I thought I knew most of what one needs to know to grow grapes but this year taught me more about how temperature affects the growing grapevine and the berry ripening process than I've learned in the past.

Before I get into the consequences that temperature has on growing grapes in short season areas, let me recap this year so far (because this has great relevence to the temperature story)...

The 2004-5 winter was milder than normal. Winter temperatures took some wide swings from January on. Luckily, the cooler swings never got that cold and caused no damage to the vines.

The biggest threat to my vineyard was early budding and spring frosts. I realized that 2005 was setting itself up to be a perfect test spring for the alginate frost avoidance experiments. I had surveyed my various varieties used previously in the alginate treatments and realized that no winter kill of the primary buds could be detected. This was a first in my growing experience at the Rist Canyon Vineyards.

Winter kill has always confused the results of the alginate treatments in past years. This year I would have little to none to complicate the data and observations.

I already knew that the alginate treatment delayed budding and thus the grape buds avoided being killed by spring frosts. In the first years experimenting with this treatment, I had learned that the alginate by itself washed off during wet spring snows and heavy rains. I had remedied this situation by adding latex house paint to the mixture to allow greater adherence of the treatment and prolong its effectiveness.

This year I decided to test the difference between regular latex house paint and "sash and trim" paint. I figured that since "sash and trim" paint protected trim on houses better it would adhere better to the grape buds through wet spring snows and rains. This proved correct and almost worked too good.

The end of March through April turned wet and cool. I enjoyed the wet snow and rain after years of drought. During this time, the plain alginate and alginate plus house paint washed off and had to be re-applied a couple of times to continue delay of budding and frost protection. The "sash and trim" addition worked much better and was never re-applied.

Temperatures began to warm considerably in April. Towards the end of that month I began to worry that the alginate was working too well. Vines in Fort Collins had already budded out and shoots were elongating in the warm spring sun. Mine were just sitting there, dormant. This was good luck though. A killing frost on the first of May was survived and the vines began budding right after that, free from damage. My friends vines in town had the primary shoots killed.

May and June were much cooler than normal. Shoots elongated slowly. It wasn't until the end of June that the summer temperatures warmed up to normal. These cool spring months delayed the blossoming of the grape flower clusters until the first week of July. The latest blooming I've experienced here.

July temps made up for the previous two months. July set a record for the warmest July ever in this part of Colorado. And the warmer than normal temperatures continued through August. I was worried about the late blooming but figured the vines were catching up in the warmth of the summer sun. Boy, was I wrong!

Here it is, the middle of September and even the earliest of grape varieties has ripened sufficiently to harvest them. In a normal growing year I would be harvesting from the first of September through the 15th, depending upon the variety. Verasion didn't even begin until the last week in August this year. Coloring of the clusters has been slow and erradic. The only luck is that the temps have remained warm and there has been no fall frosts as of yet.

Why the delay in grape clusters turning color and ripening has been perplexing to me. Especially with the warm summer since grape bloom....

This question ran through my head for weeks. My common sense told me that I had delayed the budding until the first of May and that cool spring temperatures had slowed the normal growth and delayed bloom until much later than normal. This could account for the delay in ripening. But common sense also told me that the warmer than usual temperatures in July and August should have allowed the vines to catch up and ripen at their normal time.

It would seem that what's common sense to me is not for the grapevine. What I've observed of the ripening period in past years is that the clusters of the earliest varieties begin to color in mid-August. Once color is seen on a few berries, the whole cluster colors up quickly in a matter of a week or so. Sugar accumulation and acid reduction in the berries also proceeds quickly in a normal year.

This pattern was not observed this year. Berry coloring wasn't seen until the end of the last week in August. When it finally arrived, the coloring was very slow and individual berries colored one at a time. Sugar accumulation has been slow too.

This phenomenon was very perplexing but I explained it away by the late budding and bloom. As I realized that the ripening was going much, much slower than usual, I began to get worried that it would frost before harvest. I decided to take a closer look at my temperature records for more normal years and contrast them to this year to see what the differences were.

I used the records for 2003 to be the "normal" year as it was a near perfect year for me here at the vineyard. How did it differ from this year? I was surprised to realize that bud break was within two days of this year. I had thought it was earlier. May through the second week of June that year were much warmer than this year and bloom occurred in the third week of June. The earlier bloom in 2003 only could account for about a week earlier ripening in that year vs 2005. But I'm observing almost a month delay in verasion and ripening.

One big difference between these two years is the temperature in July and August. 2005 set records whereas 2003 was cooler than normal by a few degrees. This year the temps were consistantly in the 90's and in 2003, the temps were in the high 70's to the mid- 80's. I began to wonder if this was the factor that made these two years so different in how the grapes ripened? Something was happening when the grape berries were green and pea-sized. The lag period in berry development.

I began to write emails to authorities on grape growing asking them what they had observed along the lines of temperature and berry ripening. I received the usual research chatter relating temperature to overall sugar and acidity. Temperature within the canopy vs the outside. Temperature and its effects on varieties. But not on speed of coloration and ripening. Most researchers didn't think that temperatures while the berries are in the lag phase should affect development and ripening. And most research on temperature and grape ripening concerned what was happening after verasion.

Then I found a citation for an obscure study conducted in Australia by Hale and Buttrose. These grape researchers performed an extensive, controlled, growth chamber experiment on grape ripening that followed temperature regimes during the three phases of berry development and maturation. They found that temperatures above 86F during the daytime and 70F at night during the lag phase delayed verasion 17-30 days and caused sporatic berry coloring with the onset of verasion. This is exactly what I've been observing this year.

They also found that sugar accumulation, once the berries have colored, is also delayed. But acidity was lower in grapes that underwent high temps during the lag phase when they become fully ripe.

What does all this mean on the practical level to us marginal grape growers? Temperatures during the middle of the summer are very important. A cool year means an earlier crop. A warm year means the threat of fall frost is greater and the grape crop may be somewhat under-ripe if they are to be harvested before a killing frost. There are also implications when it comes to global warming and being able to grow grapes in short-season areas where grapes haven't been grown before.

It's nice to know why I'm going to harvest later than I've ever harvested before, but it doesn't stop the worrying about whether I'll get ripe grapes (and great wines) before the frost stops this growing season in its tracks.

For those that are interested, the citation for this research is:
Hale and Buttrose, 1974. "Effect of Temperature on Ontogeny of Berries of Vitis vinifera, L. cv. Cabernet Sauvignon". Journal of American Society of Horticultural Science, Vol.99, 390-4.

I have been consulting with them on this project at every step. I'm so proud that they are carrying on the vinter tradition. Unlike me, they have chosen to grow grapes in a USDA approved grape growing area known as the West Elk appellation. I know their job of growing grapes will be so much easier than I have experienced on this side of the Rocky Mountains.

Mount Lamborn Vineyard