How does delaying spur-pruning to the onset or after bud burst impact vine performance? Insights from recent studies
By Michela Centinari
Now that harvest is finally over and wines are tucked away in the cellar, it is time to prepare for the next year. One of the first concerns that many growers feel in a new growing season is that worry of spring frost and the associated potential risk of vine injury. In the spring of 2016, for example, an unusually warm March was followed by a very cold start to the month of April, which resulted in damaging frost incidences in some vineyards of the Mid-Atlantic region.
Susceptibility to frost injury increases with advanced phenological growth stage , therefore, growers and scientists have explored different techniques for delaying bud burst of grapevines to increase the chance of avoiding spring frost damaging events. Vegetable-based oils (e.g., Amigo oil) can be sprayed on the canes/buds during the winter to slow down bud de-acclimation and delay the resumption of vegetative growth in the spring [2; 3; study at Penn State]. Delaying pruning until late winter can also be used to delaying bud burst of vines growing in frost prone areas.
Canes of cordon-trained vines can be pruned to 2-3 node spurs late in the winter or even when apical buds begin to open to delay bud burst of basal buds. Due to the strong apical dominance of Vitis vinifera cultivars, apical buds of an unpruned cane tend to burst first, which inhibits development and growth of median and basal buds  (Figure 1).
What may happen if we wait until the onset of bud burst or even later to prune the vines?
Spur-pruning the vines when the apical buds of un-pruned canes are already open may not only delay bud burst of the basal nodes, but may also postpone other phenological growth stages such as bloom, fruit-set, or even veraison with potential consequences for vine yield and fruit chemical composition at harvest .
I recently read two articles on this topic published in the American Journal of Enology and Viticulture (Post-bud burst spur-pruning reduces yield and delays fruit sugar accumulation in Sangiovese in central Italy  ) and in Frontiers in Plant Sciences (Phenology, canopy aging and seasonal carbon balance as related to delayed winter pruning of Vitis vinifera L. cv. Sangiovese grapevines ).
The studies described in these articles aimed to assess if and how delaying winter spur-pruning of Sangiovese vines to the bud swelling stage or later, after bud burst, impacted the annual growth cycle of the vines and its productivity.
The studies were conducted in Italy and the researchers were specifically interested in assessing if vines pruned around or after bud burst exhibited a delay in grape ripening as compared to those pruned during the winter, resulting in lower sugar accumulation and higher acidity in the fruit at harvest. A steady trend of increased warming is, indeed, pushing some Mediterranean grape growing regions toward accelerated ripening , which could lead to excessive or overly fast sugar accumulation in the fruit, high alcohol in the wine, unacceptably low acidity, high pH, and also atypical grape flavors and aromas .
Although excessive or overly fast sugar accumulation may not be a problem in our region, it’s still important to understand if delaying winter pruning to extremes could be used to delay bud burst and reduce risk of frost damage, as well as the impact this practice may have on vine yield, and fruit and wine chemistry. This is a topic of further interest in light of changing climatic conditions and the potential increase of unpredictable weather patterns like early spring warming and late spring frosts .
Below, I will summarize the two previously mentioned studies emphasizing results which can be of interest to wine grape growers in our regions.
Both studies were conducted on mature Sangiovese (Vitis vinifera L.) vines. The first study was established in a commercial vineyard in central Italy, whereas the second study was conducted on vines growing outdoors in 10-gal pots at a research station in northern Italy. Groups of vines were assigned to different pruning treatments. Vines assigned to the standard grower practice treatment were spur-pruned to 2 basal nodes during the winter when buds were dormant. Vines assigned to the other treatments were spur-pruned at more unusual times from the bud swelling to full bloom (Figures 2 and 3A).Did delaying vine spur-pruning to after bud-burst consistently delay the whole annual growing cycle?
Basal buds of Sangiovese vines spur-pruned when apical shoots were about 1.6″ long (called late pruning treatment; Figure 3A central panel) burst 17 days later than those of vines pruned in the winter, when buds were dormant (called standard pruning treatment; Figure 3A left panel). Pruning the vines even later, when apical shoots were about 4.7-5.5″ long (called very-late pruning treatment; Figure 3A right panel) extended the delay in bud burst to 31 days as compared to vines pruned in the winter Unfortunately no phenology data were recorded for vines pruned at bud swelling stage (study 1).
The delay in phenological growth stage decreased over the season. For example, late-pruned and very-late pruned vines reached veraison 3 and 13 days, respectfully, after those pruned in the winter (Figure 3C). Shoots of vines pruned after bud burst developed later in the season under higher air temperature than those of vines pruned during the winter. Greater air temperature may have helped shoots of late- and very-late pruned vines to reach bloom and veraison in fewer days as compared to those pruned earlier .
