Early Season Update

Dr. Michela Centinari, Assistant Professor of Viticulture, Department of Plant Science

Another growing season has started for many Pennsylvania grape growers. Unfortunately, but not surprisingly, we are seeing and hearing of situations of vine winter injury across the State. This past winter, the lowest temperatures occurred at the end of January and during the first two days in February, with values around -5 °F (-20.6 °C) here in State College (central PA) and even lower temperatures were recorded at other locations.The injury seemed to have mainly affected Vitisviniferavarieties with reports of bud kill up to almost 100% for the most cold-sensitive varieties and, in some cases, trunk splitting.Growers also noticed uneven /nonuniform budburst which is typical of winter-injured vines.  We ask that more growers share their experiences with us; in particular, we would like to know if growers made any pruning adjustments and what the results are/have been. 

Since winter injury is a reoccurring issue for the eastern US, during certain years, we have covered topics related to vine cold hardiness, injury assessment, and pruning techniques for winter-injured vines at Extension meetings. Also, we have posted an announcement that focused on Pruning strategies for cold climate viticultureon the Penn State Viticulture and Enology Facebook page in January 2019, just before the “Arctic Vortex” event hit our region. Please do not hesitate to contact us if you have questions on how to manage cold-injured vines.

We heard from several PA growers in southern and central PA that budburst occurred earlier this year, a week to 10 days is what has been typically reported, than in 2018. This was also true for the hybrid varieties grown at the Penn State research farm at Rock Springs (central PA). I checked the growing degree days (GDD), a widely used index of heat accumulation, data calculated by the Network for Environment and Weather Applications (NEWA Cornell) for weather stations located in North East, Erie (northwestern PA), Biglerville (south-central PA), and Reading (southeast PA). Although historic data are not available, I compared the average GDD accumulated from January 1 to May 15 for 2013-2017 to those accumulated for the same period in 2018 and 2019 (Figures 1, 2 and 3).

Figure1. Cumulative growing degree days (GDD) at North East, Erie (northwestern PA)

Trends across locations/regions 

Not surprisingly, it was cooler in Erie compared to south-central and southeastern PA between January to Mid-May, not just in 2019 but for each year analyzed. In 2019, approximately 158 GDD accumulated between January 1 to May 15 in Erie, while GDD were at least double in south-central and southeast PA. Differences in temperatures across regions and locations explain why budburst typically occurs much earlier in southeast PA compared to the northwestern part of the state.

Difference between years 

In Erie, the GDD accumulated between January to mid-May 2019 (red line) were slightly lower than those for the same period in 2018 (blue line) and for the 2013-2017 average (black line). Also, note that there was no accumulation of GDD for a few days in May 2019 due to cool temperatures (Figure 1). The trend, however, was opposite in south-central and southeast PA, at least at the locations reported in this post. April was warmer (higher GDD) in 2019 compared to 2018 and the 2013-2017 average. While warmer spring temperatures favor earlier budburst they also increase the chance of freeze injury to green, tender plant tissues (Figure 4). 

Figure 4.Spring freeze (frost) injury on Concord, Rock Springs (PA)

At several locations across PA, temperatures were below freezing in the early morning of April 29 and some varieties were close to or already passed budburst. Below freezing temperature does not necessarily mean freeze injury as many factors affect the temperature at which the plant tissue is damaged or killed.  However, the freeze event on April 29 did cause freeze damage to vines at several locations, while others avoided the damage by using frost protection methods, such as frost dragons.  Some of the varieties grown at the Penn State research vineyard at Rock Springs, chiefly Marquette and young LaCrescent vines, sustained freeze injury. It is too early to estimate crop losses, but at least we are seeing some secondary shoot development (Figure 5).

How to recognize a secondary from a primary shoot

A relatively easy way, especially for caned pruned vines, is to check the angle of projection from the cane. Primary shoots typically grow with an angle of 45°, while secondary grow at an angle of 90° (figure 5).

Figure 5. Primary shoot damage after spring freeze event on April 29. Note a secondary shoot developing at 90° from the cane.

You can learn more about the basics of spring freeze injury and methods of protection at https://extension.psu.edu/understanding-and-preventing-spring-frost-and-freeze-damage

Looking ahead

It is almost time for some early season canopy management practice. Please check the following articles if you need information on shoot thinning or early leaf removal: 

Early season grapevine canopy management, Part I: Shoot thinning

Early season grapevine canopy management, Part II: Early leaf removal 

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