Feb 6, 2025

Winter pruning essentials for vineyards

A Q&A with Markus Keller on how winter pruning protects wine, table grapes.

Winter pruning in grapes is crucial for producing uniform canopies and evenly distributed buds. Pruning also keeps the size of the vine consistent and manageable, allowing growers to enter vineyards with equipment and machinery for harvesting ease.

To learn how proper winter vineyard pruning can help growers assure healthy wine and table grape production, Fruit Growers News met with Markus Keller, a Chateau Ste. Michelle distinguished professor in viticulture at Washington State University’s Irrigated Agriculture Research & Extension Center in Prosser, Washington.

Keller’s research focuses on developmental and environmental factors and vineyard management practices as they influence crop physiology of wine and juice grapes. His lab investigates scion-rootstock interactions, grape development and water relations during fruit ripening, irrigation and crop load management, yield formation and cold hardiness.

How do vineyards benefit from winter pruning?

It will establish the number of buds on a plant, on a grapevine, the bud number and then also their position on the plant. That, in turn, determines the distribution of the shoot on the canopy.

How does pruning during the winter benefit canopy growth?

When you first establish a vineyard, you prune differently than once the vines are mature from about three to four years of age. When you prune an established vine, it’ll have a certain size of its root system. The more material that you prune off the canopy in the winter, the more vigorously the remaining shoots will grow.

You can manipulate the amount of vigor by how much you prune off. In an established vineyard, you typically prune off about 90% of the annual growth. You can vary that percentage depending on whether you want to increase or decrease the vigor of the plant.

Does winter pruning help vines resist diseases?

To a degree. The pruning itself doesn’t really change the pathogen resistance. What it will do during the growing season is change how accessible the canopy is to spraying. For spraying, you want those vines to maintain approximately the same size over the life of a vineyard once
the vineyard is fully established.

If your canopy is open and the shoots are evenly distributed and there are some gaps, then your spray coverage is going to be better than if you have a very dense canopy with a lot of shoots, a lot of leaves shading each other. It’s more about spray coverage and less about pathogen resistance.

What role does dormant pruning play in avoiding infections from fungal and bacterial pathogens?

During the winter, you can get infections from trunk disease pathogens, fungal species that will enter the plant through pruning wounds and then establish themselves in the plant’s vascular system and, over time, slowly tend to kill the plant. Those pathogens enter mostly during wet weather.

Typically, the risk for trunk disease or pathogen infection increases as the size of the pruning cuts increase. One piece of advice would be to try to avoid large cuts on the vine. Making more but smaller cuts is often beneficial. If there is a problem, if you have an infection of trunk diseases, you can sometimes do vine surgery. Because these pathogens grow fairly slowly down the plant, you can remove the infected portion and retrain a vine from suckers close to the ground.

What are the dangers of under-pruning?

Grapevines are good at self-regulating, so they compensate a lot. For example, minimal pruning or mechanical pruning where you don’t prune a lot during the winter used to be very common in Australia and now in juice grape production in Washington. You have a lot of shoots. Then, in the first year or two, the vines will overshoot and overproduce, and will kind of self-regulate over time. The canopy is a little bigger, but the crop balances over time so that there’s not really that much danger of under pruning. The vine is not going to get too big or too vigorous over time.

Is there a difference in pruning winegrapes versus table grapes?

In terms of timing, there isn’t really a difference in the overall pruning approach. The general rules are the same, but you’re going to prune a winegrape canopy obviously lower to the ground than a table grape canopy. In most cases, you’re going to leave fewer buds in winegrapes than in table grapes. Most winegrapes nowadays in America are pruned to spurs. They’re cordon trained vines and then spur-pruned.

Many table grapes are still cane pruned because there’s often a little less fruitfulness at the base of the cane and then the fruitfulness increases with higher bud numbers, higher bud insertion on the cane. They’re leaving longer canes on a table grape than they do in a winegrape.

Pruning, including these minimally machine-pruned juice grapes, maintains vine consistency and management and bud numbers.

What are the best times for a grower to prune vineyards for the winter?

It depends on whether they are in a winter freeze or spring frost prone area or not. If there’s no danger of winter freeze or spring frost, then the grower can prune pretty much anytime during the winter. Where danger from low temperatures exists, delaying pruning until close to or even after budbreak can be a good risk management strategy.

If you push pruning out closer to or after bud break, the delay in budbreak often means that the shoots grow in warmer weather so that shoots grow more quickly. In principle, if there’s no trunk disease pressure, then a grower can start pruning almost after harvest or as soon as the leaves fall. When there is spring frost, it’s often beneficial to delay pruning even until after bud break. A vine can make up a three week delay in bud break. What happens if you prune late is you’re going to delay bud break.

So often you tend to escape the spring frost risk period simply because the vines start growing later. They make up for that because the buds now break during a warmer period of the year and the shoots grow faster and they compensate for the lost time during the growing season. If you go too late, say, past about four weeks after normal budbreak time, they will not be able to compensate for that time anymore and then ripening will be delayed. The window is before bud break to roughly about two to three weeks after bud break for pruning under those conditions.

How does dormant pruning invigorate vineyards?

It often doesn’t. That’s because vines are so good at self-regulating and at compensating. The overall principle is that the more buds you leave on a plant, the less vigorous the individual shoots are that grow from those buds, but you get more shoots. So you tend to get a bigger canopy overall.

You leave more buds, you get more shoots. Those individual shoots grow less quickly, but you get an overall bigger canopy and heavier crop. So you get a higher crop load overall. If your vines are not vigorous enough, we tend to then prune more buds off. So we leave fewer buds on the vine and then the individual
shoots tend to grow more vigorously.

— Doug Ohlemeier, Assistant Editor