Yield and Wine Quality

What’s the deal with yields? Or terms like “crop load,” or “vine balance?” What does it mean to ”green harvest” or to “restrict yields?” Why would one do that? Is 30 hectoliters per hectare always better than 60 hectoliters per hectare, or two tons per acre better than four tons per acre? How is wine character actually changed by adjusting to different crop levels? These terms get thrown around, but what do they really mean?

The Green Harvest

Fruit thinning (“green harvesting”) is a key technique used to lower yields, and it is one of the most mysterious viticultural practices; besides science, it involves habit, compulsion, penance, fear…a whole host of impulses that have very little to do with normal farming but everything to do with wine, that most irrational of human agricultural products. Fruit thinning is extremely emotional for farmers and winemakers, and science often has very little to do with this incredibly costly and often wasteful practice.

As a farmer and a vineyard consultant who devotes a huge amount of mindshare in the summer to helping my clients figure out how much fruit to keep on the vines, I’ve struggled to understand the process of fruit ripening, and how it is affected by different amounts of fruit. This is not a trivial matter—besides the impact on wine style from fruit thinning (note that I said “style,” not “quality”…much more on that later), economics are a huge factor. I’ve calculated that for one of my clients (a vineyard owner who makes a $350 bottle of wine) each scraggly little bunch of grapes that we cut to the ground represents twenty dollars worth of wine! And this doesn’t include the labor to do the fruit thinning, which is very slow and exacting. But the client points out that fruit is only worth what you can sell it for, and if that thinned fruit cannot go into the bottle of wine it is worthless; it is, in fact, a negative. So the judgment of whether or not that fruit needs to be thinned off is obviously an incredibly important decision.

For the most part, fruit thinning as a standard quality vineyard practice is a fairly recent phenomenon, having developed over the past thirty years or so. There are plenty of exceptions, but this generality is important because it is a piece of the global wine style shift over the past few decades. The practice of fruit thinning originally evolved in high-end estates in cooler growing regions and slowly diffused out to the rest of the grape-growing world. Adoption of the practice was not overnight--cutting fruit to the ground was (and still is) something that is extremely difficult for the average practical-minded farmer to accept. A good farmer will take fruit off of young, weak, or sick vines, but this is a critical viticultural practice to relieve vines that cannot handle their fruit load, not a "green harvest"—it's more of a fine tuning than a yield restriction.

The basic principle with fruit thinning is that with less fruit on the vine, the rate of sugar accumulation is increased in the fruit that remains. Since sugar is accumulating steadily over time, the earlier in the season it is done, the more sugar ends up in the remaining fruit; or, the more fruit that is thinned at a given time, the more sugar ends up in the remaining fruit. Basically we’re talking about timing and severity: these are two of the levers that can be pulled to increase sugar in the harvested fruit. The third lever is the amount of leaf area that the vine has working for it. Leaves make sugar through photosynthesis, and the vine transports that sugar to the fruit, where the sugar slowly increases in concentration during the ripening period. Less fruit with the same amount of leaves, more leaves with the same amount of fruit, or more time with the same amount of leaves and less fruit: all of these scenarios lead to more sugar in the remaining clusters. The main thing is the ratio of leaf area to fruit load over time. 

There are other factors that influence sugar accumulation, such as the nutrient or water status of the vine, the percentage of leaves receiving sunlight, the average age of the leaves, the virus load in the vine, and so on; but if all of these things are equal, the ratio of leaves to fruit will greatly determine the rate and amount of sugar loading in the grape berries.

The biggest impact that higher sugar levels have on wine style is obviously a higher level of alcohol in the wine. There is a direct and linear relationship between sugar and alcohol. Fruit thinning and sugar accumulation is not linear, though. The leaves sense the crop level and compensate in their photosynthesis to a certain degree. A higher crop load results in more efficient leaves, and a lower crop load results in less efficient leaves. Therefore, simply cutting the crop load in half will not double the sugar production. Thinning fruit does impact sugar production, but a lot of fruit needs to be thinned from a vine before it experiences even modest differences in sugar accumulation. Cutting the fruit in half or more might result in as much as a 10-20% increase in sugar—that is around the maximum that sugar can be increased by manipulating crop load. That small difference makes a big difference in wine style though. It could increase a potential alcohol of 11% to 13%, which could be a critical difference in a cool climate like Switzerland. It might also raise a 13% wine to 15% in a warmer region. 

Greater sugar accumulation as a result of fruit thinning seems to have an impact beyond simply raising alcohol levels. This is not a well-studied area, and there is a lot of anecdotal information out there—much of which is misleading. One thing we do know is that sugar movement into the fruit around veraison seems to trigger a signal for the fruit to start manufacturing anthocyanins. Earlier anthocyanin production can result in more color in the fruit at harvest. Anthocyanins complex with tannins in the wine, softening the mouthfeel and enhancing mid-palate concentration. Other ripening parameters are almost certainly enhanced; anthocyanins are just easier to measure, so that is why they are so often studied. For a modern, dark, concentrated wine style, early fruit thinning at levels that are “luxuriously” low has a big effect on alcohol levels, but it can have a slight effect on other aspects of wine style too. If you think about the shift in style in Bordeaux (or Napa Valley) from the '80s to the present, higher alcohol, blacker color, and darker fruit character have become routine. Some suggest global warming is to blame, but you can just as easily make the case that this is the result of fruit thinning. Global warming is undoubtedly involved, but my take is that the widespread embrace of fruit thinning is probably even a bigger factor. And high end vineyards are continuing to thin even more in an effort to differentiate themselves.

