed a tradition and a craft of viticulture through trial and error which dates back thousands of years, and that craft is what I am going to attempt to share with you over the course of the year.  This will be the first of a series of detailed articles on viticulture, starting with pruning.

Pruning

Pruning is the foundation of viticulture, and it is not usually described very clearly. There is Guyot, cordon, goblet, or whatever, and that’s fine, they look different and are traditionally used in different places, but why are these different systems used, and how are they actually managed in the vineyard?  This first article will attempt to break pruning down for you so that when you look at a vine somewhere in the world you will understand why it is pruned the way that it is.

The way the vines are pruned speaks volumes about the terroir of that site, and adds another dimension to one’s understanding of the wine.  The head-trained vines of the Southern Rhone tell you something about the climate, while the cordon-trained vines of the Northern Rhone tell you something different about the climate there.  The traditions didn’t evolve in a vacuum; they evolved due to the needs of the site.  So the pruning gives insight into the wine.

cordon pruned vine at Silvio Jermann's estate in Friuli

Why We Prune Vines

The most important tool in all of viticulture and winemaking is the pruning shears. Unlike other fruiting crops, like apples or peaches, grapevines cannot shed crop on their own.  By pruning a vine we regulate the amount of fruit that it will carry, assuring ripeness.  Pruning also allows us to space the fruit out for sunlight and airflow.  Pruning determines how strongly each shoot will grow, which profoundly impacts wine quality and style.  With proper pruning there is little else that needs to be done to the vine for the duration of the season; leaf-pulling, fruit-thinning, trimming and hedging, and much of the spraying can all be reduced or eliminated with good pruning.  Unfortunately, it’s very difficult to achieve proper pruning on each vine, but many of the problems that we deal with in the vineyard and the winery—disease, slow ripening, green character, lack of concentration or otherwise unbalanced wines—can be prevented with good pruning.

The main areas that we look at with pruning are the timing, vine balance, and vine architecture. These concepts supersede the different pruning systems, like Guyot, goblet, or pergola.  The pruning systems are means to an end—different ways of accomplishing vine balance given the growth habits of the particular variety and the climate and soil of the specific terroir.  So when it comes to pruning a vineyard, the first question is when to get started—the timing of pruning.

Timing of Pruning

The timing of pruning has major implications.  Pruning can be done any time from harvest up to and even past budbreak—a four month window.  The timing of pruning within that window has a number of different implications, so in the vineyards we put a lot of thought into when to prune.

Recently, there has been greater awareness of the biology of the wood-rotting diseases that are wreaking havoc around the world by drastically shortening the lives of vineyards.  A number of these fungal diseases, such as Eutypa or Esca, infect fresh pruning wounds.  The spores are dispersed by rain, and are exhausted over the progressive winter rain events, which means that the later the pruning, the lower the danger of infection.  For this reason pruning dates have tended to be much later in the past decade or so than they used to be. Napa and Sonoma used to be pruned in December and January, whereas now most vineyards in both counties are pruned in February and March.  Later pruning makes a big difference.

Pruning timing also influences the timing of budbreak—the later the pruning, the later budbreak occurs.  Depending on the site, later pruning (such as mid-March) can delay budbreak by up to 10 days.  If the site is susceptible to frost this can make a big difference (vineyard sites have different frost susceptibility because cold air rolls off the hillsides and sinks down and builds up in low-lying areas).

Given labor issues, though, not everything can be pruned at the last minute, so vineyards have to be prioritized.  Also, late pruning can result in reduced spring shoot growth.  If the spring is dry and the pruning is too late, the soil water can start running out before the canopy has fully grown. For this reason, dry-farmed vineyards are often pruned very early, even as early as December.  Luckily, Zinfandel—the most common dry-farmed variety—is less susceptible to the wood-rotting diseases. 

There is a slight effect of pruning date on harvest date as well—only a few days, but in a marginal climate, like in Oregon, or out on the extreme Sonoma Coast, a few days can make all the difference, especially when rain is forecasted.

The pruning date can have an impact on fruit set or shatter as well.  French and British books refer to shatter as “couloure”: a bunch of berries fall off the rachis, or stem, and yields go way down.  It’s a huge problem with grape-growing; Malbec and Merlot are notorious for it, and Pinot Noir and Cabernet Sauvignon can suffer some years, creating a big vintage variable.  Depending on spring rains, early or late pruning, which results in early or late budbreak, can shift bloom one way or another by up to a week, potentially placing bloom in completely different weather.  In general, there is less chance of spring rains with a later bloom, however there is a greater chance of heat, neither of which is good for fruit set.  So the local weather risk has to be taken into consideration: is the vineyard on the coast with a lot of spring rainfall (better for late pruning), or in a hot interior region, with dry hot winds in the late spring/early summer (better for early pruning). 

Balanced Vines

Once you decide on pruning date, there is the bigger question of how to actually prune the vines.  “Pruning severity,” which basically describes how much of the vine is cut off and how much is left, may be one of the most critical factors in the entire season of the vineyard.  Pruning severity is the main way that we create “balanced vines.”  Balanced vines are everything.

