|Figure 3. Thompson seedless vineyard displaying significant frost damage. The vineyard middles were disked but were not packed and irrigated.|
Severe frosts will kill entire shoots to the cane. When temperatures are low enough to kill whole shoots, the damage is often uniform throughout the vineyard. Shoots killed by frost will turn a dark brown to black color within a few days of freezing.
Unusually warm weather in January and February will advance budbreak and expose more green tissue to frosts which may occur in March and April. Thus, early budbreak can increase the risk of frost damage. Budbreak in Fresno County normally occurs on March 15 (+/- 7 days) for Thompson seedless, which is about when budbreak was noted in 2008. Since then we have had some exceptionally warm, pleasant weather.
Such weather can give growers a false impression that the risk of frost has passed but early spring weather patterns frequently shift between highs and lows with devastating results. Growers should follow the weather forecast closely for extremes between highs and lows. In general, as growth progresses through spring, grapevines become more susceptible to frost but, fortunately, the likelihood of a severe frost decreases as the season progresses.
On clear nights when frost forms, the coldest air is found near the ground. As the night progresses, a cool layer of air continues to build and the shoots nearest the ground are the first to experience damage. Grapevines trained at one height are more susceptible than those trained at 12-15 inches higher.
For example, a Thompson seedless vineyard grown to a height of three feet will be more susceptible to frost than one trained on a open gable or overhead trellis with a head height of 48-54 inches. Previous research has shown that a temperature of 28ees Fahrenheit 1 foot above the soil can be 2 degrees warmer at 3 feet above ground. At 20 to 25 feet above the soil line the temperature will gain another 2 degrees. At some point the temperature will again begin to decrease, forming an inversion layer. The greater the temperature change between the soil line and 25 to 30 feet above ground level the greater the chance of avoiding frost damage.