During the growing season, all green parts of the grapevine are susceptible to frost. Spring is a particularly delicate period for the vines, since spring frost often damages opening buds and young shoots, and hazards the crop load. Not all vine varieties are equally susceptible to frost, as well as susceptibility depends on bud development stages. In some regions, fall frost that occurs prior to the harvest, can lead to premature leaf fall, and increase susceptibility to Botrytis attacks. In order to prevent vineyard frost damage, several passive and active methods can be used, while having weather stations across the vineyard which can provide warnings on extreme weather conditions and/or having a reliable weather forecast can be of great help as well.

Critical temperatures for grapevines

Critical temperatures for grapevines differ based on the growth stage, while during the winter, dormant buds can resist temperatures below -10°C and even down to -20°C and more, due to cold hardiness in endodormancy phase. However, new growths can be damaged at -1,1°C. In the table below are presented critical temperature values (in °F and °C) for grapevines in different growth stages, as found in the study which was made at Michigan State University for cv. Concord.

Critical temperatures values for grapevine

Tabel (adapted by FAO): Critical temperatures values for grapevine

Advective Freeze vs. Radiational frost

There are two different weather events that can harm vines  – Radiational Frosts and Advective Freeze.

  • Advective Freeze occurs when a large cold (polar or artic) air mass, accompanied with winds, cloudy conditions, and low humidity, blows into an area and replaces warmer air. In such days, temperatures often drop below 0 °C and stay in this range all day. Advective Freeze may cause more damage then radiational frost, because active protection measures are not effective against it.
  • Radiational frost occurs in clear nights with little to no winds, and as the name implies is connected with heat radiation. When more heat radiates away from the earth surface then the surface receives, the temperatures drops, and so the coolest air is closer to the ground. Apart from a clear sky- temperature inversion, low dew-point temperatures, night air temperatures below 0°C, and daily air temperatures above 0°C are associated with radiational frost.

It can also happen that a combination of advective and radiative conditions occur. For example cold air mass enters into a region, which results in the advective freeze, then for several days clear and calm conditions follow that cause radiation frost.

How can winegrowers prevent vineyard frost damage?

Winegrowers can protect vines against frost with several passive and active frost protection methods. However, these methods can prevent or limit frost damages due to radiational frost while there is very little that can be done to protect vines against an advective frost.

Passive frost protection methods

Passive frost protection methods include biological and ecological protection methods, which are often less costly than active methods and can even eliminate the need for active methods. Therefore is advisable to focus on passive frost protection methods first and foremost, as they are used to avoid frost danger.

  • The site and varieties selection

The most important element of frost protection is of course site selection and choice of varieties (late vs. early variety). Since cold air flows downhill, the mid-slope positions are warmer, if there are no trees, brush or other air dams that prevent cool air to flow out of the vineyard. Different grape varieties vary in the date of budburst, so it’s somewhat recommended to choose early bud burst varieties in locations with the lowest risk of frost.

cold-air-flow-vineyard-frost

Photo (E. B. Poling, 2008): Cold air drains downhill and settles in low spots during a radiational cooling period. You want your vineyard to be in “Thermal belt” and free of any air dams, such as trees, brush, etc. which would prevent cool air to flow out of the vineyard.

  • Soil water management

Wet soil holds and stores more heat, so low-laying air stays warmer for a longer time in this way. Therefore, in order to prevent frost in the vineyard, the soil water content should be near field capacity 2-3 days prior to the frost event. However, this method has also some drawbacks such as the risk of erosion, loss of soil organic matter, destruction of soil structure, etc.

  • Cover crops management

Although there are several benefits of using cover crops in the vineyard when it comes to preventing frost, cover crop does not affect the situation drastically. However, cover crops prevent soil to absorb and hold heat, it was reported that the temperature of the bare soil can be up to 2°C higher compared to the floor with 5 cm high grass. Tall cover crops can also slow down the movement of the cold air out of the vineyard. Additionally, cover crops tend to harbor ice-nucleation active (INA) bacteria which can cause more freezing injuries. What winegrowers can do is to mow ground cover short before the frost-prone period or if there is no erosion concerns even cultivate the cover crops prior to that period.

  • Pruning method

With vineyard management practices such as pruning, winegrowers can influence on the date of bud burst and thus reduce the risk of frost injury. This can be achieved with either a delayed pruning, since unpruned vines bud burst later than pruned vines, or with double pruning. When conducting double pruning, do early pruning to long spurs with 5-8 buds, as this will suppress the growth of basal buds, and once frost risk passes or basal buds begin to break, do a final pruning.

Double pruning-passive frost protection methods

Double pruning, one of the passive frost protection methods.

Active frost protection methods

While active frost protection methods include activities in the vineyard which increase the temperature above the injury level and therefore influence on vineyard micro-climates, such as wind machines, orchard heaters, and overhead sprinklers.

  • Wind machines

Wind machines work only with radiation frosts when there is an inversion and can provide between 1-3°F temperature differences. Wind machines basically mix cooler air that is located around the vines with the warmer air above the inversion layer, which is usually at about 40-50 ft (12-15 m) above the ground. The fans should be turned on when the temperature at 5-foot height is above the critical damage temperature or before the temperature at 5 foot (1,5 m) height falls much below the temperature at 33 foot (10 m) height. In practice, winegrowers turn on wind machines at about 32-34°F (0-1°C).

wind machines-radiation frost

Photo (FAO, 2005): Effect of wind machines on the temperature profile during a radiation frost.