For an effect on wine flavor—physiologically pushing the fruit into a riper spectrum—fruit thinning has to be done very early, well before veraison. The process is ideally conducted by seed hardening, which occurs halfway between bloom and veraison, and signals the beginning of ripening (ripening starts well before color change—veraison is just one stage in the process). If done later in the season it will increase sugar accumulation in the remaining fruit, but in my experience it won’t change the fruit character much. A loose rule of thumb for fruit thinning is that for every week after seed hardening 10% more fruit needs to be thinned to get the same result.

Pignolo in Friuli after fruit thinning

Merlot in Napa after fruit thinning

Color Thinning

There is another type of fruit thinning that can change wine character: color thinning. This is simply a sorting process, and it has no impact on the way the remaining fruit ripens. At veraison, fruit color change is not uniform; instead, the berries and clusters darken one at a time, over a period of time. It’s pretty standard practice these days in high end vineyards to try and create more uniformity by thinning off the lagging green clusters at the tail end of veraison. The idea is to get rid of the less ripe fruit, shifting the population of fruit into a riper spectrum. This has become a totally standard practice, and it’s another way that wine style has gotten less green and more ripe throughout the world over the past couple of decades. Incidentally, color thinning at the end of veraison provides a different result than sorting during harvest. By harvest, all the fruit has turned color, and there is no way to determine whether a given cluster is two weeks behind the pack without tasting it or sampling the brix. Optical sorters at the winery can only remove the extreme laggards, but not the fruit that is just a week or two behind the others.

Interestingly, a study by Mike Anderson at UC Davis showed that the fruit eventually catches up around 26° brix (26-28° brix is fairly standard these days in many parts of the world, which I find sad, because in my opinion that level of ripeness obliterates terroir by homogenizing wine style...but that’s another story). So color thinning is probably unnecessary for modern high-ripeness winemaking. If you leave the fruit out long enough, it will all get ripe. Of course, for someone trying to make that style of wine in California in 2010 or 2011, when early rains cut short the prospect of long hangtime, rigorous color thinning could have been a savior. Color thinning becomes a tool of risk management, an expensive form of insurance. When picking earlier, say around 23.5°—a pretty balanced ripeness level for a more classic red wine—color thinning can make a pretty big difference in the greenness and acidity of the wine, and that comes down to stylistic goals. For our wine, for example, we don’t perform any color thinning, because we want the freshness that is contributed by the slightly less ripe fruit. To summarize: color thinning eliminates some green character and lowers acidity for an earlier harvest date or cool climates, but doesn’t have a whole lot of impact if the fruit is given a long extended hang time in a warm climate.

Do Lower Yields Always Produce Higher Quality Wines?

When producers say that they “restrict yields,” assuming the work is done early enough or they are selecting the green fruit at veraison, the result is a shift in wine style. This is an “improvement” in quality if the goal is higher alcohol, darker color, riper fruit character, less greenness, less acid, and more concentration. If these are not desired attributes, then restricting yields can actually be a negative. It is well known, for example, that fruity, high acid wines, like rosés, are much better at higher crop loads. When I was working as a research viticulturist I did a number of experiments with Chardonnay and Sauvignon Blanc, and we always preferred the fruitiness and balance of the higher crop levels. The low crop levels showed unpleasant bitter and phenolic characters, higher alcohol, less acid, and less fruitiness. They did have more weight, though, if that is the goal. High scoring-type Chardonnay producers often thin the crop way down, emphasizing the phenolic character to intensify weight and finish while softening the wine with lees stirring and new oak.

Among European Biodynamic and “natural wine” producers, there is a move away from fruit thinning. Some of the Biodynamic producers believe that it interrupts the vines’ natural ripening process or “rhythm”, while some of the natural winemakers and other traditional or “old school” producers think that the mix of less ripe fruit with the riper fruit is an important part of the wine balance and longevity. The green, vibrant fruit contributes freshness, a critical part of wine balance. There is no doubt that wine from vineyards with no fruit thinning will often show less concentration, lower color, and higher acid. The aromas can be much more high-toned, though, with finesse and the distinctive freshness that one often finds in the wines of more traditional producers. If lower yields are necessary to achieve a minimum level of ripeness, these producers leave less buds at pruning rather than thinning fruit.

White grapes react very differently than red grapes to fruit thinning. Color and tannin are not issues. A heavier crop slows ripening, allowing more time for green characters to dissipate and for ripe characters to accumulate. White varieties can benefit from as heavy a crop as the vines can ripen by the end of the season. As mentioned earlier, the exception is when viscosity is a goal, in which case a low crop seems to work better. The modern, thick and heavy Chardonnay formula requires severe fruit thinning to increase viscosity and concentration, and a long hangtime, during which the acid drops and the sugar climbs to high levels. Balanced and fruity whites do better with the opposite approach. A big crop can slow sugar accumulation while fruit character develops.

