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Frost prevention and protection in vineyards

By: Diana Fisher, Viticulture Development Officer, Manjimup.

Viticulture in Western Australia

Frost prevention and protection are the only methods to manage frosts. There are no management techniques, such as the removal of shoots, or tools that have proved beneficial following a frost. The following information refers to radiation frosts.

Site selection and vineyard design
Variety selection
Trellis height / canopy management
Timing and pruning methods
Soil management and cover crop selection
Overhead sprinkler irrigation
Air mixing
Foliar sprays
Fuel burning
Other methods

Site selection and vineyard design

To avoid frost it is best to select a vineyard site where there is no or minimal frost risk.

This can be done by:

Review of local weather records - This will provide information on the type, severity and frequency of frosts.
Consideration of landscape - Closed valleys can trap cold air and create a thick frost zone while open plains tend to thin the frost layer. Where radiation frosts are common vineyards should be designed so that cold air may drain away from the vines. A two per cent fall along vine rows with no barriers to airflow is considered ideal (Hedberg, 2000). Barriers to airflow include windbreaks, plantations, trees along creek lines, dam banks and buildings. Fog is a good indicator of cold poorly drained areas where radiation frosts could be a problem.
Obtain knowledge of soil types - Dark soils absorb more heat than light soils while gravelly soils absorb more heat than clay soils.
Selection and location of windbreaks - Windbreaks restrict airflow and can trap cold air of a radiation frost. Frosts need to be considered when windbreaks are selected and located.

Variety selection

The probability of spring frost damage declines as the season progresses. Earlier bursting varieties (e.g. chardonnay and pinot noir) are more likely to be affected by frosts than later bursting varieties (e.g. cabernet sauvignon).

Some varieties will also recover better after a spring frost than other varieties as they have a more fruitful secondary bud (e.g. gamay).

Trellis height / canopy management

There is a greater risk of frost damage the closer the buds or shoots are to the ground. Trellising systems such as Te Kauwhata Two Tier whose bottom cordon wire is 0.4 m from the ground and Scott Henry whose bottom cordon wire is only 1.0 m from the ground and the shoots are trained downwards have a greater risk of frost damage. Trellising systems such as Smart-Dyson whose cordon wire is 1.2 m from the ground and Vertical Shoot Positioning whose bottom cordon wire is about 1.0 m from the ground but the shoots are trained upwards have a lesser risk of frost damage.

It has been suggested that moving the trellis height from 0.9 m above ground to 1.8 m above ground is to move into air about 1o C warmer (Vagnarelli 1998).

Timing and pruning methods

Timing - Budburst may be delayed by pruning later and thus reduce the risk of frost damage in spring. To be effective this pruning needs to be very late. The idea is to prune the earliest bursting varieties (i.e. the most susceptible) last. Earlier bursting varieties include chardonnay and pinot noir. On large properties or where there is labour or machinery shortages delayed pruning may be difficult to implement to be of benefit.
Long-cane pruning - The principal behind long-cane pruning is to increase yield from vines that may not yield as high if are spur-pruned. An adjunct to this is that long canes also tend to display the physiological feature of apical dominance. Apical dominance is when the distal buds on the cane shoot first leaving the buds closest to the trunk dormant for a longer period. Under most circumstances this is undesirable due to the varying growth along the vine. Where late spring frosts are prevalent this delayed shooting may be beneficial, as the dormant buds should survive the frost and still provide a crop for that season.
Double and insurance cane pruning - Double cane pruning is when canes that are eventually to become spurs are pruned to lengths of 40 to 50 cm while all other canes are removed. Insurance cane pruning is when the vine is spur pruned but two long canes are retained. This first pruning pass occurs during the normal winter pruning period. After the frost vulnerable period these long canes are shortened back to the normal two-bud length. These methods are also based on the principal of apical dominance. In spring the distal buds shoot first. Retaining the longer canes at the first pruning delays bursting of the more fruitful buds that occur towards the base of the cane by as much as 10 days (Winkler, 1974). The major limitation of these methods is the labour requirement of two pruning passes.

