Interspecific Hybrids versus Vitis vinifera L. Bud Hardiness, Viability, and Postfreeze Pruning Implications in Cane-Pruned Vines

Hybrid versus V. vinifera bud hardiness and viability

While the appropriate bud hardiness ranking of each individual cultivar, including where the hybrids and V. vinifera meet (Geisenheim 318-57 and Pinot noir), is not conclusive, the finding that hybrids as a whole are hardier than V. vinifera is conclusive (Figure 3). This finding was reinforced by the higher bud viabilities observed in the hybrid versus V. vinifera cultivars after the 2020 and 2022 freeze events. While such a finding is not new (Vandal 1986, Dami et al. 2012), the cultivar hardiness range and ranking provide interesting information from a region that grows both hybrids and V. vinifera in high numbers. The range in peak hardiness differed between the two vine types: hybrids −5.6°C, V. vinifera −1.7°C (Figure 3). This result is dependent on the basket of hybrid and V. vinifera cultivars grown in this region. The lower extent of the hybrid hardiness range could have been extended if some additional hardier hybrids (e.g., several relatively recent releases from Minnesota: Frontenac, La Crescent, Petite Pearl) had been included in this survey. While these cultivars are grown in Nova Scotia, their plantings are more limited. Additionally, despite adequate hardiness levels, some hybrids are not well suited to the Nova Scotian climate because of their heat and season length requirements. Regional climate limitations are a greater factor in determining which V. vinifera cultivars are suitable for the area. Less-hardy V. vinifera (e.g., Malbec, Merlot, or Syrah) are missing from this list because they are not grown in numbers in the region. Finally, while the general trends observed in Figure 3 would largely reflect those found in other regions, it should be noted that cold temperatures are needed to drive a vine to its peak hardiness and that the relative hardiness of some cultivars may also be dependent on the region in which they are grown (Howell 2000, Ferguson et al. 2014).

Temperature inversion variability, climate change trends, and winter damage risk

The 2020 and 2022 deep freezes were both temperature inversion events. Despite similar low-temperature averages in both years with damaging low temperatures, there were distinct differences in individual site temperature minima across years. This is characteristic of temperature inversions, where cool, dense air is allowed to settle near the ground, often concentrating in low areas or against tree lines, and warmer air is found above. A range of static and more dynamic variables determine the temperature minima experienced at each site. Several Nova Scotia vineyards are located near the Bay of Fundy, home to the highest tides in the world; these tides, which may exceed 16 m, can move huge volumes of air, influencing temperatures at the local level. The stratification of cold and relatively warmer air (Figure 2A) suggests that air mixing strategies, such as the use of wind machines, may have helped increase the temperature at the level of the vine and reduced damage in both 2020 and 2022 (Fraser et al. 2009).

The finding that the warming trend is being disproportionately driven by an increase in minimum temperatures is consistent with other studies (Jones 2005, Shaw 2017, Hewer and Gough 2020, 2021). The average annual extreme minimum temperature—a factor in determining a region’s “hardiness zone”—is considered one of the most important factors in determining where more sensitive cultivars can be grown (Willwerth et al. 2014, Shaw 2017). The frequency of damaging deep-freeze temperatures, like those experienced in both 2020 and 2022, has dropped dramatically, most markedly since ~1980 (Figure 2B). However, the reduction in risk from low-temperature frequency would be tempered by reduced hardiness in the vines related to warmer winter temperatures and the trend in planting less-hardy V. vinifera cultivars. This same trend has been observed in other Canadian grapegrowing regions (Hewer and Gough 2020, 2021). While growing V. vinifera is now commonplace in the region, a large cultivar evaluation trial that occurred in the 1960s did not include V. vinifera and even concluded that the region was unsuitable for most hybrids due to insufficient heat (Bishop et al. 1970). However, it is important to note that the timing of this trial coincided with a period of climatic cooling in Nova Scotia (Figure 2B) and that production methods, cultivar choice, and wine styles may now be better suited to the region.

Pruning implications for yield and vine architecture

Despite some differences on account of the degree of freeze damage, V. vinifera (Chardonnay) and hybrid (Vidal blanc) vines responded similarly to the four postfreeze pruning treatments (Table 1). However, increased fruitfulness (a hybrid trait) in Vidal blanc secondary buds relative to Chardonnay, may also have been a factor. For both cultivars, the yield was inversely correlated with the amount of pruning material removed. However, only the HO treatment (maximum pruning) was different from the other three pruning treatments (Table 1). In principle, this finding agrees with the conventional advice that the amount of pruning material removed should be inversely proportional to winter damage (Keller and Mills 2007, Moyer et al. 2011, Dami et al. 2012, Willwerth et al. 2014). Fruitful primary buds are typically the first to freeze in a lethal winter event, increasing the odds that the shoots that emerge in the current growing season will be less fruitful secondary, tertiary, or lesser latent buds (Khanduja and Balasubrahmanyam 1972, Howell 2000, Zabadal et al. 2007, Willwerth et al. 2014). In practice, however, no pruning treatment resulted in a crop worth harvesting in the more damaged Chardonnay. Retaining four as opposed to two canes (i.e., 4-CH—the second highest amount of pruning material removed in this study) was nearly as effective at bolstering yield in damaged vines as the 10-NC and 3-NS treatments (i.e., minimal pruning). Furthermore, there was capacity to retain five or six canes (Table 1), which would have further bolstered the crop. In light of this finding, further metrics should be explored to see if there are other reasons to recommend minimal pruning (i.e., 10-NC and 3-NS) in the case of severely damaged cane-pruned vines.