Restricting yields by thinning fruit or limiting bud numbers at pruning can be critical in cold, marginal sites—without a severely low crop the fruit may not be able to get ripe. On the other hand, in hot growing areas it is important to have enough fruit on the vine to temper sugar production, so that the alcohol doesn’t totally dominate the wine. This assumes that there aren’t other restrictions on the vines, like low water availability. There is a three dimensional matrix between crop load, water and nutrient availability, and climate.

There is a point of diminishing returns with fruit thinning, no matter what the goals. The difference between 2.5 tons per acre and 3.5 tons per acre in the Napa Valley, where the vines could easily ripen 5 tons per acre to conventional standards of sugar and acid, is probably nil. Yet people insist on thinning to an extreme that makes no sense. I often feel that there is a “no pain, no gain” mentality, that through suffering one achieves greatness. Or, “if a little bit is good, a lot is better.” Beyond the wastefulness of thinning more than is necessary or advisable, there is a sustainability issue: vineyards are farmlands that have been taken away from nature. They are poor wildlife habitat. They need to be sprayed, cultivated, etc., and even organic sprays have an impact on the environment. So the lower the crop level, the more environmental impact per hectoliter of wine. That is the operative formula. Sustainability needs to be looked at per unit of wine, not per acre, and it is wasteful to throw perfectly good fruit on the ground. Cutting the crop in half is equivalent to doubling the diesel, pesticides (organic or not), and acreage under cultivation.

There is a trend in high-end winemaking towards thinning fruit to a level that has no scientific grounding. It causes grape growers and viticulturists incredible consternation. There is a constant tension in the business between the grape growers and the winemakers. When the grapegrower and the winemaker are the same person, this tension results in sleepless nights. Even with a decent understanding of the science it is difficult to determine exactly how much fruit to thin, or even to thin fruit at all. The current dominance of tradition and conformity muddies the waters tremendously. It is difficult to have an open discussion between vineyard and winemaking stakeholders about how much fruit to thin, and when to do it. Many winemakers feel incredible pressure to make wine that “wows”, and there is a great deal of fear involved in the process. Thinning fruit is one way they can assert “control” over the “quality”, and ease the anxiety a bit. 

To summarize, other than in the case of a few Biodynamic and natural producers who don’t want to meddle with the natural ripening cycle of the vine (and a good number of viticulturists like myself who question the wastefulness and resulting high ripeness levels), most winemakers would agree that moderate fruit thinning is a standard modern practice in almost all cases. Thinning the fruit from the weak vines, the weak shoots, defective or diseased fruit, really green fruit, bunched up fruit, and heavily loaded vines, is universally accepted, and more about farming and managing vine health than wine style. Further fruit thinning to reduce yields far below the “balance” level (which I define as the point at which the vines can easily ripen to 13-15% potential alcohol), if done early enough, may result in darker, more concentrated, and riper wines. Ultimately, this marks a change in style, but not necessarily quality.

Some Useful Conversions:

  • 1 ton per acre = 13 hectoliters/hectare = 2250 kilograms/hectare
  • 1 ton = 5.6 hectoliters = 2.5 barrels = 60 cases

General Comparisons of Cabernet Sauvignon Yields in California

  • Napa = 2-5 tons/acre
  • Lodi = 5-8 tons/acre
  • Fresno = 8-12 tons/acre
  • We use alot of the cut grapes for verjus, but I'm fortunate to have a restaurant and greenhouse composting at the vineyard I work at. I'm a little on the fence, but I personally like a green Bordeaux, some may not. I do feel for the winemaker in the sense that he or she never wants to make an inferior product, the whole awards system, blah blah, yet as much as consistency plays a role, sometime Mother Nature is not kind to you after you've gone through this process and makes me wonder sometimes how less catastrophic it would be to lose grapes if we let more hang or what if we went for higher acid than less. I'm surprised our winemaker has a full head of hair.

  • Steve,

    Thanks for the article!  I appreciate the way you ascribe yields to 'wine style' rather than quality, but I did have a question:  Regarding yield, how much do you take into account the rootstock and clonal selections used when fruit or color thinning? Or are there more important factors to consider?

  • Thanks for the great insights Steve! I had always assumed that less fruit on the vine contributed to higher quality, but it seems quality is viewed within a certain context of stye preference among those that share that opinion. Very good stuff, keep it coming!

  • Thanks Steve for such a valuable article.  It's great to hear an objective (even if somewhat biased) perspective so one can make decisions more effectively on their viticultural practices.  It's also nice to understand when producers explain they thin to the 1.5-2 tons/acre, how this is more about style than quality.


  • Steve; the article offered a perspective I've never actually heard or considered. Total game-changer for me and I really value the personal insight you instilled into the piece alongside clearly described other industry perspectives and practices!