FACT SHEET Frost damage, control and prevention - fruit and vines Introduction Adequate winter chilling is essential for the normal seasonal cycle of winter dormancy and spring growth of deciduous trees and vines. Most horticultural crops are sensitive to frost in early spring as buds and shoots emerge. A severe frost in spring can cause damage, resulting in partial or total crop loss. In extreme cases plant death may occur. Storing heat in the soil is the best form of protection against frost damage. What is a frost? The term “frost” is rather a loose one. In its most general sense it refers to the deposit of crystals of ice on the ground and other exposed surfaces when the air temperature falls to the freezing point of 0°C. However the temperature at which ice forms varies with humidity and as a result of this we have two forms of frost – a white frost and a black frost. White frost If the air is moist at the time of the frost, dew will form on the ground before the temperature falls to O°C. When this temperature is reached, the dew freezes to give a coating of whitish ice crystals on exposed surfaces and hence the name “white frost”. Black frost When the dew point (ie, the temperature at which dew forms) is below 0°C, frost develops without any visible sign of white ice appearing on exposed surfaces. Frost danger period The frost danger period for deciduous crops extends from bud burst to about the 7th of November. However, records over nearly 40 years show that the most critical periods in the Riverland occur between September 18th and October 12th. By September 18th most of the fruit shoots on vines and stonefruit have emerged and frost at this stage can cause great damage. After October 12th, the weather gradually warms up and an increasing amount of heat is stored in the soil. Furthermore the nights are shorter, reducing the time that the soil loses heat by radiation. Type of frost There are two types of frost, radiation and advection frost. The radiation frost is the most common in South Australia. The two types of frost are not distinct and many nights have characteristics of both. Radiation frost Radiation frosts begin at ground level and gradually rise. Radiation frosts occur when there is very light or no wind, temperature inversions, clear skies and low humidity. During the day the soil and plants absorb and store heat. At night the heat is lost by radiation particularly when there is no barrier (ie clouds) to prevent this heat loss. Clear skies and light winds allow any stored heat energy in the soil to escape easily. As a general rule, air temperatures drop by about 1°C per hour when conditions favour the development of a frost. Advection frost This type of frost occurs when a cold air mass moves into the area displacing warm air. In Australia this usually occurs when a cold Antarctic air mass moves over the Australian mainland. When this type of frost occurs some characteristics of radiation frost are also seen as factors causing radiation frosts can still be present. Advection frosts can occur earlier in the night and while the freezing process is the same as for radiation frosts the effects are usually seen on the upper part of the plants. Frost prediction The Bureau of Meteorology considers a number of factors when estimating the probability of a frost. These factors include: General weather situation Particularly favourable for frost development is the end of a depression, which is associated with a clearing of cloudy and possibly rainy conditions during the day to give a clear, still night. Wind Conditions of no wind are most favourable for radiation frost development. Cloud A cloudy day followed by a cloudless night produces the greatest frost risk. The cloudy day reduces the sun’s ability to heat the soil surface and when followed by a clear night what little heat is stored quickly dissipates. However, only a small amount of cloud at night is required to prevent a frost from developing. Dew point The dew point is the air temperature at which dew or condensation begins to form. Low dew point indicates dry air, which means that very little heat is released as the air cools and as moisture, condenses. This in turn leads to a rapid fall in temperature and a greater likelihood of frost. Experience has been that if the dew point in the evening exceeds about 5.5°C then there will be sufficient radiation reflected back by the atmosphere (of heat lost from the ground surface) to retard the rate of cooling and limit the probability of a frost. A dew point of less than 2.2°C indicates a high probability of frost. Orchard design As cold air tends to drain to the lowest point, avoid barriers across the line of airflow such as buildings and banks of trees, which may pool cold air. Frost sensitive crops should not be grown in low areas where cold air is trapped by natural topography, vegetation or by manufactured obstacles that inhibit drainage of cold air. Site selection is a primary decision to reduce frost damage in sensitive crops. Select tolerant varieties for frost prone areas. Late pruning in stonefruit and vines delays bud burst and crop development and reduces exposure of the crop during high frost risk periods. Trellis height of grapevines Temperature increases significantly from ground level to a height of 1.5 m. The difference in trellis height from 0.75 m to 1.5 m may mean an increase in air temperature of 1°C. This can reduce the frost risk significantly. Frost damage The extent of damage depends on the stage of growth of the crop and the time that the temperature remains below the danger point for that stage of growth. Damage occurs when the plant tissue is frozen. Water within the plant cells expands as it turns to ice rupturing the cell walls. Young spring shoots and blossoms are more easily damaged than the older limbs because the cells have thinner walls and higher water contents. Symptoms Minor frost damage appears as small whitish – yellow areas on young growth leaves, which eventually turn brown and fallout leaving tattered and distorted leaves. Severe frosts cause very distinct damage, shoots and flowers of stonefruit turn a brown colour and appear limp only hours after freezing. Fruit develops a blackened heart and may fall over the next few days. Damage to citrus fruits can be identified a day or two after a frost by the watersoaked appearance inside the fruit. Rind burn can occur, but often there is damage within the fruit without visual symptoms. The tables below illustrate the critical temperatures for stages of development of crop, at which significant damage can occur. Stone Fruit Growth stage °C Apple Apricot Pear Peach Plum Buds closed but showing colour -2.8 -1.1 -2.2 -3.9 -1.1 Full Blossom -1.7 -0.6 -1.7 -2.2 -0.6 Small, green fruit -1.1 0 -1.1 -1.1 -0.6 Citrus The temperatures at which tissues begin to freeze are: Variety °C