Soil management and cover crop selection

Soil management is one of the easiest methods to help reduce frosts in the vineyard. A compact, bare, moist soil can store solar heat radiation during the day and re-radiate this heat during the night a lot better than freshly cultivated soils or those soils planted with cover crops. These compact, bare, moist soils can actually increase the air temperature by up to 2o C above the minimum temperature (Vagnarelli, 1998).

Although cover crops offer many benefits to the vineyard their retention into the spring period can pose a frost risk to vines. A tall, dense cover crop has the potential to reduce night time temperatures by up to 5 to 6o C while a closely mowed cover crop is usually only 1o C colder than bare soil (Thomas et al, 1993). It is difficult to quantify the risk, but in frost prone areas cover crops should be mown or rolled prior to budburst. If there is any uncertainty whether mowing will be practicable, consider planting a low growing cover crop. If a self-regenerating annual is to be planted be sure to select one that has a short growing season, so that seed may be set for the following season before it is mown or select a species such as subclover that does not need to aerially seed.

The other factor to consider with cover crops and vineyard flora composition is that they vary in the amounts of ice-nucleating bacteria that inhabit their surfaces. The Pseudomonas family establishes well on grasses and plant species with thin cuticles. Legumes with smooth, thick waxy cuticles do not support as much ice-nucleating bacteria. Grapevines do not support as much ice-nucleating bacteria as other crops such as pears and citrus but if large populations are present in the cover crop they can be moved by wind and air currents onto the emerging vineyard foliage. Close mowing reduces leaf area and thus reduces the bacteria levels. Close mowing also helps to maximise daytime solar radiation absorption into the soil.

Overhead sprinkler irrigation

Overhead sprinkler irrigation provides frost protection by the release of latent heat. Latent heat is released when droplets of water on the vine tissue reach 0o C and begin to freeze. As the freezing continues latent heat is released which prevents the vine tissue from being frozen. As the ice reverts to a liquid heat is drawn from the plant tissue. This necessitates the sprinklers to operate until after all of the ice has melted which is usually after sunrise.

The standard practice for overhead sprinkler frost protection is for sprinklers to be turned on when air temperatures in the vineyard have dropped to 1o C. The sprinklers are run continuously until the air temperature outside the sprinkler area is above 0o C and all the ice has melted from the shoots.

Water application rates are normally between 2.5 and 3.5 mm per hour. Higher water rates may be applied but the higher water run-off can lead to vineyard management issues such as waterlogging, poor drainage, poor trafficability and inefficient use of water and power. Dam capacity and annual rainfall should be considered if using overhead sprinkler irrigation for frost prevention. Regulated Deficit Irrigation techniques would also be compromised, as the extra irrigation would fill up the soil profile.

The water rates applied for frost protection are considerably less than those applied for overhead canopy irrigation. The sprinkler systems are usually designed so that they may be used for both purposes. Some overhead sprinkler systems can alter temperatures by 6o C (Vagnarelli, 1998). Under vine irrigation systems can have an effect on vineyard temperature but they tend not to be as effective as overhead sprinklers.

Air mixing

Radiation frosts have an inversion layer that contains warmer and usually drier air than that found at the soil surface where there is the susceptible vine tissue. Night breeze can mix these two airflows and reduce the risk of frost. If no or insufficient breeze is present man made machines such as wind machines or helicopters may be used to mix the air.

If man made machines are to be used to mix the inversion layer noise pollution, especially in built up areas, needs to be considered.

Wind machines - Depending on the contour of the land and vineyard layout a single wind machine has the potential to protect 4 to 6 hectares of vines (Walsh, 2000 and McAulay, 2002). Multiple wind machines increase the range of effectiveness of individual machines. They also offer better protection if they are used to maintain temperatures in the vineyard rather than to try and raise the temperatures from below freezing to above. This technique is often referred to as a 'slow-freeze-slow-thaw method'. Thus, wind machines tend to be turned on before frosts occur 1.5 to 0.5o C and are turned off after the threat of a frost has gone. This is usually after sunrise. Wind machines ability to be beneficial quite often depends on the height of the inversion layer. If this warmer air is not reached the wind machines could potentially be delivering more below zero air to the vineyard and exacerbate the problem. Wind machines have the added advantage in that they can be designed as auto start units. When the temperature drops below a preset level for a period of time the wind machine will start automatically. If batteries are the power source, solar panels could be a good investment to ensure the battery is always charged.
Recent research funded by the Grape and Wine Research and Development Corporation (GWRDC) titled 'Frost management in cool climate vineyards' found that wind machines generally create a stable mean increment in air temperature of 1.0 to1.5o C.

