Myth #1

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Myth #1:  Hot Days and Cool Nights are Good for Wine Grape Growing

How many times do we hear California vineyard owners telling us that their sites “benefit from very warm days which develop our fruit, followed by very cool nights which preserve acidity?”

It is repeated like a mantra in California but it is almost certainly wrong.

In California, the number one challenge in grape growing is to get the fruit physiologically (phenolically) ripe before sugar levels get too high.  That is, you want your grapes to taste great before they are overly sweet.

But having large day-night temperature swings (also known as “diurnal temperature changes”) is counter-productive for flavor development in the grapes.  Or, more precisely, large diurnal temperature swings favor sugar development over phenolic (flavor) development.

Note these quotes from famous viticulturist John Gladstones:

  • “The best wine-producing regions of Europe are all characterized by narrow ranges of day-night and day-to-day temperature variation… as are, even more, the best vineyard sites within them.”
  • “My interpretation is that relatively constant, intermediate temperatures during ripening specifically favour the biochemical processes of colour/flavour/aroma development of the berries.”
  • “A general conclusion can be drawn.  The narrower the range of variation about a given mean or average ripening temperature, the greater the grape flavour, aroma and pigmentation will be at a given time of [sugar] ripening.”

Why is this true?  The answer is likely this:  biological chemical reactions are driven by enzymes.  These enzymes’ activity levels are a function of temperature.  So it is likely that the enzymes relating to phenolic development do not work as well in extremes of temperature.  So if you have a site that is often very hot, and is usually quite cool at night, then your sugar development is likely to outpace your phenolic development.  That would leave you with two choices:  either pick early and have underdeveloped phenolics (herbaceous or other unattractive flavors); or, pick later with much higher sugar levels which must be dealt with at the winery with water-back or de-alcing strategies.

California generally has high day-night temperature swings due to the breeze from the frigid Pacific Ocean which turns upside-down the notion of a “maritime climate.”  Most California vineyards are latitudinally in very warm sites, but they are strongly cooled in the evening by very cold ocean breezes.  Bordeaux, on the other hand, has a true maritime climate in which the ocean moderates the day-night temperature fluctuations.  Because of this, Bordeaux has always been able to ripen grapes at far lower sugar levels than California.

Does this mean that all of California is not ideal for grape growing?  Not at all.  But it is important to find sites where there is less diurnal temperature fluctuation.  The best way to do this is through elevation.  While not always a panacea, it is generally true that at higher elevations, the vineyard will experience cooler days and warmer nights.  It does not take much elevation – because the cool still night air will sink down the hillsides and make for warmer nights for the vines.

I developed these ideas while I was at Fresno State’s Enology program, after having worked a harvest in Bordeaux.  One important clue for me was that fact that I could walk around St. Emilion nearly every night in the late summer and early fall without a coat on – you can’t do that in California.

When we arrived in California to develop our vineyard, I was introduced to viticulturalist Daniel Roberts and his team of affiliated companies.  Daniel and his associates were the only ones talking about the problems of large diurnal temperature swings.  As I heard Mark Greenspan say at a recent conference, “I guess it is good to talk about cold nights and cool foggy mornings if your site is extremely hot during the day or is particularly subject to heat spikes;  but it would be best if there were less extremes of heat and cold.” (paraphrased).  Nevetheless, vineyard owners keep trumpeting warm days and cool nights as a key to success.