Oranges, Grapefruit, Mandarins Green Oranges –1.4 to –1.9 Half ripe oranges, Grapefruit and Mandarins -1.7 to –2.8 Ripe Oranges, Grapefruit and Mandarins –1.7 to – 3.9 Lemons Buds and Blossoms -2.8 Button Lemons ( up to 12mm diameter) –0.8 to – 1.4 Green Lemons (greater than 12mm diameter) –1.4 to –2.8 Tree Ripe Lemons –0.8 to –3.3 Reference: (Opits,K.W., Brewer, R.F., and Platt,R.G., 1979) Grapevines For grape vines the temperatures at which damage occurs are: Growth Stage °C Woolly bud stage -3.5 Early Bud Stage -1.1 Shoots up to 150 mm -0.5 Shoots over 150 mm 0 Predisposing factors Frost damage predominantly occurs in low-lying areas, where there is dense ground cover, or where sunlight cannot warm the ground during the day. Control The key principle to preventing frost is good soil management which maximises heat storage in the soil during the day, for release at night. Control measures should be aimed at increasing the temperature around the buds and the shoots. Cultural control measures A weed free, compact, moist, soil maximises the amount of heat that can be stored during the day. Clean up the soil surface Most orchards have an inter-row sward, consisting of either a cover crop or combination of weeds. The use of this system to benefit soil structure is preferred to frequent cultivating. However, if vegetative cover is present this insulates the soil during the day, not allowing any substantial bank of heat to accumulate. Slashing close to the ground or spraying with a knockdown herbicide before bud burst is essential in known frost prone areas. This is a sensible compromise to cultivation which can harm soil structure. Compact the soil surface If cultivation is carried out, the soil must then be compacted and irrigated. Air pockets, as a result of cultivation insulate the soil and reduce its ability to store heat. Cultivation should not be carried out during the frost prone period as it also removes heat from the soil because water brought to the surface evaporates and removes heat in the process. Irrigate lightly Moisture in the soil stores a considerable amount of heat, and moist soil conducts heat more readily than dry soil. Heavy irrigation gives no more frost protection than light irrigation, with the added danger of damage to feeder roots. Aim to wet up only the top 30 cm to 40 cm of soil. This can be achieved by applying about 10 to 15 mm of irrigation on a weekly basis. Full cover irrigation will give up to 2°C more soil temperature than drip irrigation due to the greater area of moist soil. Mechanical frost control Mechanical methods include the use of overhead sprinklers, burning oil in heaters or frost pots and wind machines. Mechanical systems of frost control are generally expensive to operate. Before undertaking mechanical measures for frost protection every possible effort should be made to reduce the frost hazard through the cultural means as outlined earlier. Overhead irrigation systems The most common method of active frost protection is the use of overhead sprinklers while the frost risk is present. When water is applied to crops under zero or sub zero temperature conditions, it freezes. On freezing the water releases heat, which offsets the heat lost by the crop to its cooler surroundings. To achieve successful frost control a film of continuously freezing water must be maintained on the surface of the formed ice. When the ice begins to melt, usually after dawn when the air temperature begins to rise, heat must be applied to the ice or this heat will be drawn from the crop. This heat is supplied by water applied to the crop via the irrigation system. Generally an application rate of 3.5 mm/hr is adequate for frosts down to –4.0°C. Undertree sprinklers used during frost will give some protection but not at the same level as overhead sprinkler systems. Although overhead sprinklers are an effective means of frost control, they can have two major disadvantages. Firstly, during severe or prolonged frosts, ice can build up on the tree limbs to the extent that limbs break. Secondly during a long series of frosts, and when frosts begin early in the night, sprinklers must be operated for many hours; soils can become waterlogged promoting the development of root rots such as phytophthora. Orchard heaters Heaters or frost pots are only effective in radiation frost conditions. The burning of oil in frost pots heats up air creating localised convection currents which cause a mixing of the layer of air from the ground up to the inversion layer thus keeping the air warmer at crop height. Many pots are required to create this convection, approximately 120 to 170 per hectare. Frost pots are expensive to operate and buy. They are also not readily obtainable. However frost pots maybe worthy of consideration for crops of high monetary value located in a small-localised high frost prone areas such as nurseries. Wind machines The effectiveness of wind machines needs to be assessed for individual properties. Wind machines operate by drawing warm air down from above the inversion layer that exists at times of a radiation frost condition. They force cold air from the treatment area and replace it with warm air from above. Before investing in a wind machine, a grower should research several factors including orchard topography, height of the inversion layer and the temperature gradient to the inversion layer. Depending on conditions and topography wind machines can provide approximate temperature increase of around 2°C and protect an area of around 4 to 5 hectares. It is important to check with neighbours on the noise factor of these machines. Frost alarms In recent years electronic frost alarms have become available. Electronic sensors are placed in the orchard and record the temperature changes. The sensors can be pre- set to a certain temperature. When that temperature is reached, an alarm sounds via a radio link, alerting the grower to a possible frost, enabling him to take preventative steps such as activating the irrigation system. The placement of these sensors in known frost prone areas minimises the risk of substantial crop losses. Last update: August 1998 Agdex: 200/10 Author: David Pocock and Ashley Lipman, Industry Consultants, Renmark and Waikerie. Disclaimer Use of the information/advice in the Fact Sheets is at your own risk. The Department of Primary Industries and Resources and its employees do not warrant or make any representation regarding the use, or results of the use, of the information contained herein as regards to its correctness, accuracy, reliability, currency or otherwise. The entire risk of the implementation of the information/advice which has been provided to you is assumed by you. All liability or responsibility to any person using the information/advice is expressly disclaimed by the Department of Primary Industries and Resources and its employees.