Vine Vigor: Use It or Lose It
As the summer progresses, growers often get frustrated with the high vigor of grapevines grown in our region. While vigor does require effort to manage, its preferable over vines with no vigor. Vigor is a term that can have different meanings, so defining it can be a good starting point.
Vigor in grapevines is generally referred to as the rate of growth of the vine or its shoots. The quicker they grow, the higher the vigor. Another useful term is vine capacity. Capacity of a grapevine is the total growth of a grapevine, including the fruit and the canopy. Capacity is tied with yield potential, while that is not always the case with vigor. Vigor is more of a transient trait than capacity, as we can manipulate vigor by adjusting the amount of shoots and fruit on a vine.
In an ideal world, the grapevine canopy would only fill its trellis space, and then stop growing. We all know that it rarely works out that way. Vigor is affected by the number of shoots and the amount of fruit on the vine. The grapevine canopy produces sugars (most of the time), and the fruit is a sink for sugars. If we have too few shoots and/or a small amount of fruit, the vine reacts by putting most of its resources into the shoots that are there, and we get vigorous growth. This is where a low yield can hurt fruit quality because there is too much canopy, and the fruit is not in the sunlight.
In Figure 1, we have diagrams of two shoots. In this example let’s assume that the shoots come from vines with the same capacity, so they have the same amount of resources available to them. The shoot on the right has moderate vigor, because a lot of the resources are going into the fruit, which moderates the shoot growth. The shoot on the left has little fruit so those vine resources are now available to increase shoot growth, as evidenced by the abundant laterals on this vine, which will shade the fruit. So, the shoot on the right has a better balance between fruit and shoot growth.
Balance is a vague term that can be quantified by various vine measurements, but some of those measurements can be tedious to track in an entire vineyard. However, I would encourage you to consider tracking fruit yield and pruning weights on a few sentinel vines in your vineyard, so you have some numbers to help determine where your vineyard is at. I am willing to assist with this, so please contact me if you have interest.
While yield is a good indicator of vineyard profit, it does not tell the whole story. To some degree we can just look at a vine to see if it’s balanced, but that can be difficult to do without a reference point. The vines in Figure 2 both have the same amount of fruit on them, but very different amounts of canopy growth. Vine A has moderate vigor shoots, while Vine B has a high amount of vigor as evidenced by the large shoot growth on the vineyard floor.
Many grower reactions to Vine B are to remove the excessive growth. While that may have some merit, the question is why is there excessive growth, and how do we prevent that in the future?
In the Figure 3 there are three vine scenarios. The top is low in capacity, the middle is medium, and the bottom is a high-capacity vine. The width of the triangle represents the abundance of fruit (purple) or canopy (green) at various vine yield situations. Generally, the more canopy, the higher the vigor. On the left edge are high vigor vines that have very little fruit and a lot of foliage (under cropped). On the right-hand side are over cropped vines have excessive fruit and shoots that are low in vigor. Note that what is considered over cropped is relative. For a very low-capacity vine, 3 tons per acre could be over cropping, while on a high-capacity vine this could be under cropping.
When you transpose the vines from Figure 2 onto Figure 3 it shows that Vine A is a balanced, medium capacity vine. In our example its producing 4 tons of fruit/acre or around 15 lbs. of fruit/vine and at the end of the year is has a pruning weight of around 2.5 lbs./vine. One way to look at balance is by calculating the crop load ratio, where we divide the fruit yield by the pruning weight. Vine A has a crop load ratio of 6 (15lbs fruit /2.5 lbs. pruning weight), which is in the range of where we want to be (~5-10).
Vine B on the other hand has a high capacity (Figure 3). It is farther to the left on the scale because it is also producing around 15 lbs. of fruit/vine, but it is producing more foliage and at the end of the year it has 5 lbs. of pruning weights/vine. Visually we can see (Figure 2) that the vine is out of balance due to its high vigor and Vine B also has a crop load ratio of only 3 (15/5).
As our crop load ratios decrease, we are increasingly under cropping vines, compared to what they might be able to handle. Some sources indicate that when a crop load ratio gets below 5 we are losing yield and creating quality issues by having excessive shade. To bring Vine B into balance where it has medium vigor shoots (Figure 3: shift it to the right), we need to increase the amount of fruit on that vine in the future. That can increase our profit by increasing yield, but it may also potentially decrease our labor managing the canopy and increase fruit quality since the fruit is likely in less shade.
Ways to Bring High Vigor Vines into Balance:
- Leave more buds at pruning. A way to increase fruit yield and moderate shoot growth is to increase the number of buds left on the vine at dormant pruning. Some sources indicate that around 6 shoots per foot of cordon is an idealized maximum, but some cultivars might handle more or less than that. One option is to always leave more buds than you need and thin off the least fruitful shoots.
- Summer canopy management. If you have reached the maximum numbers of shoots you want on your vines, and they are not getting the amount of fruit you need to bring the vines into balance, you may need to increase the amount of sunlight getting into your canopy. In early summer the buds that we prune to at dormant pruning are determining how fruitful they will be. The buds in more direct light generally will have shoots with more clusters and larger clusters. For high-wire trained vines that may mean combing those shoots downward rather than let them grow horizontally on the trellis. For vertically shoot positioned vines, that may mean hedging the canopy if it gets too thick. This may initially require more labor, but in theory the vines will be more balanced the next growing season and not require as much canopy management.
- Reduce nitrogen rates. If you have genuinely tried the above-mentioned items without success, reducing the nitrogen fertilization rate may assist in reducing vigor as well. Be cautious on eliminating nitrogen fertilization all together since we will still see nitrogen deficiencies in grapevines grown in the fertile soils of our region.
- Adjust vineyard floor management: If items 1-3 are still not enough, we are increasingly seeing growers with high vigor vines reducing the width of (or eliminating) the plant-free zone under their mature vines. However, in some situations this can provide excessive competition with vines and also reduce yield capacity.
Randall Vos, Ph.D.
Commercial Fruit Crops Field Specialist
Iowa State University Extension & Outreach
210 N. Iowa Street
PO Box 409
Knoxville, IA 50138