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By: Maria Smith and Imed Dami, HCS-OSU
It is no surprise to anyone who has been out in the vineyard pruning that hydraulic pressurization and sap flow (“bleeding”) from pruning wounds has been seen earlier than normal due to the above average temperatures over March (Fig 1, photo).
We monitor the warmth of the growing season by recording the growing degree days (GDD). Last Friday (22 March 2024) the GDD at Wooster, Unit 2 was 43, which is higher than the historical average and is about two weeks ahead. However, when we checked the stage of development of the earliest bud breaking varieties, the majority of buds were still at the closed stage with some at the wool stage (see photos). The cool/cold weather we have experienced in the past few days has helped slow bud growth, which is a good thing. Although the recent weather event had no negative impact on bud survival, we are still concerned by the potential for early budbreak this season.
As an annual reminder, we are freeze prone (< 32F) from roughly April through mid-May, with the latest observed freezes occurring through early-May to early-June, depending on region (Fig 2).
Figure 1. Temperature departures from the 30-year average for March 1-March 24, 2024. Temperatures have ranged between 5 to 11F above average across Ohio during this period.
Vine “bleeding” from pruning wound. March 13, 2024, Unit 2 Wooster. Photo from Diane Kinney
Figure 2. Median date of last 32F freeze (top) and latest date of observed 32F freeze (bottom). Figures from https://mrcc.purdue.edu
Cabernet franc grapevines grown at the research vineyard in Wooster. Photo taken by Imed Dami on 22 March 2024.
Brianna grapevines grown at the research vineyard in Wooster. Photo taken by Imed Dami on 22 March 2024.
Itasca grapevines grown at the research vineyard in Wooster. Photo taken by Imed Dami on 22 March 2024.
Current susceptibility to freeze injury
Several growers reached out this week with concerns about potential bud injury due to temperatures reaching near 20F (Fig 3).
Figure 3. Minimum low temperatures on 3/21/24. Most regions hit lows in the low-20s, with some areas in the 10s.
Across the Eastern US, buds are undergoing “cold deacclimation”, which is the transition from maximum winter cold hardiness to a cold-sensitive state. With the recent cold temperatures last week, there have been concerns that these temperatures may cause damage, even though buds are still dormant.
The cold hardiness, or LT50 (lethal temperature that kills 50% of primary buds), of the cold sensitive vinifera, Sauvignon blanc, was -1.8F on 12 March 2024. Its cold hardiness was -9.9F on 11 January 2024. In the past 10 days, the minimum air temperature at Wooster, Unit 2 was 17.8F (on March 21).
A bud cold hardiness model released by Dr. Jason Londo’s lab at Cornell, seeks to provide broader guidance throughout the Eastern US on real-time estimates for bud cold hardiness. This model provides estimated cold hardiness for several key regional wine and juice grape cultivars. Combined with our own bud DTA cold hardiness measurements in Wooster, we are highly confident that buds should not have experienced injury from this past week’s cold temperatures. However, if you remain concerned, it is advised to select a few buds to dissect to verify if any injury occurred at your site.
Planning for another year of spring frost risk and damage
Given that vineyards will be at risk for dormant and/or green tissue injury for several weeks to come, it is important to be prepared for mitigating and responding to injury. We have experienced some level of spring frost in Ohio for the past five years in a row! Be prepared by reading up on some of our previous posts on preparing for spring frosts and managing injury or obtaining a copy of the spring frost bulletin from OSU Extension.
We will be keeping you updated over the next couple of months for the emergence of bud break and any major spring frost events. Although we may not personally enjoy these cold spells, it is helpful for pushing back bud break and our spring frost risks.
By: Maria Smith, HCS-OSU
It’s still winter, but thanks to this El Niño year, it sure hasn’t felt like it in Ohio. Daily average temperatures over the past 30 days have ranged between 6-10 °F above the 30-year mean (Fig. 1). Many of our vineyards are likely taking advantage of this weather to prune the vines for the upcoming year. To those with smaller vineyards (< 10 acres), there is still plenty of time to get your pruning done through March and early April before buds begin to break dormancy. For a year with this mild of a late-winter, delaying your dormant pruning as long as possible can be considered advantageous to staving off bud break among your preferred yield-producing buds.