For both cultivars in the year of the freeze damage, pruning strategies that increased the number of shoots arising from the previous year’s canes and spurs (i.e., minimal pruning treatments: 10-NC and 3-NS) did so to the detriment of the number of shoots emanating from the head. Canes emanating from the head of the vine are important in a cane-pruned system as the source of the following year’s fruiting canes but are less important in spur-pruned systems. The two minimal pruning treatments had lower vigor (i.e., lower maximum cane diameter) and below normal cane numbers at the head of the vine (as compared to the year after the freeze), with roughly half the number emanating from the head (mean = 3.5 canes) as the two maximal pruning strategies (4-CH and HO) (mean = 6.2 canes) (Table 1). This finding aligns with the classic work of Winkler et al. (1974), which describes pruning response in undamaged vines as an interplay between vine vigor and capacity. Pruning more severely may lower overall capacity, and this was observed as lower total cane mass in the maximum HO treatment, but it increases the vine capacity-to-aboveground wood ratio in the remaining structure, as evidenced by more and larger canes emanating from the head (Table 1). It is speculation that a higher vine capacity-to-wood ratio could benefit the recovery from vascular damage. Dami et al. (2012) found that pruning mass increased with pruning severity on winter damaged vines, which differed from the results of the present study. The difference may be the degree of winter damage and the degree to which the capacity was dwarfed by the crop. Pertinent to cane-pruned vines, all pruning treatments in the present study averaged approximately one weak cane (unsuitable for use as a fruiting cane) arising from the head. This suggests that for the two minimal pruning treatments, there were few to no options for these vines when selecting two canes to be retained in the subsequent year.

In the year after the freeze, there was little carryover effect from the previous year’s pruning treatments (Table 1) after all vines were converted back to a two-cane system (Figure 1D), the regional norm. The relationship between pruning treatment and cluster number in shoots emanating from the head did not affect overall yields (Table 1). This finding could reflect a subtle carryover in shoot number from the previous year (Table 1). However, differences in the unmeasured carbohydrate status at the head could also be a factor. The greater vigor (cane diameter) and cane number at the head (Table 1) indicate a greater concentration of resources in this part of the vine in the 4-CH and HO treatments. Other studies have found that the fruitfulness of the buds may be correlated with the carbohydrate status of the vine (Lebon et al. 2008, Jones et al. 2013). It is uncertain whether the carryover treatment differences in pruning mass (Table 1) and in the lower TA values in the HO treatment—both only found in Vidal blanc—were statistical anomalies or had some basis in physiology.

Finally, in agreement with Dami et al. (2012), converting the minimally pruned vines (10-NC and 3-NS) back to a cane pruning system took more time than the pruning required for the maximally pruned treatments (4-CH and HO). The large amount of wood greater than one-year-old that was removed from the minimally pruned vines was two to three times more than that found in a normal cane-pruned system. Furthermore, the poor placement of canes in the minimally pruned vines (e.g., at the top of a vertical 10-node cane [Figure 1B]) often resulted in long, horizontal canes that became entangled with neighboring vines. The minimally pruned treatments also required larger cuts to be made (greater than two-year-old wood) than maximally pruned vines (less than two-year-old wood) when vines were converted back to cane pruning in year two. Larger pruning cuts are associated with increased levels of fungal infection and trunk diseases (Gubler et al. 2005, Pouzoulet et al. 2017, Wine Australia 2019). It was further speculated that in cases where winter damage was severe enough to include vascular damage, reducing vine size through increased pruning could reduce the type of vine collapse that occurs when the amount of growth on the plant outpaces the ability of the damaged vascular system to provide water and nutrients. Vine mortality was primarily observed in the year after the freeze event (five vine deaths), when vine mortality rose above average levels, but not in the year of the freeze event (one vine death). Anecdotally, the highest level of vine mortality was found in the minimal 10-NC treatment (accounting for four out of six vine deaths spread evenly across the two cultivars). Previous studies did not find a link between pruning strategy following winter injury and vine survival and future performance (Keller and Mills 2007, Dami et al. 2012), but they were challenged by the same limitations as the present study.