Freezing stress, cold hardiness and managing winter damaged vines

Winter damages are a challenging issue for sustainable viticulture in Michigan.

Cold injury to grape buds and canes.
Figure 1. Cold injury to grape buds and canes. A) Healthy compound bud; B) Discolored tissues indicating injury to primary bud; C) Compound bud with cold injury to primary, secondary and tertiary buds; D) Healthy cane tissues; E) Moderate cold injury to cane indicated by discolored cambium tissues; F) More advanced symptoms of cold injury to cane. Photo by Paolo Sabbatini, MSU.

The 2021 winter in the Midwest and Eastern U.S. was impacted again by the polar vortex; temperatures plunged into the sub-zero digits and the extended duration of the cold events severely affected the grapevines. Minimum temperatures dropped to 3 degrees Fahrenheit (-16 degrees Celsius) at the Northwest Michigan Horticulture Research Center in Traverse City, Michigan, on Feb. 15 and -3.7 F (-19.8 C) at the Southwest Michigan Research and Extension Center in Benton Harbor, Michigan, on Feb. 17.

Many cultivars that normally survive our winters with little to no damage were probably severely injured by the extreme cold. The extent of the damage will depend mainly on cultivar and location. This winter will show dramatic differences among cultivars and the spring will reveal what type of damage (bud, trunk and vine death) growers will have in different vineyard locations; the Michigan State University grape team is currently assessing the damages in several cultivars and locations. Once the damage is assessed, the challenge is to determine how to successfully manage the vines during the 2021 growing season.      

The strategy for coping with winter injury should start with delaying pruning as long as possible during the dormant period. The delay should be used to assess the extent of winter injury and then adjust the pruning strategies in relation to bud and vine damage and mortality levels. Therefore, before pruning, grape growers should carefully evaluate each cultivar for bud damage.

Each bud (or node) is a compound bud, or a complex of three primordia. The primary primordia (or “primary bud”) is the largest meristematic tissue in the middle of the compound bud. The secondary bud is located towards the base of the cane, while the tertiary bud (which is generally not fruitful) is located towards the apical portion of the cane. For most of the varieties grown in the East and Midwest, the primary bud carries 70-75% of the cropping potential of the compound bud (primary plus secondary plus tertiary).

Knowing this simple bud morphology and making cross-sectional cuts (with a sharp razor blade) through the bud, growers will be able to identify the health status of each bud. If the buds are alive and healthy, they should be green; brown or black color is unfortunately an indication of mortality (Figure 1).

How many buds need to be assessed is related to the amount of damage that the vineyard suffered. Start by collecting a sample of 40-50 buds; if they are all dead (brown-dark), the chances of finding living buds is very slim and there is no need to continue the assessment. Contrarily, when the results are highly variable (alive and damaged buds in samples coming from the same cultivar and the same vineyard location), check about 100 buds to get a more accurate idea of the level of damage. If the vineyards are not uniform (different vine size, slope, soil), it is better to keep samples from vines/areas separated to evaluate the potential impact of those variables.

Secondary bud mortality is usually similar to primary bud mortality, yet some indication of the amount of secondary bud damage is important, especially when over 60% of primary buds are dead. When the assessment of bud damage is complete, the next step is to adjust the pruning strategy for the 2021 season accordingly. General suggestions are reported in Table 1.

Table 1. Suggested pruning strategies in relation to different levels of bud mortality.

Bud mortality (%)

Suggested strategy

10-15

No need to adjust your winter pruning.

 

20-50

Leave a higher number of buds (more than 20-30%) at winter pruning; e.g., prune to four to five bud spurs rather than the standard two to three bud spurs and leave more spurs/canes per vine.

60

Double the number of buds of your standard pruning strategy.

More than 60

No dormant pruning or just reestablish the bearing structure of the vine.

