For help in planning your frost protection program this spring, take a look at Table 1. It’s based on research reported by Matthew Fidelibus, University of California Cooperative Extension specialist, based at the Kearney Agricultural Research & Extension Center, and Stephen Vasquez, formerly Fresno County UCCE farm advisor for viticulture.

This table shows how the relative susceptibility of grapevine tissue varies at different growth stages and critical temperatures.

Table 1. Frost damage to various growth stages of grape

Growth Stage                                 Critical Temperature*

Buds—tight, closed                         25-27°F

Buds with wool (eraser stage)          < 26°F

Bud break—green tissue showing    < 30°F

Shoots< 6” in length                        < 31°F

Shoots > 6” in length                       < 32°

Keep in mind that these critical temperatures are based on research under controlled environments. Such vineyard characteristics as location and cultivar, for example, may increase or decrease susceptibility to frost damage. These numbers should only be used as a point of reference when developing a frost protection program.

Training grapevines to higher heights decreases their susceptibility to frost damage, the authors note. On clear nights, the coldest air is found near the ground. As this cool layer of air builds through the nights, shoots nearest the ground are the first to suffer from freezing temperatures. That’s why, for example, a Thompson Seedless vineyard trained to a height of 4 feet will be less susceptible to frost than one trained to a height of three feet. 

Research also shows that during radiative freezes, temperatures generally increase with height above ground before decreasing again at some point to form an inversion layer. For example, air temperature may be 28 F 1 foot above ground, but 30 F three feet above ground. At twenty to twenty-five feet above the soil line the temperature will gain another two degrees. The greater the temperature change between the soil line and the inversion layer, the greater the chance of avoiding frost damage. Wind machines can help mix the cold and warm air, increasing the temperature and providing relief for green tissue.

 

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If frost is predicted prior to bud break, vineyards may not require special attention. However, when green shoots are longer than 6 inches, soils need to be prepared well in advance of cold weather. That includes preparing vineyards soils for maximum heat absorption during the day and release at night.

As shown in Table 2, moist soils are better able to absorb heat during the day and radiate it at night as ambient temperatures drop. Soil texture will also have an impact on heat absorption. Vineyards planted on sandy soils are more prone to frost damage because they retain less water than firm soils.

Table 2. Comparisons between optimal soil conditions for frost

Soil Characteristics    Vegetation                       Temperature        Benefit

Bare, firm, moist          None                                  Warmest               Optimal

Moist                           Shredded cover crop           0.5 °F                  

Moist                           Low growing cover crop     1-3°F colder        

Dry, firm                      Freshly disked                    2°F colder           

Dry to moist                 High cover crop                 2°F colder             Least optimal

If winter precipitation has not been adequate to maintain soil moisture, more water may be needed, say the Extension authors. Prior to a predicted frost, the goal should be a uniformly distributed irrigation that allows for maximum heat absorption.

Soils that have been recently cultivated or disked do not retain heat well because they are dry and have numerous air pockets. That’s why they should be irrigated soon after cultivation.