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By: Maria Smith, HCS-OSU

This past week, I have received several questions on when, where, and what to shoot thin (ST), so now is the perfect time to highlight the importance of this practice. In 2017, I co-authored an in-depth article on shoot thinning with Dr. Michela Centinari at Penn State that can be used as a cited companion resource to this blog post.  

Shoot thinning is the first important canopy management practice following dormant pruning, especially throughout Ohio and the Eastern US where high humidity, temperatures, and precipitation pose significant challenges to vigor control and disease management. This practice reduces the density of the canopy through the selective removal of excess shoots, in turn opening the canopy to increased sunlight exposure and airflow. The main benefits of ST include:

• Reduce excess yield and decrease fruit shading, thus improving fruit composition at harvest  
• Improved winter bud cold hardiness through increased sunlight exposure during bud development
• Lower disease pressure due to a less humid, faster drying canopy environment and, again, more sunlight penetration through the canopy

Timing ST:

ST is best performed when shoots are between approximately 6-12” in length  and, if possible, following the last date of frost. However, as Hickey, 2021 noted, constraints for timing ST may be dependent on labor availability. Also, hybrid varieties can be thinned before Vinifera due to the highly fruitful nature of secondary and latent/basal buds in the event of a spring frost. Very early ST (< 4”) can reduce the practice benefits by inducing emergence of secondary and latent buds, while late ST (> 12”) can be difficult due to the lignification of the shoot base. Late thinning should be performed using pruning shears to reduce injury to the vine.

As of this week (5/16/22), we are approaching the start of the window for shoot thinning at Unit 2 in Wooster (Fig 1).

Fig 1. Shoot growth at E-L stage 12 on 5/14/22 on Marquette. At this stage, shoots are about 5 leaves unfolded and 4” in growth.

Density of shoots:

For the sake of speed, keep in mind that these are estimates and visual inspections are sufficient as you are thinning. Only in academia will we stand around precisely counting shoots!

• Vinifera: 3 to 5 shoots per foot of cordon (= 18 to 30 shoots per vine at 6’ spacing)
• Hybrids: 4 to 6 shoots per foot of cordon (= 24 to 36 shoots per vine at 6’ spacing)
• Native or Labrusca varieties: up to 15 shoots per foot of cordon (= up to 90 shoots per vine at 6’ spacing)

As Dr. Patty Skinkis mentioned at the OGWC in February, spur vs. cane pruning tends to shift labor demands in the vineyard: spur pruning, while quicker to perform during dormancy, often requires more labor for ST than cane pruning. This is because spurs from older cordons tend to produce latent shoots that require removal, while cane pruning will typically produce one, or occasionally two shoots depending on secondary bud break, at each node position along the cane (Fig 2, Fig 3).

Fig 2. Shoot density of 2-bud spur (left) vs. canes (right). Note: increased density often arises from latent buds of older cordon wood or on vines with excessive bud retention in spur-pruned vines. Photos from Unit 2, Wooster. 5/14/22

Fig 3. Ideal shoot density, representing approximately 4 shoots per 1’ of space in V. vinifera Chardonnay. Photo from Unit 2, Wooster. 5/14/22

What shoots to keep/remove?

In a spur-pruned vine, the fruitful primary shoots from count nodes 1-2 on each spur should be prioritized (Fig 4). If a spur is becoming stacked, one well-positioned healthy shoot closer to the cordon may be prioritized for spur renewal. Fruitless basal and latent, diseased, weak-growing, and poorly positioned shoots should be the first to go, especially in highly dense portions of the canopy (Fig 5).

Lastly, unless retaining a shoot for potential cordon or trunk renewal, vine trunks should be suckered to prevent dense growth from shading the fruitful shoots (Fig 6). This can be accomplished easily at the 6-12” shoot growth stage by simply running a hand down the trunk of the vine to remove excess shoots.

Fig 4. Shoot emergence on a 3-bud spur on V. vinifera Cabernet Franc. 5/14/22, Unit 2, Wooster. Note: shoots that emerge on “non-count” buds are often fruitless in Vinifera and can be thinned. Fruitful shoots from nodes 1 and 2 should be prioritized for retention due to proximity to the cordon.

Fig 5. Example of a weak, fruitless latent shoot that should be removed during thinning

Fig 6. Example of a vine with dense growth on the trunk that requires suckering to remove the excess shoots and prevent canopy shading. 5/14/22, Unit 2, Wooster