During the growing season, all green parts of the grape vine are susceptible to frost. Spring is a particularly delicate period for the vines, since spring frost often damages opening buds and young shoots, thus affecting the crop load. Not all vine varieties are equally susceptible to frost as susceptibility depends on bud development stages of the different varietals. A closed bud during dormancy is protected from the cold by its thick walls, which enables it to resist temperatures as low as -15°C. Critical temperatures for grape vines 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. However, new growths can be damaged at -1,1°C.
There are two different weather events that can harm vines –Radiational Frosts and Advective Freeze.
Advective Freeze : This takes place when a large cold 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 than radiational frost, because active protection measures are not effective against it.
Radiational frost : This 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 than 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.
Winegrowers can protect vines against frost by employing 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 : These include biological and ecological protection methods, which are often less costly than active methods and can even eliminate the need for active methods. Therefore it is advisable to focus on passive frost protection methods first and foremost, as they are used to avoid frost danger
Site & varietal 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.
Cover crop 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 from absorbing and storing heat, and bare soil can be 2°C higher compared to the floor with 5cm high grass. Tall cover crops can also slow down the movement of the cold air out of the vineyard. 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 have an 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,early pruning to long spurs with 5-8 buds suppresses the growth of basal buds (buds from the base of the main grapevine), and once frost risk passes or basal buds begin to break, a final pruning can be carried out.
Active frost protection methods : These include activities in the vineyard which increase the temperature above the « danger » level by influencing the micro-climates in the vineyard. Obviously winegrowers focus on passive frost protection methods first and foremost, as they are used to avoid frost danger.
Wind machines : These work only with radiation frosts whenthere is an inversion and can provide between 1-3°F temperaturedifferences. Wind machines basically mix cooler air that is locatedaround the vines with the warmer air above the inversion layer, whichis usually at about (12-15 m) above the ground.
Wind machines are very expensive and are profitable on sites wherethere is a high probability of damaging spring frost – such as on 1 in every 5 years.
Helicopters : 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.
Heaters : Heating the vineyard air is one of the oldest practices to protect vines against spring frost. Some winegrowers still burn cuttings from pruning to heat the air, while fossil-fueled heaters are not largely used as they are expensive due to the cost of fuel and labor, as well as being rather inefficient with lot’s of energy being lost to the sky, not to mention the environmental concerns. 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 throughout 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.
Burning straw or hay : This is a similar technique which consists in burning bales of stray around the edges of the vineyard.The smoke that is given off provides a « protective blanket »which limits the loss of heat from the soil in absence of cloud cover.
Over-vine sprinklers : Vines can be protected against frostalso with the help of water sprinklers. When water is sprayed andfreeze around green tissues it releases heat and thus protects vines.For this method, a large volume of water is required since constantliquid water is needed to form a freezing coating around vines budsand shoots to release heat and rise the temperature. This is the onlymethod that can provide frost protection during the advection frost as well, if used properly.
Conclusions : Passive methods often eliminate the need for active methods, plus they are much less costly. Active frost protection methods are 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, heaters with wind machines or sprinklers with heaters, to efficiently protect vines and lower the costs.