Bloom or Bust!

By Jessica Cortell

Slowly but surely, wool gave way to pink-fringed leaf tips as the shoots emerged from the grape buds. Finally, bud break.

It occurred around May 1 in the Willamette Valley, depending on the variety and vineyard site, but it can happen as early as mid-March or as late as the start of May, contingent on the temperatures. Generally, bud break will commence when the average daily temperature reaches 50°F, but it has been so cool this spring, the first day we hit that temperature mark, the calendar read May 1.

How is this late bud break going to affect the vintage? Ultimately, the harvest date primarily comes down to when we reach bloom.

As Greg Jones predicted at the Oregon Wine Symposium, strong La Niña conditions have resulted in a cool, wet spring similar to last year. He stated the last two springs have been cooler that the previous 10 to 15 years, and we should break out of this pattern by mid-June and the rest of the summer looks normal.

Jones summarized by saying, “For the vines, things are clearly behind, with growth about on track with last year in Southern Oregon. This means that things are 10 to 20 days behind. I would expect that the hope is for a warm-enough summer to catch up and an October to ripen things out.”

What does it take to get from bud break to bloom? According to Al MacDonald, viticulture instructor at Chemeketa Community College, it takes around 500 growing-degree days (GDD, heat units) and shoots with around 15 leaves.

In “General Viticulture” by A.J. Winkler, he reports bloom occurs when the mean daily temperature reaches 68°F. Bloom often occurs about 60 days past bud break in Oregon. In 2008, the average number of days from bud break to bloom was 63 across several vineyard sites. However, with cool temperatures last spring, it took 101 days from bud break to bloom since bud break was at the end of March. Although it was early last year, little to no growth occurred for three to four weeks.

At the Northwest Viticulture Center (NWVC), there were only seven heat units in April. In comparison, the average for April is around 100 GDD, going back to 1986 (Al MacDonald, NWVC data). In the last couple of cool vintages, there were 31 GDD in April of 2008 and 34.5 in April of 2010. Using the current weather forecast through the end of May, we are likely to accumulate only around 100 heat units by the end of May. This will be 120 GDD off the May average of 220.

I made a toast at my recent “May Day Bud Break Party” to get to bloom before July 4 for Pinot noir in the Willamette Valley. Is this possible?

If we need 500 heat units to get to bloom, we would need to accumulate 400 heat units in June or 11.76 heat units per day to hit bloom by July 4. In 2010, there were only 251.5 GDD in June or 8.38 per day. Last year was the latest bloom on record, occurring around July 10. The average at NWVC going back to 1986 is 400 GDD in June. This doesn’t bode well for hitting bloom before July 4 this year if temperatures are still below average through mid-June.

We need to cross our fingers for June temperatures closer to average and, unfortunately, plan for another late harvest for the 2011 vintage.

Why is the timing of bloom so important? Because there are a typical number of days needed to go from bloom to ripeness based on variety. For early ripening varieties such as Pinot Noir, it usually takes around 100 to 110 days from bloom before the fruit is ready to pick. Randy Gold of Pacific Crest Vineyard Management in Talent said they usually pick Syrah, Merlot, Malbec and Viognier in 110 days and Cabernet Sauvignon and Cabernet Franc in 120 days.

For example, taking an average of a number of Pinot Noir vineyards in the Valley in 2007, bloom was on June 20 and harvest on Oct. 5, with 105.5 days from bloom to harvest. In 2008, bloom was on July 1 and harvest on Oct. 15, with 104 days from bloom to harvest. In 2010, bloom was around July 10, and most of the fruit in the Valley was picked on Oct. 21, which was 103 days after bloom. For 2011, it looks like we will need to have a warm September and October to bring in high quality fruit.

Since we can’t control the weather, what can be done in the vineyard? Early root and shoot growth is entirely dependent on nutrient reserves in the permanent structure of the vine. The vine is dependent on nutrient reserves until the fifth or sixth leaf stage before leaves become photosynthetically capable of producing and exporting carbohydrates. Where possible, provide nutrients to support shoot growth while temperatures still remain cool. Foliar sprays of calcium and kelp can be used to promote shoot growth. Kelp is a growth stimulant. Once temperatures warm up, limit excess vine vigor or stress where possible.

Additionally, there is high susceptibility to powdery mildew and botrytis infections at bloom, particularly under cool, wet spring conditions. Ideally, pull leaves in the fruiting zone early to help improve spray coverage, reduce disease pressure and increase temperatures and development of the fruit. A botrytis preventive spray can be included in the spray program at bloom.

Doing all vineyard tasks in a timely way will promote good vine health and fruit ripening. Growers should plan for an October harvest with disease and bird pressure similar to 2010. If only we could control the weather … but then that wouldn’t be farming.

A Growing Glossary

Learn viticulture’s vocabulary to better understand the vine and wine.

Growing-degree days (GDD), also known as “heat units” are a heuristic tool in phenology. GDDs are a measure of heat accumulation to predict plant and pest development rates such as the date that a flower will bloom or a crop reach maturity. GDDs are calculated by taking the daily average temperature (high plus low) divide by 2 and subtract 50°F. If the number is negative, there were 0 heat units accumulated. Seasonal GDDs are counted from April 1 through Oct. 31. 

La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3° to 5°C. In the U.S, an episode of La Niña is defined as a period of at least five months of La Niña conditions.