Have you ever wondered how vines that survive Iowa’s warmest winters fail to break bud in the spring when it warmed up? Or, after four days in mid-February with 60–70°F temperatures, these same vines haven’t started to break bud?
Of course, since Wednesday had freezing temperatures and snow, bud break would NOT be a good thing right now! And this is after four days with 60°F temperatures.
In the simplest sense, the season for a grapevine ends as temperatures drop in the fall. At that time, the vine goes dormant with energy in the form of sap dropping down from the leaves and the trunk of the vine to be stored down in the root system. The vine stays dormant all winter and only starts to leave dormancy when temperatures warm up.
This description of temperatures is too simple, however, because as we are experiencing right now, there may be a cold snap in early November followed by warmer weather that lasts through December. That can be followed by very cold weather in January, a warm period in February, cold weather again in early March and then another cold period in late March. What prevents the vine from starting to bud-out in mid-December only to have tender buds frozen off in January, for example?
Here is where “chilling hour” models enter the calculations. These models assume that once the vine goes dormant in the fall, it needs a certain number of hours of cold weather before a warm period will cause it to leave dormancy. While there is a lot of evidence that the number of chilling hours is important in determining whether the vine will bud out during a warm-up, we don’t know the precise number of chilling hours the vine needs before it will leave dormancy. Observation indicates that about 1,000 hours of cold temperatures is required before warm weather will cause the vine to bud-out but at this time, that is a guess and it appears that different cultivars may react differently.
The concept of a “chilling hour” timer makes it seem that vines are like little computers. They keep track of temperatures and they know that until they reach 1,000 chilling hours, winter is not over. But, when they do reach 1,000 chilling hours, warm weather will tell them to get the sap flowing and to form and break buds. At that point, the vines are extremely vulnerable to a cold snap that could kill emerging buds.
So, even with the vine’s “chilling hour” timer, they can bud out just before temperatures drop low enough to kill the primary buds. Fortunately, there are secondary and even tertiary buds however these buds are progressively less productive. So, we really don’t want vines to bud out before the chance of a hard freeze is past.
Chemicals that can delay bud break by a few days are being developed and data is being gathered to tell us exactly how many chilling hours each grape variety needs. Hopefully, in the near future, we will be able to take advantage of the grape vine’s weather computers to avoid losses from early bud-break.
