Upcoming Bloom Considerations
Weather had been warm through Memorial Day, advancing growing degree days above last years at this same period, however temperatures have returned to around normal for this time of the year. Check out http://fruit.wsu.edu/Grapeweb/gdd.htm for the latest update on growing degree days. This warm weather has jumpstarted the bloom period with plentiful sunshine. In the lower central Columbia Basin, bloom started the week of May 23rd, with growers in the Walla Walla Valley reporting bloom around the 27th of May. West side growers anticipate bloom in about another two weeks, but still ahead of normal compared to previous years. Bloom should continue through the next month as late-blooming varieties set fruit.
Many vineyards have been showing good signs of shoot growth with about 10-12 inches of shoot growth. As summer approaches, watch for signs of water stress during hot periods, indicated by slowing shoot growth rates, tendril browning, and acute leaf to petiole angles (sun avoidance).
One note of caution for Concord and Niagara growers: Concord vines may overcrop this year because of winter damage last year and low cropping. There have been reports of higher numbers of clusters/shoot than compared to average years. Growers may want to consider some mechanical thinning by running a mechanical harvester through the vineyard to knock some of the berries off after fruit set to alleviate this potential problem. The ideal time for cluster thinning is between about pea size berry stage and just before veraison. Once berries are knocked off the vine, the energy of the grapevine will be directed towards the existing crop, rather than into laterals and secondary growth. It is very important that the crop on the vine will ripen, and with winter damage requiring retraining of some vines, the leaf area this year needs to support the cropload.
Dr. Mercy Olmstead, Extension Viticulture Specialist
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