Special Crop Load Report by Dr. Kevin Ker

 

Crop Thinning for Crop Balance Equals Vine Balance

After the tumultuous year of 2023, most growers are reporting that vines (for the most part) appear to be growing well, especially the hybrids. However, there are also reports of some vines growing poorly or have little crop with good vegetative growth, or in some cases-much more fruit than expected- to the level that the vines are not likely to ripen all of the clusters present.

All of these observations illustrate different ways a vine responds and compensates after winter injury followed by a very wet growing season. 

Poor vine growth is symptomatic of carryover winter injury where trunk injury and/or root damage is finally showing up. Last year, many vines that normally would have collapsed from stress in season did not; the wet conditions and early harvest did not stress the vine, but reserves expected to be accumulated did not happen.

Understanding natural vine biology can help us make sense of what we were seeing, especially where we are seeing lots of clusters. In the wild, grapevines are biennial bearing – meaning a large crop one year followed by a smaller crop the next. Part of this is due to its growth habit and the vine self regulating the balance of shoot and root growth (vegetative) with fruit crop load (reproductive) .

We manipulate the vines through cultural practices and site capacity to strike a balance between canopy growth and crop load to get a consistent crop each year and enable the vines to mature enough and acclimate to survive the maritime winters.  Through canopy management (shoot positioning and thinning, leaf removal, hedging, etc.)  we work to ensure good sunlight exposure on shoots to initiate fruit buds for next year's crop while ripening the hanging crop on the vine this year.

 

How we mange the vines this year can have big implications for fruitfulness for next year if we allow the vines to overcrop

Dropping fruit clusters or fruit thinning in a year after a really poor crop year is the most difficult thing to do when you are looking for a quick financial recovery. However, this short term (one year) gain will lead to long term pain (reduced crop next year) by taking the vine out of balance. As well, excess crop often results in poor fruit quality, delayed ripening, poor vine acclimation for winter and reduced winter hardiness.

Crop thinning to achieve crop balance, fruit bud initiation for next year and vine balance should be your prime objective

Vine capacity (amount of fruit produced that ripens well and achieves maturity) is directly related to cultivar potential, vine age, vine size along with site attributes to achieve adequate vine vegetative growth. Hybrid cultivars can usually carry a much larger crop than vinifera, and hybrid vines are generally larger and more vigorous.

How do I make crop thinning decisions after a poor growing season (2023) and vine injury

The vines in 2024 are still in a recovery phase, and should not be expected nor pushed to crop levels achieved prior to 2023. Many hybrids where trunks were intake and had a moderate crop in 2023 (50-60% of normal) can likely handle a crop of 80 to 90 % in 2024 allowing for more recovery and building of reserves in the trunk and roots for future production. Vinifera cultivars, where fruit is present in 2024, should probably only target at most 50% of a crop or less- if trunk renewal/ re-establishment is taking place.

In literature, viticulturists measure vine crop yields and dormant pruning weights to determine if a vine is weak, moderate or extremely vigorous.  After 2023 season where vines were left “wild” doing pruning weights for 2024 would not have helped us this year. Weak vines have low pruning weights, observed in season with weak or stunted shoots, smaller than normal leaf size and poor leaf colour. All of these attributes are indicative of a damaged vine and is likely carrying too much crop for its capacity. Vigorous vines in season often have very long shoot growth, large numbers of laterals, dark green leaves and often require lots of canopy management and not much crop.

This year, it is highly encouraged for you to get your crop yield, know the actual number of bearing vines in the block and calculate an average yield per vine.  Then in spring you can take some vine weights and  use the information to get crop weight / pruning weight ratio to assess vine balance . This ratio is known as the RAVAZ Index.  A general rule of thumb ratio of 5 to 10 ( 5 to 10 lbs of fruit per pound of pruning weight) is good but this will also depend on the cultivars grown and site capacity.

A generally accepted concept is that a vine needs 15 to 20 mature leaves on a single shoot to ripen a primary cluster on the SAME shoot!

In general, hybrids are very vigorous with multiple lateral shoots growing on the original primary shoot can handle two to three clusters as there are enough mature leaves from veraison to harvest to ripen the fruit. 

On vinifera cultivars, only very strong shoots can carry two clusters to maturity and depends on vine age, overall health and vine density.  In high density situations, many growers often thin to one primary cluster per shoot as overall vine size ( canopy and root system!) is smaller  ( smaller vines should have smaller crop load per vine – optimum  Vine Balance!)

Also. be aware that leaves on a shoot with no fruit does not help mature fruit on neighbouring shoots. It will help feed the trunk and roots and reserves but does not help ripen the current crop on the rest of the vine. 

How do I assess what a vine can ripen? Experience is the best way to understand what your vine can or cannot carry but the following are some generalizations.

Vine observation	# clusters to keep on a shoot
·      Stunted shoot growth in mid July (< 50 to 60 cm) ·      No new tendrils growing on shoot tips ·      Few lateral shoots growing ·      Shoots less than pencil size diameter ·      Poor leaf size and colour	NO FRUIT
·      Moderate primary shoot growth (1 to 1.25 meters) ·      Some lateral shoot development on primary shoot ·      Pencil size shoot diameter or slightly above ·      Normal leaf size and leaves with dullish green appearance	1 to 2 clusters per shoot
·      Vigorous primary shoot growth (more than 1.5 meters) ·      Numerous lateral shoots growing vigorously ·      Large leaves with deep green colour ·      Large diameter shoots (>15 mm) “bull wood developing”	2 to 3 clusters per shoot

 These are just generalizations and grower experience will always be more accurate than any formula. Site history (crop production records) and capacity is your best information to guide ongoing and future production.  The following are some additional web resources on this topic.

 

·      Cluster Thinning – Cornell University  (PDF)   Cluster Thinning (cornell.edu)

·      Hellman, E.W. and T. Casteel. “Crop Estimation and Thinning” in Oregon Viticulture, ed. Edward Hellman. Oregon State University Press, 2003. http://winegrapes.tamu.edu/grow/tentative%20pdf/thinning.pdf

·      Pool, R. 2001. Estimating and adjusting crop weight Finger Lakes vineyards. http://www.fruit.cornell.edu/grape/pool/cropsize.html

·      Skinkis, Patty. 2019. “ Basic Concept of Vine Balance”  http://www.extension.org/basic-concept-of-vine balance/

·      Skinkis, Patty. 2019  “Crop Thinning: Cluster Thinning or Cluster Removal”  http://www.extension.org/pages/31767/crop-thinning:-cluster-thinning-or-cluster-removal