Sudden increases in daytime temperatures in mid-May and again in mid-June had a dramatic impact on the progression of bloom in California winegrapes. If you were caught at bloom time in one of these high temperature periods, bloom began and ended in three days versus a normal seven- to ten-day period.
So far, 2008 has been a tough year for the grape industry in California. We have experienced a very late spring freeze, very cyclic spring temperatures, high winds, low winter rainfall, and a drought. Almost all growers have been affected by at least two of these problem situations and some are describing their situation as the “perfect storm” (all four).
An unseasonable cold snap in mid-April affected winegrapes primarily in the northern San Joaquin Valley, the Sacramento Valley, and on the Central Coast. As with most frost situations, damage was highly localized. In affected areas, frost damage to clusters ranged from 10% to 30% with a few instances of losses up to 80%. The southern San Joaquin Valley, where 60% of California’s winegrapes are produced, was mostly unaffected; therefore, total winegrape production in the state will not be impacted as significantly as it could have been.
With cooler early spring temperatures, grapevine growth was running four to five days behind normal in 2008. By bloom time, we had almost caught up and bloom occurred at near normal times for most varieties and areas. However, temperatures in May and June were very cyclic (see “Hot Streak”).
In some areas, these cyclic temperatures resulted in unusual nutrient levels in the vines at bloom time. In areas where canopies were developing slowly prior to bloom, the sudden exposure of the fruit to high temperatures caused sunburn damage. In addition, some areas on the Central Coast have reported excessive post-bloom shatter on Merlot and Cabernet Sauvignon of up to 80%.
Just to make life more aggravating, most of California experienced higher than normal winds. The effect was most severe on first-year vineyards with young shoots just coming up the stake. For mature vineyards, cane breakage and shredded leaves were common. Of the last 20 years, 2008 had the third highest average wind speed for the months of March, April, and May (1991 was the highest and 1999 second highest).
Low Winter Rainfall And Drought
When we receive normal winter rainfall, the soil profiles are typically filled, or at least partially filled, with moisture. The North Coast and Northern San Joaquin Valley were OK this last winter, but the Central Valley and the Central Coast areas were about 50% of normal. In these grape growing areas, there is a greater reliance on surface water supplies and deep well pumping.
We have two types of droughts in California â€” “natural” droughts and “regulator” droughts. Many growers, particularly in the Central Valley, receive surface water from reservoirs that store winter snow runoff; however, with lower snow packs in the Sierra Nevada Mountains last winter, supplies are limited. We don’t like natural droughts, but we learn to live with them.
A “regulatory” drought is also occurring in 2008 that involves the transport of surface water supplies. Almost all of the water used by agriculture on the western side of the Central Valley comes from the California Aqueduct. Due to regulatory limitations, the water supply for agriculture is limited in 2008 to a total of 6 inches per acre for all of June, July, and August. The normal irrigation need for grapevines during these months is at least 12 to 14 inches per acre.
Whether the drought is natural or regulatory, growers must minimize their use of irrigation water and turn to deep well pumping. This of course results in a strain on the groundwater pumping reserves.
Overall Yield And Quality
While the raisin and table grape industries are the least affected by the abnormalities of this year, the winegrape industry is most affected. Our summer weather will have an effect on total winegrape yield and quality. Given a “normal” summer, most observers estimate that the 2008 crop will be about 5% less than 2007.
The weather pattern in California that typically results in the highest quality fruit consists of:
1) Early bud break;
2) Warm springtime temperatures resulting in grape bloom being seven to ten days early;
3) Moderate temperatures during bloom;
4) A moderately warm post-bloom period as the berries are formed and enlarge;
5) A moderate summer that allows the berry to mature without undue vine stress; and,
6) A warm, dry fall so that the grapes can develop strong flavor.
So how would one judge 2008 so far? Of the six steps to success, we have struck-out on Steps 1, 2, and 3; and Step 4 was OK, but not great. And as I write, we are in the middle of a heat wave, so the jury is still out, but it doesn’t look so great.