Since grapevines are long-lived perennial plants, the potential growth of an individual vine doesn’t change much from year to year.  They become fairly well regulated to their growing conditions and only change their potential growth slowly from year to year.  This is partly because the carbohydrates that fuel the spring growth were stored the year before, and partly because the conditions where they are growing (rocky soil, deep soil, sandy soil, cool climate, hot climate, dry climate) don’t tend to change.  In my way of thinking, grapevines just aren’t risk-takers—they pretty much just do the same thing each year.  A vigorous vine will tend to be vigorous, and a weak vine will tend to be weak.  So, staring at a leafless dormant plant with pruning shears in hand and assessing the way it grew the previous year is the first step in deciding how “hard” to prune it.

The goal is for each vine to grow a certain amount—not too much, not too little.  Weak vines tend to make wine that is hard, thin, and lacking in generosity.  Strong vines tend to make wine that is green, light, and insipid.  If there is one point to be made in this entire article, it is that “balanced” vines tend to make balanced wines—literally thousands of years of viticulture have confirmed this.  The Romans knew this, the monks in Burgundy knew it, and it is still the rule.  Everything else being equal (soil type, climate, water availability, etc.), the best wine will be made from vines with proper balanced growth.  If you were to weigh an individual cane it would be somewhere around 60 to 90 grams.  It would be almost as thick as a Sharpie at the base and as thick as a pencil two feet out.  It would stop growing naturally at around 40-50 inches. This is true in Châteauneuf du Pape and in the Mosel.  Our goal with pruning is for each cane to grow just the right amount.

We can achieve vine balance by leaving the correct number of buds.  Bud number is so important that most AOC regulations in France dictate it.  Bud number is relevant because the number of potential new canes is determined entirely by the number of buds left at pruning.  We leave more buds on a stronger vine to dilute the growth over more shoots, and we leave fewer buds on a weaker vine to concentrate the growth into fewer shoots.  If we get the bud number right at pruning, both vines will have canes of equal length and girth, but the strong vine will have a lot more canes and the weak vine will have less canes: the number of canes is different, but growth for each cane is the same.

Here is where is gets a bit technical and geeky: the way we determine how many buds to leave is to count the canes grown by the vine the previous year.  Imagine that two adjacent vines were pruned the previous winter to 16 buds.  If one vine grew 12 normal canes and 4 weak canes last year, it should only get 12 buds this year when pruned: one bud for each healthy cane and no more.  This growing season, everything else being equal (rainfall etc.), it won’t have any weak canes, and therefore will be more balanced.  If the other vine grew 12 normal canes and 4 “bull canes” (overly-strong canes) then it should get 20 buds this year when pruned (doubling the number of bull canes and repeating the normal canes).  It too will be more balanced, without any bull canes.  Things should be done in the vineyards to strengthen the weak vines and subdue the strong vines, but that is a long-term strategy, and pruning needs to be used in the short term to restore vine balance.

This is one of the reasons that cane pruning provides more flexibility than spur pruning.  A quick definition: “cane pruning” is the practice of leaving one of more canes, typically 5 to 9 buds long (e.g. Guyot) and “spur pruning” is the practice of leaving a certain number of two bud spurs along a cordon or on a head (e.g. goblet).  There are pros and cons with each system, but there is no doubt that cane pruning is the best bet for balancing the vine.

Cane pruning gives the pruner tons of options: to leave just a short cane, a long cane, two long canes, multiple canes, or any combination thereof.  Spur pruning can be flexible too, but not on an annual basis—canes can be altered every year, but once the vine is set up with spurs they are pretty well fixed.

Vine Architecture

Vine architecture, especially spacing, is the second most important principle of pruning after vine balance.  It varies by variety, climate, and the economic realities of the production needs, but in general the goal is to prune the vine in such a way that no fruit is touching and the vine canopy will be even—no clumps or bare patches.  This means that each shoot should about every four to five inches apart.  Areas where the shoots are bunched up will result in clumps of fruit that are plagued with uneven ripening and fungal disease issues.  In addition, clumped up leaves adversely affect fruit flavor and acid balance even if the fruit is thinned out and well-spaced. Shaded leaves will turn yellow, dumping their potassium into the ripening fruit, which kills the acidity, causing tartaric acid to precipitate with the potassium as an insoluble salt.  Fanatical attention to light distribution is one of the ways my own Matthiasson wines have high natural acidity without green characters.  It’s not rocket science or slight-of-hand, its just good pruning. 

Bare patches in the canopy caused by excessive spacing are problematic too.  They expose fruit to direct sun, which robs aroma and color through overheating of the fruit, or they may even cause sunburn, which contributes bitter and prune-like characters to the wine.  It also means that you get less fruit—gaps in the canopy here and there add up over the entire vineyard.

