Tender green shoots are sensitive to frost

Dormant grapevines can withstand temperatures that are much lower than those normally encountered in the San Joaquin Valley.  However, warm weather causes the dormant buds to deacclimate to the cold, and the succulent green tissue that emerges in the spring has little cold tolerance and can be killed by temperatures that would not injure dormant vines.  It is common in the San Joaquin Valley to have warm, pleasant weather following budbreak, which instills the false impression that the risk of frost has passed.  However, the weather in early spring often shifts between warm temperatures that promote shoot growth and cold temperatures which could potentially injure or kill tender young shoots. In general, as growth progresses through spring, grapevines become more susceptible to frost but, fortunately, the likelihood of a severe frost decreases as spring weather becomes consistently warmer.  Nevertheless, in the spring, growers should closely follow the development of their vines, and the weather forecasts.

The incidence and severity of frost damage in vineyards depends on shoot growth stage at the time of a frost, the minimum temperature reached, and the duration of time that the shoots are exposed to critical temperatures.  Buds can withstand temperatures of 25 to 27 ?F, but emerging shoots are less cold tolerant and may be damaged or even killed by sub-freezing temperatures.  Thus, growers should take note of their vineyard’s growth stage when a frost is predicted. If the vines are still dormant, no preparation may be needed, whereas precautions are needed in vineyards having green shoots longer than six inches. 

Frost damage in a Thompson Seedless vineyard.

Trellising can also affect frost tolerance.  On clear nights when frost forms, the coldest air is often found near the ground. As the night progresses, a cool layer of air may continue to build and the shoots nearest the ground will be the first to experience damage. Under such conditions, grapevines on taller trellises are less susceptible to frost than those on shorter trellising. 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 28?F one foot above the soil can be two degrees warmer at three feet above ground.

Frost damage can be minimized by preparing vineyard soils to maximize heat absorption during the day and release at night.  Soils that are moist, firm, and free of vegetation confer the most frost protection.  Thus, in preparation for frost season, native vegetation or cover crops should at least be mowed--before budbreak, and again as often as is necessary after bud break.  In vineyards with access to surface water, the row middles may be disked instead of mowed, and then irrigated.  However, growers with drip irrigation, and no surface water, should not cultivate the row middles unless a significant rain is predicted to occur before the risk of frost because freshly disked soil confers little, if any, frost protection. 

Typical effects of soil conditions on vineyard temperatures.

Irrigating during a frost event can also be beneficial.  On nights that low temperatures are expected pumps should be turned on early enough to cover the entire vineyard. Depending on the size of a vineyard, it may only be feasible to irrigate the most susceptible areas (low lying) where cold air tends to drain. Doing so will provide a measure of frost protection.  If a drip system is the only source of water in the vineyard, it should be turned on early, to wet as much soil as possible. Growers with drip will have to be especially vigilant to the weather forecast in order to start irrigating well in advance of the frost event.

By Matthew Fidelibus, viticulture specialist in the Department of Viticulture and Enology, UC Davis, and Stephen Vasquez is the UC Cooperative Extension viticulture advisor in Fresno County. Photo credit: UC Regents