Helicopters - is another way in which the inversion layer may be mixed. Helicopters come with a high price tag whether they are to be hired or owned. However, depending on the distance to the helicopter base, the area to be protected, the severity and frequency of frosts helicopters may actually be a viable option. There are regulations governing their use and some safety issues to be considered such as the location of large trees, hills and power lines to the vineyard and the ability of the pilot to see the ground, especially if a fog is present.

Foliar sprays

There are several foliar sprays that have been reported to help with frost protection. These include:

Oil sprays - Oils such as canola can be sprayed to cover all green tissue. This oil-coated tissue has been reported to up to 1,sup>o C warmer than surrounding unsprayed tissue (Vagnarelli, 2000). This practice has limited benefits, as the shoots tend to outgrow the protection sprayed. This technique could be useful very early in the season when a forecast frost coincides with a fungicide spray.
Application of copper - Copper sprays have the ability to reduce the amount of Pseudomonas bacteria and thus provide some frost protection. Copper is not registered for this use, but applications used for pest management may be of some benefit.
Anti-freeze solutions - There are several commercial products that claim to have frost protective properties when applied to vines (e.g. Antistress, Envy, Seasol and Teric). These products are meant to have some impact on the ice-nucleating process. Recent research funded by GWRDC titled 'Frost management in cool climate vineyards' found that these commercially available frost protectant sprays were ineffective in either laboratory or field trials, but they did suggest further trials were required (refer to GWRDC Annual Reports).

Care would need to be taken that any foliar sprays applied were registered for use in vineyards and that no Maximum Residue Limits would be exceeded.

Fuel burning

Burning fuels, such as hay and kerosene, can be an effective means of frost prevention if an inversion layer is present. There is some contradictory information as to how fuel burning helps to warm the air. Some say that protection is provided by air stirring as well as by the generation of heat and the insulating layer of smoke that collects in the inversion layer slowing heat loss from the ground (Corby, 2000). Others have said that the benefit against frost comes from the heat of the burning fire not from the smoke. The fire sends up a column of heated air. At the inversion layer this heated air meets other air of its own temperature and density and rises no more. As the fire continues to produce warm air the cold layers of air below the inversion layer are heated gradually, from the top down. If the fire is too strong it could actually produce a strong draught of heated air that could break through the inversion layer and ascend to a height of no benefit. For fuel burning to be effective the heat produced is to be no less than the rate of radiative heat loss (Vagnarelli, 1998).

Fuel burning could have some environmental concerns and the Department of Environmental Protection should be contacted before this method of frost prevention is undertaken. Also, fuel burning is reportedly to be more effective using a number of smaller burners than a few large burners. This could increase the labour intensity of the method.

Other methods

Other methods that may be used to reduce frost damage or losses from frost include:

Insurance - this will ensure a reliable method to obtain an income from the vines even if frost occurs. It can however be quite expensive.
Frost Alarm - this is where electric sensors are placed in the frost prone areas of the vineyard to help reduce the risk of a frost. These sensors are designed to record temperature changes that have been pre-set. When that pre-set temperature is reached the alarm is activated by technologies such as a radio link or mobile phone. This alerts the vigneron to the possibility of a frost so preventative steps can be undertaken. Some alarms are directly linked to the management method (e.g. wind machine or overhead irrigation system) so that once the pre-set temperature is reached these management tools are immediately activated.
Fog machines - these may be used to increase the humidity in the vineyard.
Vine guards - have been reported to provide some protection to young vines to -1o C (Corby, 2000).
Physical coverings - Physical coverings such as greenhouses and netting have also been reported as providing protection to vines from frost. This type of protection would only be suitable on small plantings.
Electric currents - these are passed through an insulated cable that runs along the fruiting wire on which the canes are held. The cable is meant to warm the sap flowing inside the cane by up to 5o C. The increase in vine temperature protects the vine tissue from inside the vine (e.g. the buds and shoots). This is a world patented system known by the name ViniCable.

Page updated September, 2005

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