Good spacing is one of the main virtues of spur pruning—cane pruning may give you better vine balance, but spur pruning gives you better shoot distribution.  The vines are trained when young to have their spurs spaced six to eight inches apart.  You have to carefully moderate the growth of the young vines to get them to grow that way—if they grow too fast they get leggy, and the spacing is too wide.

After that the trick with spur pruning is to maintain the proper spacing: we’re dealing with mother nature, and the spurs tend to wander around over the years; the pruner has to select spurs each year that maintain or improve the spacing.

Here is where we get geeky and technical again, but bear with me so I can explain how to spur prune.  Since two canes grew out of each two-bud spur from the year before, there are two choices per position (each spur location is called a “position”), an upper cane and a lower cane—either can be cut back to two buds for the spur.  The other cane gets cut off.  To keep the position from climbing too high over the years, the lower cane is typically chosen for the spur and the upper cane is cut off. Occasionally, though, for better spacing, an upper cane is turned into a spur and the lower one is cut off.  If there is a big gap next to the position, both canes from that position can be left as spurs creating a “rabbit ear.”  If two spurs have grown too close together, one is cut out entirely. The pruned vine should have a nice even distribution of spurs all the way along it.

The downside of cane pruning is spacing. Mother nature gives us buds that are unevenly spaced along the cane, and everything is bunched up around the head of the vine (the “head” is the top of the trunk where all the canes sprout out from).

The first part of dealing with spacing on cane-pruned vines is selecting the cane. We’re looking for a cane that has buds spaced three to six inches apart.  If the buds are too close some of them can be cut off to space them out. Selecting this cane can be tricky, because it also needs to be round (not flattened—that means it was poorly nourished), have received plenty of sun (this is important for fruit production), and be in a good position to be bent over and wrapped onto the wire without breaking.

The second issue is to figure out how to deal with the head.  Cane pruning typically involves leaving one 2-bud “replacement spur” in the head area for each cane. This is to assure that there will be a cane for the next year in the right place, and also a spur to continue the process for the following year.  This is a simple concept but it can be hard to explain.  The process is this: the two-bud spur grows two shoots; they are cut back to one cane and one spur; the next year the cane is cut off (it has born fruit and has now outlived its usefulness) and the two shoots coming from the spur are again cut back to one cane and one spur; the following year the cane is again cut off and again the previous year’s spur is cut back to a cane and a spur…and the cycle continues annually.

Leaving a cane and a spur works great with a traditional Guyot, wherein you leave one cane and one spur.  The cane grows out one direction and the spur grows out the other direction.  It’s a great system—it’s used in most of Burgundy—but the vine vigor has to be low enough so that just one cane can handle all of the growth.  If there is too much vigor (back to vine balance) you need to leave two canes, and with one spur per cane the head of the vine is now totally congested and screwed up. There is no physical space for the spurs and both canes all coming out of one single head. This is a big deal, and it is where a lot of cane pruning goes wrong. 

An example of a single Guyot vine, with cane and spur, at Grands Echézeaux

An example of a Double Guyot vine (two canes and no spur) at Littorai

With two canes it is best not to leave spurs, a much more advanced viticultural technique reserved for high-end vineyards.  The selection of the replacement canes for next year must be done when the vines are suckered in the spring. All vines have to be suckered every spring, but instead of just ripping through it, the person doing the suckering has to be sure to leave the two replacements from the head—or the next year there won’t be any good canes for the pruner to select. With no spurs the finished vine should have an even spread of shoots along its entirety, with no extra density at the head or the ends of the canes.

The third main training system, and one that I absolutely love, is head pruning.  Head pruned vines are tough to manage, they are an inefficient use of vineyard real estate, and they are only suited to varieties with an upright growth habit and good fruitfulness on spurs instead of canes.  Nonetheless, head pruning is a fantastic system for a warm climate and for varieties with large clusters.

The two main systems for head training are the vertical cordon and the goblet.  The vertical cordon is more common mainly because it’s easy—and that’s the only thing it has going for it.  Basically, you just run the vine up a stake and leave spurs up and down the trunk.  It creates a shaded and non-uniform canopy.  You see it a lot in California, and much less so in Europe, where the pruning tends to be better.  The goblet is the way to go.  It takes a few years to develop, but is well worth it.  The spurs are trained out like a wagon wheel, ideally spread out to a diameter of a foot and a half or so, and all on the same vertical plane.  Diffused light reaches all of the fruit, and the clusters are not in a glob.  One of the beautiful advantages of the goblet is that spurs can easily be added or taken away to balance the vines, unlike in cordon pruning.  Yet the vine benefits from the nice spacing and regular shoot growth that you get from spurs.

Next installment: “Springtime in the Vineyard”

Steve Matthiasson makes classically balanced wines under the Matthiasson label, and consults on vineyard practices for a number of wineries, including Araujo, Spottswoode, Stag’s Leap Wine Cellars, Chappellet, Hall, David Arthur, Long Meadow Ranch, Limerick Lane, Duckhorn, and others.