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From the Lake Erie Regional Grape Program Crop Update of 1/17/01

Cultural Practices -  Dr. Terry Bates

  More Than One Way to Skin a Grapevine

Pruning, the age-old practice of controlling crop level and vine growth in vineyards, is still one of the most debated issues in pole barns and coffee shops along the Lake Erie belt. The controversy over how to prune Concord grapevines has been as general as "cut some and leave some" and as specific as "leave 'x' amount of buds for 'y' pounds of dormant cane pruning weight." It is no surprise to me that the debate over minimal vs. mechanical vs. manual pruning systems would be any less controversial than the debate over fixed node vs. balanced pruning.

How many buds do I prune to?

This question comes up a lot and my usual response is, "it depends." It depends on vine performance, site potential, and the goal of the grower. Although I can't give you a single number that all vines should be pruned to along Lake Erie, I can describe the relationships between bud number, yield, quality, and vegetative growth in a healthy Concord vineyard. Understanding pruning concepts will help you make a better decision for your vineyard than me telling you a meaningless single bud number.

Let's start with the conservative extreme and say a grower prunes a healthy vine to only 5 buds (at the standard 8 x 9 vine spacing). The 5 buds can only grow so much leaf area and fruit, even if each bud grows to its maximum potential. In this case, pruning limits both vegetative (leaf area) and reproductive (crop) growth. Since the leaf area does not fill the trellis space, total light interception is limited and the maximum growth potential is diminished.

Since 5 buds per vine does not make the grower any profit and since his grower neighbors laugh him out of the coffee shop, let's say he decides to go with the industry standard of 60 to 80 buds. Assuming the grower is average at staying on top of fertilization, weed control, and pest management, the increase in bud number both fills the trellis space with leaf area and increases his yield to the belt average. In most years, the grower is ready to harvest in the beginning or middle of the harvest season. An overall average situation.

The Crop Load Effect

Now the grower thinks, "If 70 buds are better than 5, then 120 must be better than 70." So the grower prunes to 100-120 buds per vine. Leaf area growth still fills the canopy with leaves so total light interception is not a problem. He notices that shoot density is getting tight but it does not seem to be a big problem yet. The vineyard yield continues to increase but some other negative things start to happen. The higher crop load delays the sugar accumulation in the fruit so he cannot harvest early in the season anymore. Everything being equal, for every extra ton of grapes that he hangs, he must wait 3 extra days to harvest. In addition, he notices that the vines do not ripen as much wood as in the past.

Two things are happening in this case. 1) Increasing the crop load delays fruit ripening. 2) Maximum leaf area and maximum crop increases vine water and nutrient demand. While assimilating carbon dioxide from the air to make sugar during photosynthesis, Concord leaves lose a lot of water. The greater the exposed leaf area, the greater the photosynthesis, but also the greater the water loss. Even on a sunny day during a wet year, leaf photosynthesis can decrease during the middle of the day. Why? When the vine becomes water stressed, the tiny pores in the leaves (stomata) where carbon dioxide and water are exchanged start to close to conserve water. As a result of water conservation, photosynthesis decreases. What is the impact on the vine? Less ripe wood! Fewer ripe nodes of periderm (fewer ripe buds) leads to a lower return crop at a given bud number.

Can the grower do anything about this? Yes! Maintaining leaf photosynthesis is a matter of good vineyard management. Better weed control during dry years or dry blocks and cleaner pest management in high stress years are the two practices that are on the top of the list. Irrigation, mulch, and crop thinning have also been options for some growers to relieve water and crop stress on the vines, usually as spot treatments in poor blocks.

Now, let's say the grower follows this lesson and really pays attention to management practices. He keeps the vineyard totally weed free during the month after bloom and even uses mulch or irrigation in some dry or eroded blocks. Possibly, he also adopts mechanical thinning in years where the crop is inappropriate for the weather conditions. What he starts to notice is that he can still ripen a large crop somewhere in the middle of the harvest season but he also brings the ripe wood back to where he wants it (somewhere around the 500-600 ripe nodes of periderm level or 2.5 pounds of dormant cane pruning weight for you balanced pruning followers).

Minimal Pruning and Canopy Inefficiency

Despite the higher sustainable yield, all the extra attention to detail is costing the grower money. He hears that other grape growing regions around the world are finding success with minimal pruning, a huge cost saving practice. So, he tries minimal pruning in his vineyard in New York.

