Purdue Wine Grape Team Newsletter

Dodging the Frost Bullet

Growers across the state are breathing a sigh of relief after recent frost events. Frost and freeze advisories were issued for much of the state April 5-12. It is normal to have frosts in April. However, because temperatures were well above average in February and March we were on pace for very early bud burst.  We were about 2 weeks ahead of normal in growing degree-day (GDD*) accumulation through the end of March.  Luckily, this past two weeks have been cooler and vine development has stalled. That kept grapes at the early to full swell stage across most of the state. At that stage they are able to tolerate temperatures in the mid to low 20s.  Some growers in central and southern Indiana experienced damage on early budding varieties, but hopefully not enough to affect yields. There was some damage to other fruit crops such as strawberry, peach, apple and blueberry. Again, it is too early to tell if the damage is enough to affect yield. So, for now, we feel lucky to have dodged this potential disaster. Growers to the east were not so lucky. Pennsylvania, Maryland, Virginia and North Carolina are reporting significant damage to vineyards in some locations.

Don't miss  the  Spring Workshop   on Wednesday, May 4.  Advance registration of $50 is required. Lunch will be provided, and attendees are encouraged to bring a bottle of their favorite wine to share. For more information or questions, contact Jill Blume at 765-494-1749 or blume@purdue.edu.

Best regards,
Bruce Bordelon
Professor of Viticulture 

* For grapes we use base 50˚F. The average daily temperature minus the base temp equals the number of GDDs accumulated for that day. We accumulate them for the season. For "growing season" we start counting April 1 and end Oct 30 based on the California model. That tells us how warm our climate is for ripening a crop and what varieties are adapted. But that obviously doesn't count all the important heat units we achieve since we are already showing some development by April 1.  For bud break we have to count earlier, as soon as chilling requirement is met. That might be late Jan or even late Feb in some years depending on how cold the winter was, different crops and varieties, etc. So at some point the GDDs start making vines deacclimate and grow. Those are the ones I'm talking about in this article. I'm just counting all from Jan 1 but I doubt those all are effective since some chilling may not have been achieved. Based on our long term observations, it takes 110-130 GDDs for early varieties to bud out. 150-180 GDDs for later varieties. But this is just guesswork as we don't know the exact chilling requirement for all the different varieties. 

Christian Butzke

Jill Blume


Bruce Bordelon