The goal of this post is to remind you and your crew about pruning best practices following last week’s Wooster grapevine pruning workshop. In 2023, we published a pruning primer article about the what, when, and whys of pruning. In this companion article, we’ll touch on vineyard sanitation and a few routine pruning errors so that you can maximize the long-term health and productivity of your vines.
Fig 1. 30-day Average temperature departure from mean Feb 9, 2024 – March 9, 2024. Photo from https://mrcc.purdue.edu
Vineyard Sanitation:
Did you have grape diseases in the vineyard during 2023? Fungal diseases such as Anthracnose, Phomopsis, and black rot are well-known to overwinter on woody tissues (canes, cordons, trunks) and persistent mummified fruit that was left in the vineyard overwinter (Fig. 2). These infected tissues go on to serve as a source of pathogen propagules for infecting new, healthy tissue over the next growing cycle. Similarly, crown gall bacteria (Agrobacterium vitis, Fig. 3) can persist for years in infected tissues and is managed through renewal of vine parts during pruning.
Vineyard sanitation, or the act of removing and destroying dormant, infected tissues while pruning can aid in the reduction of pathogen populations available to infect in subsequent seasons. If disease was a significant issue in your vineyard, it is best practice to destroy tissue by burning, burying, or removing completely from the vineyard rather than composting, since inadequate temperatures during composting can fail to kill propagules. Sanitation should also extend to tools through regular cleaning and sharpening (sharp-cut pruning wounds heal quicker) of pruning tools.
Fig 2. Phomopsis lesions (left) will persist on shoots that lignify and become canes; black rot mummy berries (right)
Fig. 3 Grape trunk infected with crown gall, a bacterium that causes tumor-like growths (galls) on infected woody tissues (trunks, cordons, roots).
HOWEVER, if your vineyard was clean of disease, cuttings may be mulched using a flail mower or brush hog (Fig. 4) directly in the row. Avoid using a regular lawn mower deck to chop up pruned wood, especially larger pieces of trunks or cordons.
Fig. 4 Flails on a rotating drum. Flail mowers and brush hogs can chop up small diameter woody materials. Photo from: https://en.wikipedia.org/wiki/Flail_mower
Routine pruning and training issues:
Grape pruning and training are not intuitive, and even experienced practitioners make mistakes in the process. Here are a few of the most common issues that I encounter when out assessing vineyards in the late-winter:
- Training weak wood: healthy wood is 1) ¼ to ½” in diameter, 2) sun exposed in the previous year, 3) has dark brown periderm (outer bark). Canes that are small and weak or large and vegetative are less cold hardy and less productive than optimal wood quality.
- Retaining excessive bud numbers after final pruning: Pruning should remove approximately 80-90% of the vine growth from the previous season. Retaining too many buds after final pruning sets the vine up to produce shoots with smaller clusters and less shoot growth than vines with an “optimal” number of buds. Refer to the Midwest Grape Production Guide for guidance on balanced pruning practices.
- Training cordons that are too long: Cordons are best established in segments of 12 to 15” in length, especially in Vinifera cultivars. This is to encourage all buds along the cane to develop into healthy shoots. Leaving excessively long cane segments while training or retraining cordons can result in blank regions where shoots do not develop, thus leading to canopy gaps and a less productive vineyard.
- Making poor pruning cuts: Wounding on the vine is an entry to pathogen infections. This includes pruning wounds. Avoiding large flush cuts on older (2+ year-old wood), using sharp pruning shears, and pruning in dry conditions are important to reducing risks for spur/cordon/trunk infections and dieback.
- Consider the spur, cane, and bud position: We want to select canes, buds, and spurs that help us best conform to and maintain our training system. When training young vines, bud positioning should be considered when making cuts to ensure that shoots and subsequent canes are positioned along our fruiting wires without strong bends or breaks at the base that can restrict sap flow through the vascular tissues. Selecting shoots/canes along the trunk that can be easily arched upwards from under the fruiting wire or those that conform to the fruiting wire when extending/replacing cordons are preferred to shoots/canes that emerge from buds positioned above the fruiting wire.