As the bud number jumps from 120 to 350-500 buds per vine, several things happen. The vines still fill the trellis space with leaf area so total light interception is not a problem; however, the shoot density is very tight and there is a significant proportion of leaf area that is constantly in the shade. The grower also notices that the yield does not increase further by leaving up more buds. What is more disturbing is that the dense canopy further delays fruit ripening over and above the normal yield-brix relationship. In other words, the grower is now harvesting the same yield later in the harvest season with minimal pruning. To top it off, the ripe wood (or ripe nodes of periderm) is much lower than in the 120 node vines.

If 120 node and minimal pruned vines under the same environment both intercept the same amount of light and have the same crop, then why do minimal pruned vines have a later harvest date and less ripe wood? The answer appears to be the inefficiency of the canopy. The shaded leaves of minimal pruned vines are "not water use efficient, " meaning that they use water but do little photosynthesis. Therefore the whole canopy (exposed leaves and shaded leaves together) use more water than the 120 node vines with very few shaded leaves. Therefore, the minimal pruned vines come under water stress, shut down their stomata, and decrease photosynthesis earlier in any given day than the hand pruned vines (or minimal pruned vines with the shaded leaves removed) in the attempt to conserve water. Less photosynthesis = lower rate of fruit and wood maturation.

Having identified its shortcomings, is minimal pruning "bad?"   Not necessarily! Minimal pruning is a higher stress system and vineyard management can be used to decrease the stress of the system. Again, weed management, pest management, irrigation, and/or fruit thinning are all techniques used in successful minimal pruned vineyards in New York. If a grower is not able or willing to recognize and manage the characteristics of minimal pruning then the grower should not use it! We must also recognized that some sites will never be able to handle minimal pruning and the best management practice for those sites is not to use it.

Why does minimal pruning work in other parts of the World? Minimal pruning success in other grape growing regions has to do with growing season length. We know that the minimal pruned system is later ripening but if you have the luxury of a long ripening season then the vines have plenty of time to ripen both the crop and the wood for the next season. This is NOT the situation in Western New York where a short ripening period is the norm and an early frost is possible.

Mechanical Pruning and Minimal Pruning:  Not the Same Thing!

So where are we now? Can the grower combine the cost saving technique of mechanical pruning with the viticulture soundness of appropriate bud number? Growers are now using mechanical pruning to target more reasonable bud numbers for Concord vineyards. The major drawback of pruning with a machine is in bud selection. Hand pruning offers the luxury of selecting only the best, most fruitful buds. Minimal pruning has no bud selection at all. Mechanical pruning is a compromise between the two but it is still impossible to select only the best buds by machine. Therefore, two techniques have developed to address the bud selection issue. 1) Mechanically prune the vineyard and leave up a few more buds than you need. When I say "a few" I am talking about leaving 150 buds per vine NOT 500 buds per vine as in minimal pruning. Then use mechanical thinning 30 days-after-bloom to make the crop load appropriate with the season and vineyard. 2) Mechanically prune to approximately 150 buds and then do a hand-pruning follow-up to improve the bud selection. Theoretically, thinning will be used less often or not at all with the hand follow-up technique.

Current research addresses these mechanical pruning questions by comparing the following pruning systems. 1) Traditional hand pruning to the best 100 buds. 2) Mechanical pruning to approximately 150-160 buds with hand pruning follow-up to approximately 110-120 buds. 3) Mechanical pruning only (150-160 buds) with mechanical crop thinning on the years that need it. 4) Minimal pruning with thinning on the years that need it. Notice that the treatments address not only bud number but bud selection and that crop control is addresses by pruning, thinning, or a combination of the two.

What is the Answer?

"It depends!" It depends on what the grower is comfortable with doing. Hand pruning is labor intensive with less risk. Minimal pruning is less expensive but high risk (some might argue that is a manageable risk). Mechanical pruning alone appears viable if the grower is comfortable with mechanical thinning. Mechanical pruning with hand follow-up has given us identical results as hand pruning in our research plots.

What is the Concept?

Bud number, crop load, and shoot density are decisions made by the grower that increase or decrease stress in the vineyard. Machines are just tools used to implement the grower's decision.

There is more than one way to skin a grapevine. The choice is up to you.