Assessing Frost Damage: Paso Experts Say It's Too Early To Tell

By Nancy Hoover

If you consider yourself a wine aficionado then you've probably already heard about the recent frost that blanketed much of California wine country.  Paso Robles was no exception to this, experiencing a series of freezing cold nights on April 9, 10 and 11 that dipped in the mid to high 20s and low 30s and had many vineyard managers up in the wee morning hours on frost patrol. 

What you might not know is that a bitter cold frost, especially one that lasts three days, can adversely affect wine grape production.

Now mind yourself, I am not a vineyard consultant, but I did receive a minor in wine and viticulture, so I will do my best to shed some light on this subject (and make my alma mater, Cal Poly, proud).

Bud break is referred to the period of time when quite literally vines begin to break buds.  In Paso Robles, vines typically go dormant sometime in November and stay dormant until about the end of March.  There are some variations to this that mostly depend on macro- and microclimatic conditions, of which Paso Robles has many.  Grape variety and pruning techniques can also affect the length of time a vine stays dormant. In general, most vines come out of dormancy at the end of March or the beginning of April when the days get longer, the temperatures begin to rise and the water table is high. 

A vine experiencing this type of climatic change will jump start into production, producing new growth that will ultimately produce berry clusters.  These berries will ripen over summer and come harvest, will be picked, crushed, destemmed, pressed and fermented into the wine that we all enjoy.

There are a lot of risks involved with producing any agricultural commodity.  The largest of these risks is climate or what growers lovingly refer to as Mother Nature. No doubt, Mother Nature had her way when the temperatures dropped so low that most frost protection systems were ineffective.  Overhead sprinklers seemed to be the most beneficial in providing frost protection (you may be thinking how on earth? read this), but heaters and wind machines never stood a chance.  The idea of circulating warm air from above the inversion layer, or off the warmer hillsides, down into the colder valleys would simply not work in this case because this particular storm caused an advection frost, meaning the inversion layer was too high for circulation. The cold front was so cold that it "burned" a good portion of buds that were just beginning to develop into shoots.

Ever wonder what a frost damage looks like? Take a peek at the first photo above...and note the burned, tinged look.

So what does this mean for Paso Robles?  Reports indicate that up to 40 percent and in some vineyards, 90 percent of the new growth was lost.  The damage is significant on the outer vineyard rows and in young vines.  Don't lose hope though, the vine is resilient and vigorous in nature.  Most industry professionals are optomistic that secondary and tertiary buds will begin to break as spring resumes its normal pace. 

So, really it is too early to assess vineyard damage and how this will affect crop yields.  The good news is that not all vineyards in Paso Robles were past bud break.  In fact, my Cabernet Sauvignon is sleeping quietly as I write....

Photo 1: Pinot Noir vines are "burned" by the advection frost.  This damage occured even with an overhead sprinkler frost protection system in place.

Photo 2: Cabernet Sauvignon sleeps quietly post frost.  Today, it is beginning to waken.