When bud mortality is over 70%, pruning effort should be directed to reestablish the fruit-bearing zone of the vines and to balance the growth of the vines during spring and summer. When vines are severely damaged, base buds close to pruning cuts have the potential to break bud and grow. This physiological phenomenon is very useful for increasing the number of shoots per vine during the spring and consequently increasing the total leaf area. Having a higher number of shoots in established vines, which have large root systems and plenty of reserves in the permanent structures of the vine, will avoid excessive shoot growth (bull canes). Vigorous shoots tend to have long internodes and poor lignification, which makes them less resistant to winter injury and poor choices for next year’s pruning (both cane and spur pruning).

The extreme cold during this winter could also have permanently damaged cordons and trunks. Injured cambium tissues will be a brown to black color (Figure 1). When damage on permanent cordons is present, the pruning effort should be directed at reestablishing the structure of the vine. Renewal canes from the base of the trunks are the best option for replacing injured bearing structure. Severely injured trunks need to be replaced because in cold climate locations, they are very sensitive to crown gall disease (especially vinifera cultivars), and diseased portions of the vine needs to be removed and replaced with new healthy tissues.

Severely winter-damaged vines often have shoots, called suckers, coming from the base of the vine. Sometimes, due to very extensive damage, the suckers could be the only resource for leaf area for the vines in the spring. Several extension bulletins and growers’ experience suggest that the suckers should not be removed or even thinned to one or two per vine. Instead, the better option is to leave at least five or six suckers and to try to guide their growth towards the trellis system. Other suckers, if present, can be allowed to grow without training, leaving them on the ground, because they are source of additional photosynthetic leaf area resulting in additional carbon production during the summer and they will help prevent excessively vigorous growth on the potential replacement canes. Again, overly vigorous growth during vine recovery from cold damages, is not desirable.

Several other viticultural strategies can mitigate the impact of winter cold, such as (1) using multiple trunks, sometime defined as “spare-parts viticulture”, (2) always having replacement canes (suckers) growing under the vine, which is fundamentally important for grafted Vinifera cultivars, and (3) covering the graft union with soil (or snow) during winter.

Vineyard trunks
Figure 2. Multiple trunks provide a protection to extreme cold temperature and for variety particularly sensitive to cold. A) Vinifera and B) Hybrid. Not all the trunks of a multi-trunked vine would be killed during a harsh winter. Damaged trunks are removed and the trunks that survived are the bearing structure for the following spring. For cold-hardy varieties or in viticultural regions (or sites) characterized by moderate winter minimum temperatures, two trunks are adequate. Photo by Pat Murad, MSU.

Covering (and uncovering) the graft union of the vines every year is very labor intensive but the most efficient technique to guarantee vines and fruiting canes survival for the following season. Soil serves as an excellent method of insulation. Cold winter temperatures few inches below the soil surface are rarely damaging and they generally remain around the freezing point with much colder air temperatures just above the soil surface.

Commercial growers implement a grape hoe to mound the soil in the fall and they remove the soil in the spring, before the vines restart their active growth (to prevent scion rooting; roots growing from the scion variety instead of the rootstock variety). Graft unions on grafted vines should be covered with few inches of soil for the winter months, protecting scion buds close to the grafting point that could be very important in the case of severe winters damaging or killing the buds above the mounted soil.

Insulation of the graft union
Figure 3. Insulation of the graft union in Vinifera vines with soil (or mulch) by hilling up above the graft union. This method is going to provide the best protection from cold injury especially to the graft union area and those viable scion buds above the union. Image from: Winter injury to grapevines and methods of protection. Michigan State University Extension Bulletin E2930, 2007.

In 2012, the USDA released a new plant hardiness zone map because of the need to be in line with the temperature increase around the U.S. We are using the extra heat and the longer growing season to ripen cultivars that were impossible to grow only few decades ago in cool and cold climate viticulture. Unfortunately, this warming trend is also producing extreme winter cold temperatures (polar vortex), and going back to the basic methods of grapevine protection during the winter is important for a sustainable viticulture in the East of U.S.

The 2013, 2014, 2015 and unfortunately 2021 winters are a forceful reminder that cultivar choice and site selection are still the most important tools we have against low winter temperatures. While we need to prune vines to mitigate damage as best as possible for the 2021 growing season, we should also keep in mind the 2022 season and crop, and the effect our choices during pruning and training will have on it.

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