By harvest the delay was fully off-set for the late-pruned vines: they reached the sugar level set for ripening (~ 19 ºBrix) three days before those pruned in the winter. Grapes of very-late pruned vines reached 19 ºBrix 6 days after those pruned in the winter.
Spur-pruning vines after bud burst significantly reduced crop yield compared to standard winter pruning
Vines pruned at bud swelling growth stage had similar crop weight, number of clusters per vine, and cluster weight than those pruned when buds were still dormant. Pruning vines after bud burst, however, reduced yield as compared those pruned during the dormant season. For example, late pruned vines (spur-pruned when apical shoots were about 1.6″ long; Figure 3A central panel) had 26% lower crop yield as compared to those of the standard pruning control group (spur-pruned before bud burst). Reduction of crop yield was related to lower cluster weight and lower number of berries per cluster. While there is not a clear explanation on why late-pruned vines had fewer berries per cluster, several hypotheses were presented including increased production of gibberellins during the initial flush of growth in the late-pruned vines .
Waiting even longer to prune the vines had a detrimental effect, reducing not only cluster weight but also the number of clusters per vine. For example, vines pruned to two basal nodes when the apical shoots were already flowering had no crop at harvest. When vines were pruned so late the basal shoots did not develop flowers and remained vegetative after pruning. I am not sure why any growers would want to wait until bloom to prune the vines, but it’s still interesting to see how such a drastic treatment may limit sources of carbohydrates for developing cluster primordia .
Although uncertainty still exists, the authors suggested that delaying spur-pruning until after bud burst, but not to extremes, may have the potential for reducing crop yield in high-yielding cultivars such as Sangiovese planted in specific regions of Italy. However, long-term field studies are necessary to assess if it is possible to calibrate winter pruning date for managing yield reductions and/or fruit maturation rate.
Did delaying vine spur-pruning to bud swelling stage or after bud burst consistently impact fruit chemistry?
The effect of the timing of spur-pruning on fruit composition at harvest varied between studies and with the extent of the pruning delay. For example, Sangiovese vines late pruned (apical shoots 1.6″ long) had higher total soluble solids (+ 1 °Brix), total anthocyanins and phenolics than winter-pruned vines. However, vines growing in a commercial vineyard (study 1) and spur-pruned to two basal nodes later in the season, when inflorescences of apical shoots were already swelling (Figure 3C), had lower sugar concentration (-1.6 °Brix) and higher TA (+1.8 g/L tartaric acid) than those pruned during the winter, but at the same time they also had higher anthocyanins and phenolic concentrations. This result suggests that spur-pruning canes after bud burst may decouple the accumulation patterns of total soluble solids and anthocyanins, phenolic metabolites. This could be intriguing for growers trying to delay fruit sugar accumulation and acid degradation, while maintaining wine color, but on the another hand it could also come with a reduction in crop yield, quantified to over 50% in this study .
Did delaying winter spur-pruning have negative carry-over effects on the following season?
Pruning vines at the bud swelling stage did not have negative effects on vine growth in the following year. It did not impact bud fertility (number of clusters per shoot) or winter carbohydrate storage, which is important for winter vine survival and following year resumption of growth. However, pruning the vines to two-nodes after bud burst, specifically when inflorescences of apical shoots were already swelling (Figure 3C), reduced bud fertility by 50% in the following year. Those vines were able to recover once standard winter pruning was applied again at the end of the study.
These studies conducted on Sangiovese vines grown in Italy found that:
- Winter spur-pruning can be applied up to bud swelling without adversely affecting vine yield, grape composition at harvest or bud fertility in the following year.
- Vines pruned after bud burst show pronounced delay in shoot development at the beginning of the season, which increased as the pruning time was further delayed. Under the warm conditions of these studies the delay in phenological growth stage decreased or even disappeared over the season. This could be partially explained by the fact that late-pruned vines needed less time than vines pruned during the winter to reach maximum photosynthesis efficiency.
- Delaying spur-pruning to after bud-burst may reduce vine yield, decrease sugar accumulation and bud fertility in the following year.
Delaying winter pruning of vines located in frost prone areas to the onset of bud burst or shortly after that may be used as frost avoidance technique. However, we need to further understand how a delay in shoot development and potentially a shorter growing season (number of days from bud burst to harvest) may impact fruit ripening, yield component, vine over-winter carbohydrate storages and susceptibility to winter cold temperatures, as well as the following year growth. The studies summarized here were conducted in a warm region with a growing season longer than many areas of Pennsylvania. Further research is necessary to corroborate those results under our regional climatic conditions.
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