Wind machines are very expensive and are profitable on sites where there is a high probability of damaging spring frost – such as on 1 in every 5 years. So instead of purchasing wind machines, winegrowers sometimes prefer to make use of helicopters which also can mix inversion layers. This is a much more expensive method to protect vines from frost damages, but in areas with a low probability of spring frost more than a welcome option.

  • Heathers

Heating the vineyard air is one of the oldest practices to protect vines against spring frost. Some winegrowers are still burning prunings to heat the air, while fossil-fueled heaters are not largely used as they are expensive (cost of fuel and labor costs), inefficient (lot’s of energy is lost to the sky), and pollute the environment. Heaters work best with temperature inversions, if there is little to no inversion or wind is blowing the heaters may not provide adequate protection. In order to properly protect vines, heaters should be uniformly distributed through the vineyard and turned on before the critical temperature is reached. To lower the cost of operation heaters can be used in combination with sprinklers or wind machines. In combination with wind machines, the temperature can be raised for 3 to 4 degrees.

  • Over-vine sprinklers

Vines can be protected against frost also with the help of water sprinklers. When water is sprayed and freeze around green tissues it releases heat and thus protects vines. For this method, a large volume of water is required since constant liquid water is needed to form a freezing coating around vines buds and shoots to release heat and rise the temperature. This is the only method that can provide frost protection during the advection frost as well, if used properly.

 water freeze around green tissues to prevent vineyard frost damage


Photo (AccuWeather.com): Water is sprayed on grapevines with over-vine sprinklers to freeze around green tissues, in order to prevent vineyard frost damage.

In order to know when to turn sprinklers on and off, make use of temperature data, Dew Point temperature, and Wet Bulb temperature. Winegrowers usually turn on the sprinklers when the temperature drops to 34°F (around 1°C) unless there is a low dew point temperature. Once dew point temperature drops to 28 °F (-2,2 °C) white frost will appear on vegetation, while at dew point temperatures above 45°F (7,2 °C), frost is rarely a problem. Following the wet bulb temperature, over-plant sprinklers should be turned on when the wet bulb temperature reaches 32°F (0 °C) and turned off when the wet-bulb temperature is higher than the critical temperature. Wet-bulb temperature can be measured with a psychrometer or estimated from the dew-point and air temperatures. If using eVineyard – vineyard management system wet-bulb and dew point temperature are only two clicks away. Moreover, eVineyard supports also the latest, state-of-the-art sensors for measuring the actual frost temperature.

READ ALSO:  Climate, weather, and vineyard management

In order to know precisely when to turn on and off the irrigation system for frost protection, check the table below which shows minimum starting air temperatures (°F and °C) for frost protection with sprinklers based on wet-bulb temperature (°F and °C).

Critical wet-bulb temperature to turn on sprinklers and prevent vineyard frost damage

Tabel (Rober G. Evans, 2000, ASEV): The wet-bulb temperature will help you determine at what air temperature you need to start and stop over-vine sprinklers in order to protect vines and prevent frost damage.

Conclusion

We have presented passive and active frost preventing measures that winegrowers can use to protect vines. Most of the passive methods should be considered before planting the vineyard, such as gathering long-term weather data and frost events, especially if planting a vineyard on a new site, carefully choosing vine variety, cultivation and pruning methods, etc. Passive methods often eliminate the need for active methods, plus they are much less costly.

Any active frost protection method should be chosen based on vineyard site specifics and winegrowers financial capabilities. Often winegrowers are combining different methods to protect vines and use for example sprinklers in combination with wind machines, heater with wind machines or sprinklers with heaters, to efficiently protect vines and lower the costs. In the most critical moments, winegrowers often reach for spray materials, which supposed to protect vines against frost. Although in laboratory studies many spray products reduced freezing temperatures, results in simulation trials show no effect at frost temperatures of -6°C and -4°C. Therefore the practical value of spray-on frost protectants is questionable. The reason for that might be that the tested temperatures have been below the limit of protection and/or there was no sufficient time for protective effect to develop. However, results give some optimism, therefore we will pay attention to new research results about spray-on frost protection, as they might be a useful and low-cost alternative compared to other frost protection options.

Winegrowers, which frost protection methods do you use to prevent vineyard frost damages? Let us know in the comment below.

 

Sources:
Frost Injury, Frost Avoidance, and Frost Protection in the Vineyard, by Ed Hellman from Texas AgriLife Extension, eXtension.org [online]
Ted Goldammer. 2018. Grape Grower’s Handbook. A Guide To Viticulture for Wine Production. Apex Publishers.
V. Minton, H. Howerton, and B. Cole. 2017. Vineyard Frost Protection a Guide for Northern Coastal California. Sotoyome RCD
C. Hickey, E. Smith, and P. Knox. 2018. Vineyard Frost Protection. University of Georgia Extension
Loder, H., 2008, Frost Protection in Viticulture: A Users Guide for South East Irrigators, Wingara Wine
Group Pty. Ltd.
R. L. Snyder and J. P. de Melo-Abreu, 2005. Frost Protection: fundamentals, practice, and economics. FAO, Rome 2005.
E. B. Poling. 2008. Spring Cold Injury to Winegrapes and Protection Strategies and Methods. American Society for Horticultural Science 2008, Volume 43: Issue 6
Understanding and Preventing Spring Frost/Freeze Damage – Spring 2016 Updates, by Michela Centinari. On Penn State Wine and Grape, [online]
Featured image: From Presentation of Frost Protection & Recovery in Vineyards, 2009 Review; Cooperative Extension San Joaquin Country [online]