Pruning: Bank on It
By Jessica Cortell
It seems like we just finished harvest, and now it’s time to start pruning, one of the most important practices in growing grapes. It can have a big impact on vine health and fruit quality.
In its simplest terms, pruning maintains the vine’s size and structure, encourages exposure to light and airflow in the canopy and improves ease of vineyard management. Cane pruning is commonly done with Pinot Noir while spur pruning is often done in Eastern or Southern Oregon for other varieties.
The concept of “balanced pruning” is to cut back to a specific number of buds based on wood weight and past performance of the vine. This practice is effective because the trunk and root system act as a bank account for the vine, meaning they store the carbohydrates and minerals needed for new growth in the spring. The first eight to 10 leaves on the shoot are primarily supported by these reserves. Consequently, if you leave too few buds, you’ll have only a few vigorous shoots; if you leave too many, shoots will be numerous but weak.
From my experience, a balanced pruning formula of 14 + 10 seems to work well in Oregon for Pinot Noir. This means leaving 14 nodes for the first pound of dormant wood and 10 nodes for each additional pound.
Richard Smart lists a number of desired values for vineyard canopies contributing to high quality fruit in his book “Sunlight into Wine.” I’ve tweaked a few of these to fit cool-climate production of Pinot Noir in Oregon.
We often have four to five feet of canopy, so an ideal cane weight should be in the range of 1 to 1.9 ounces per cane. If the average cane weight is below 1 ounce or above 2, adjustments should be made. If the number of short canes is above 25 percent, the cause should be investigated, as the approach in pruning might need to be different. If the average cane weight is above 2 ounces per cane, the vine vigor is too high.
If there are short canes near the head of an average or larger diameter with a zigzag pattern of nodes, scarring on the first few internodes and a loss of the terminal growing point, this is likely caused by rust mites in the buds. These are sometimes also associated with low-vigor vines but not always. The challenge with these vines is finding good quality wood to prune to near the head. Creative pruning sometimes needs to be used to find both replacement spurs for the next season and adequate length canes for fruiting canes.
One observation is that the stunting occurs on the main buds on the fruiting cane and spurs in the spring. If this is evident, suckers can be left later in the spring below where the stunted shoots are. This will allow for high quality canes for pruning the next winter.
If the short canes are the result of leaving too many buds the previous year during pruning, over-cropping, lack of nitrogen, drought stress, phylloxera or other issues, the bud number should be adjusted during pruning. A substantial response can occur by reducing the number of buds per vine by three to four. The vines will show a strong response by having fewer shoots that need to be supported by the energy reserves in the spring.
If the vines are too vigorous, the best way to decrease the energy is to add more shoots per vine; this can be done by pruning to a higher number of nodes. If the vines are planted too closely, another tactic called “kicker canes” is needed.
Kicker canes can be used to pull extra energy from the reserves. A kicker is an extra cane left on the vine with six to eight buds that can grow until prior to bloom, at which point it is cut off the vine. If the vigor is excessive and out of balance with the site, a transition to a Scott Henry trellis system can be considered.
It is important to focus on balancing each vine in the first three to four years of production to achieve a uniform stand of vines by full maturity. Remember to check what is in the bank account! On young vines, leaving too many buds can result in serious debt, as they don’t have the reserves needed to support the shoots. On the other hand, excess vine vigor can result in higher disease pressure, inadequate ripening, poor fruit quality and lack of proper hardening off of the wood in the fall and winter.
Keep in mind that balance is the equilibrium between vegetative growth and fruit production and this balance can be achieved in vines of all sizes. It is the same idea as keeping your checking account balanced between savings and spending.
COLLECTING PRUNING WEIGHT DATA
1. Determine sample stations in the vineyard: four to six vines between trellis posts in a couple locations, as needed. Ideally, the same vines are sampled each year.
2. Count and record the total number of canes on your data vines.
3. Count the number of weak shoots less than 18 inches long. This is useful because fruit is often dropped off of these, as they are too weak to ripen the grapes.
4. Prune the “data” vines in the sampling stations.
5. Collect and weigh all 1-year-old wood. Do not include the 2-year-old fruiting cane from last season. A hanging fish scale works well to weigh the wood.
6. Do calculations for average number of canes per vine, percentage of short canes, pruning weight per vine, pruning weight per foot, cane weight (in ounces) and other calculations as desired.
7. Decide how to prune the rest of the vines in the block based on the information from the data vines.