Left: With a bloom time of only one or two days, this Chardonnay cluster in New York shows even development. Right: With a bloom time of only one or two days, this Chardonnay cluster in New York shows even development.

Left: With a bloom time of only one or two days, this Chardonnay cluster in New York shows even development. Right: With a bloom time of only one or two days, this Chardonnay cluster in New York shows even development.

If you don’t have a bloom date circled on your calendar, you’re not alone.  Bloom means different things to different people, according to Dr. Michelle Moyer, Washington State University plant physiologist. New York growers might look for 75 percent cap fall; in California, the benchmark is 5 percent cap fall in at least half of all clusters; other areas call it bloom at 50 percent cap fall per individual cluster.  

Definitions may vary, Moyer said, but it’s not good enough to say “I’ll know it when I see it.” Defining bloom is the first step in managing a future crop.  

It’s important to identify when bloom begins and ends in order to make informed decisions about irrigation.  Water stress can limit fruit set, or even cause sterility if it occurs before flowering, and too much water can result in too much canopy growth.  

The same thinking holds for disease management, Moyer said. The window of susceptibility for powdery mildew is from bloom to three weeks past bloom, and botrytis bunch rot can be a danger from cap fall to bunch closure.

With so many important decisions to make, why hasn’t the scientific community come up with a better definition of bloom?

“Definitions vary because individuals define bloom to suit their needs, and there are regional differences in the timing and duration of bloom,” Moyer explained.   
The important thing is to define it consistently from season to season, she said. For practical purposes, it’s easy to define it as a window, from earliest cap fall to the time when most clusters at a specific location are at full bloom. So, when reading information that references a practice that is related to “bloom,” consider the location and what that practice is, such as determining set, irrigation strategies, and disease management, she advised.

Day length, temperature, and genetics can all determine the timing of bloom, and those differences can appear at all levels—from cluster to cluster, shoot to shoot, or vine to vine. Uneven blooming in clusters might indicate uneven pollination. Within shoots, clusters at the base could bloom as much as two weeks before secondary clusters, Moyer said, and the clusters closest to the vine’s trunk often are the earliest to bloom.

Temperatures and day length combined probably play the biggest role in duration of bloom, Moyer said, with ideal daytime temperatures hovering between 68° and 78°F.  Temperatures lower than 60 degrees can reduce blossom opening, and weather warmer than 95 degrees slows it significantly. Winter dormant temperatures are also important.   In Geneva, New York, for example, where the mean midwinter temperature is 24.6°F, the bloom cycle can begin and end in as few as three days. The same process might extend over ten days in Adelaide, South Australia, where midwinter temperatures average 53.2°F.

Moyer described several ways to define bloom, from random sampling around a vineyard, to documenting bloom from season to season at sentinel vines. Any method works, she said, as long as it’s consistent.