Bud Break at Stake

By Jessica Cortell

Have you ever wondered what causes grapevines to lose leaves and go dormant? What influence the previous season might have on the next vintage? What triggers the vines to reawaken and burst forth with bud break? Phenology is the sequence of seasonal plant development and of the relationship of these phenomenons to climate and changes in the season.

In autumn, cooler temperatures and shorter days cause the vines to drop their leaves and begin the process of cold acclimation, which occurs in response to repeated exposure to chilling temperatures that cause physiological changes allowing the grapevine to resist cold winter temperatures. The process is initiated when the canes become brown and lignified. Ideally, this process starts near veraison when green shoots start transitioning to brown and woody. These canes are important as they contain the buds that will produce next season’s shoots and fruit.

It is particularly important in cold climates such as Eastern Oregon, Eastern Washington and Southern Oregon to manage late-season irrigation and vine vigor to encourage the transition from green shoots into brown canes by harvest. Vineyards in Eastern Washington and Oregon sustained a cold snap before Thanksgiving. Growers are reporting variable damage and, luckily, many vineyards saw only low levels of bud death.

Vineyards closer to the Columbia River had warmer temperatures and consequently less damage. Vineyards to the north and farther from the river saw substantial damage in areas where temperatures dropped below -10°F. While the extreme cold temperatures would have caused damage regardless, the late harvest may have contributed to the damage. Vineyards with the worst damage will probably not have a crop in 2011.

Early, cold acclimation is usually not as critical in the Willamette Valley since we don’t often have low enough temperatures to cause winter cold injury. In the cold snap before Thanksgiving, the Willamette Valley and Southern Oregon experienced temperatures drop to the high teens. However, the initiation of shoot lignification by veraison is an indicator of vine balance and high quality fruit.

In 2010, May and June were unusually wet with record rainfall of 8.95 inches in Portland and high rainfall throughout the Willamette Valley. This caused high soil water content and elevated vine vigor combined with a very cool season leading to a late harvest in the Willamette Valley. While we managed to pull off a crop with nice flavors, the effect of this is now apparent during pruning, where many canes did not harden off very well and often the upper half of the cane died.

Pruning crews needed skill in choosing good canes for next season’s crop. An ideal fruiting cane is one that is brown and well hardened off, of adequate diameter, internodes spaced about 4 inches and well positioned on the vine.

Dormant pruning weights are also a way to assess vine vigor and determine how many buds the vine can support in the next growing season. Average cane weight and pruning wood weight per foot of fruiting wire are useful in evaluating vine vigor. If the average cane weight is above 2 ounces, you need to consider leaving more buds per vine or other methods to reduce vine vigor. Excessively large canes are often called “bull canes” and are not as fruitful.  If the average cane weight is 1 ounce per cane or less, you might need to consider leaving fewer buds to improve vine vigor.

In some blocks that had cane weights of 2 to 3 ounces per cane, a “kicker” cane was left to help reduce vine vigor in the spring of 2011. The kicker cane will help spread the energy of the vine into more new shoots in the spring when growth is rapid. The kicker cane will be cut off around the time of bloom.  The reason this can work in reducing vine vigor in the spring is that new shoot growth is supported by carbohydrate reserves in the trunk and roots. There is a direct relationship between how many buds are left and how vigorous the new shoots are. 

Another issue is that because of the cool spring that didn’t warm up until after July 4, rapid growth occurred resulting in internodes (space between buds) that are often more than 4 inches apart. In fact, some internodes of 8 inches were observed; this can be a problem in vines closely spaced in the vineyard as it is not possible to leave enough buds to help lower the vigor and balance the vine. Once the pruning is done and the new fruiting canes are tied to the wire, it is time to relax momentarily before the whole phenological cycle begins again with bud break.

Vines, like most plants, have an internal clock that monitors time by changes in day length. Temperature also plays a role in influencing when bud break will happen. Generally, bud break will commence when the average daily temperature reaches 50°F. This is why bud break often happens a month earlier in California as compared to Oregon. We typically expect to see bud break around mid-April in the Willamette Valley, but it has occured as early as March 15 and as late as May 1.

Once the new green shoots emerge from the buds, they are very sensitive to frost damage at temperatures below freezing. Each node is actually a compound containing three buds. The primary bud is the most fruitful, and its shoot emerges first. If this one is killed by frost, the secondary or tertiary bud will grow but with fewer and smaller clusters that will ripen later than clusters from the primary bud.  Prior to bud break, the buds will be in a “woolly” stage. Sometimes frost damage occurs during this stage but may not be noticed by the grower until later after bud break.

Last season, growers in Southern Oregon experienced a frost on May 6, which resulted in damage to clusters on primary shoots. While they had some fruit on secondary shoots, it created difficulties in fruit thinning and at harvest as there were differences in ripening between the primary and secondary clusters. Rhone varietials were more affected and had up to a 50 percent crop loss.

In one to two months, there will be enough warm days for the vines to push forth with new shoot growth. For now, we can wait with anticipation to see what the 2011 vintage will bring.

Welcome spring.

Jessica Cortell received a M.S. degree in Horticulture and a Ph.D in Food Science and Technology from Oregon State University. Currently, she owns her own consulting and vineyard management company, Vitis Terra Vineyard Services, and teaches at Northwest Viticulture Center in Salem.