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Local vineyards devastated by Perfect Storm conditions

Photo by Greg Butler: A late freeze severely damaged the primary bud sprouts in thousands of local vineyards such as these pictured above. By Greg Butler, President, Shasta Cascade Viticulture Association Most valley floor grape growers are reporting at least 30 percent frost damage from the recent cold spell, and many have near 100% vegetative loss. Growers in Trinity County, where their bud break is further behind, say they will perhaps have no frost damage at all, since the susceptible shoots were not exposed. We could call this cold event a very rare thing for the time of year because of several coincidences. The biggest factors contributing to the damage were the very low humidity--essentially a very low dew point--and lack of ground moisture. Moist ground retains daytime heat which is released during the night to keep temperatures up. Our dry spring caused the low ground moisture, and since it is too early in the year to begin irrigating, most of us allowed this dangerous condition to exist. The low dew point meant little moisture in the air (humidity also retains heat) and the lack of humidity allows very rapid cooling or chilling when there is no cloud cover. Cloud cover is also a factor in holding heat, and on the worst night, April 19, we went from cloud cover to clear very rapidly in the wee hours of the morning. Without cloud cover, temperatures at my house dropped from 40 degrees at 3:30 AM to 26 degrees shortly afterwards. As one grower pointed out to me, it was an almost "Perfect Storm" scenario for disaster--a culmination of events that magnified the problem and worsened the effects. Growers often are at the mercy of such a cold snap unless they take aggressive action ahead of time. Ground preparation is a good first step. Tall cover crop or grass around the vines is actually colder than hard packed earth because the grass doesn't allow sunlight to reach the soil. Moist earth is better yet as explained above since it retains daytime heat and releases it slowly at night. So, mowing the grass short and watering are two good steps done in advance. Loosely cultivated earth is not as good as firm earth since the loose dirt acts like insulation, absorbing less heat in the day and then rapidly cooling at night because of greater exposed surface area. Sprinkler systems also provide heat to the plant during frost. I was surprised to learn that 50 gallons of water at 52 degrees will release 60,000 Btu's (British Thermal Units) of heat in cooling to 32 degrees. And sprinkling water over the vines causes the plants to become encased with ice which by definition remains at 32 degrees--one degree above the point at which freezing damage begins. However, if sprinkling is suspended while the temperature is below 32 degrees, the latent heat will be released and freezing will occur. There are economical methods of sprinkling such as using small, micro-pulse sprinklers which conserve water by providing a small amount of spray directly to the plant. Any sprinkling method will generally preserve the vine down to temperatures of about 25 degrees. Below that the plant probably will be damaged. And as was the case last weekend, the low dew point made even sprinkler systems ineffective since the water was rapidly evaporated from the plant and evaporation has a chilling effect. Air circulation with fans or heating with smudge pots are also effective but somewhat less practical, and in the case of smudge pots, environmentally unacceptable and expensive. On the positive side, grapes have secondary and even tertiary buds which often remain latent after the primary buds sprout, and the primary takes the plant energy into producing foliage. In our situation today, following the freeze, the secondary buds are popping out and generally are equipped with additional fruit clusters. Most grape growers who have experienced freezes in the past say the secondary clusters will probably be smaller in size and won’t reach the same sweetness; in the business it is called "Brix," referring to the percent of sugar in the grape. So, in all likelihood the crop will be smaller than and not as good as one would expect.