UGA Extension Viticulture Blog

Considerations for crop thinning

We talked about crop load management at the recent mid-season vineyard workshop at Wolf Mountain Vineyards and Winery.  Whether you were there or not, I hope this will serve as good food for thought if you are considering crop thinning.

Depending on variety, site, and local weather patterns, some will be starting veraison within the next week to a month.  At this time, many may start thinking about crop thinning or “green harvesting” in their vineyards.

When to crop thin.

We have typically practiced crop thinning around a week or two before veraison, but no later than say 25% color (in red grapes).  This is done in hopes of limiting subsequent berry size increases associated with earlier cluster thinning, and to potentially improve the maturity of remaining fruit.   In addition, a second round of “fine-tuning” can take place before or during harvest – by discarding green, unripe, or otherwise diseased clusters.

How to judge if crop thinning is needed.

Many may ask “How do I know if I should crop thin?”  That is a good question, and an impossible question to answer for all scenarios.  So, here are some practical scenarios where one might think about thinning some crop from their vines:

1. Shoot thinning.  To me, this is the best way to regulate crop – you are removing vegetative and reproductive tissues in tandem, thereby shaping your canopy and setting your crop yield potential all at once.  Neglecting shoot thinning will increase crop potential without a correspondent linear increase in functional/exposed leaf area – especially in single canopy, tight VSP-type systems where there is often lots of shaded, interior leaves.

This Merlot vine had not been shoot thinned, hence the high cluster density.

2. Canopy health.  Canopy health and, thus, photosynthetic efficiency is compromised by downy mildew, other grapevine diseases, and/or nutrient deficiency. In these cases, crop thinning may be warranted.

This downy mildew-infested canopy will have compromised photosynthetic efficiency and, thus, fruit-ripening capacity (photo courtesy of Phil Brannen).

3. Cluster density.  Fruit zones that are overcrowded with grape clusters often have a high incidence of grape clusters laying on top of one another.  In humid regions, the area in between two touching clusters has a high bunch rot incidence.

This Cabernet Sauvignon cluster was touching a neighboring cluster where this rot developed.

4.  Variety.  Chambourcin, Vidal blanc, and Chardonel bear high crops.  While I have harvested 7 tons/acre of very ripe Vidal blanc from a single VSP system, this may not be achievable in every vintage.  Chambourcin bears large crops and often needs thinned to optimize fruit quality and maintain perennial vineyard health.  Crop load management may be more important in vinifera varieties.  For example, Cabernet Sauvignon is difficult to ripen in many vintages in the eastern US.  Thus, crop thinning may be more warranted in Cabernet Sauvignon than say Petit manseng, which often has modest, but ripe crops.

Records of previous crop yields and fruit composition can help determine if your crop needs thinned. Vineyard managers who have farmed the same vineyard for several years can often visually tell if the crop load on their vines is above average.  However, if the same pruning and shoot thinning practices are conducted seasonally, there should be no drastic surprises, sans a drastic change in vine fruitfulness. While I don’t have data to back this up (does anybody?) – crops much higher than 4-5 tons/acre are probably tough to consistently ripen on a single canopy tight VSP system in the eastern US.  There are variety, vintage, and trellis system exceptions, of course, but this may be a rough guideline.

Tying it together

The below figure, courtesy of Terry Bates, shows Concord soluble solids (Brix) at harvest vs. leaf area: fruit weight ratio.  This figure shows that there is a threshold at which thinning crop will not result in a concomitant increase in Brix levels – and if you thin past this point there will be diminishing returns in fruit ripening and you will basically be throwing fruit (and money) away.  In this example, it appears to be around 15 cm2 of leaf area / g of crop weight, and this produced grapes that were of commercial maturity in the Concord industry (>/= 16 Brix).  This figure also demonstrates that soluble solids is not linearly related to crop amount (i.e. you will not gain 10% Brix if you thin 10% of your crop).  It’s just not that simple.

Leaf area: fruit weight ratio and soluble solids at harvest in Concords grown in the Lake Erie region. (courtesy of Terry Bates)

Kliewer and Dokoozlian (2005) suggest that 8 to 12 cm2 of leaf area / g of crop weight are a good hallmark for vine balance. What does this relationship look like in your vineyard?  It is difficult to say given the impact that several factors can have on this relationship, including climate, weather patterns of the current vintage, variety, and training system.  Keep in mind that both Terry’s and Kliewer and Dooozlian’s data are primarily based on soluble solids, which is far from the only compositional attribute that determines fruit and wine quality.  Many of the variety-specific metabolites that determine varietal character and quality are affected by direct radiation and temperature on the clusters, and may have less to do with actual crop load.  In fact, a recent study published by Frioni et al. (2017) found that crop thinning alone was not sufficient to change fruit composition, but that crop thinning coupled with fruit zone leaf removal resulted in greater anthocyanins and phenolics, albeit only in the vintage characterized by relatively cooler temperatures over the ripening period.  This is yet another example of how cultural practices have greater impact in some vintages compared to others – especially in the highly variable vintage-to-vintage weather patterns in the eastern US.  Since we can’t predict the weather, it is advised to implement best management practices (i.e. shoot thinning and positioning, fruit zone leaf removal, canopy hedging, and variety- and winemaking goal-specific crop thinning) every year.  This could not be more true for many Georgia vineyards this year, a year that has so far been characterized by infrequent clear days, very frequent rainfall, and high disease pressure.

The only real way to know if crop thinning could benefit fruit maturation is to experiment in your vineyard –  thin some vines and leave others with a full crop.  If you crop thin all or none, you will never for sure know if crop thinning had an effect as you will have nothing to compare it to.  I would recommend doing this in varieties in which you have historically observed high crops coupled with attenuated ripeness.  In closing, I would encourage asking yourself the following questions: “Is my canopy healthy?”, “Did I prune to high bud numbers?”, “Did I shoot thin?”, “Does my fruit zone look highly crowded with fruit clusters lying on top off one another?”, “Have my hybrid or vinifera varieties been historically picked under-ripe?”, “What are my winemaking goals?” (rose wines and sparkling wines are made with more acidic / less sweet fruit). Answering these questions may help you decide if crop thinning is necessary in your vineyard, and may limit unnecessary crop thinning (and unnecessary cuts in production and economic returns). Note that in the Frioni et al. (2017) study, cluster thinning alone had very rare and inconsistent effects on fruit composition, yet reduced crop yield by 40-44% compared to the un-thinned control. Thus, wine quality potential was likely unaffected by crop thinning alone, but wine quantity potential was highly and negatively affected.  If I dropped 40% of the crop in my vineyard and made 40% less wine with no no perceived increase in quality, I would not be a happy vineyard / winery owner.


Further reading:

Kliewer and Dokoozlian 2005

Frioni et al. 2017


Good luck out there, and let’s continue to hope for drier weather.