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Protecting your orchard from frost this season.
Steven Lorimer DPI Swan Hill
Current risks
Depending on the district, Northern Victorian fruit growers are at risk of two types of frosts, radiation and cold air invasion. Drier flat inland areas of Victoria such as the Goulburn Valley are often at risk from radiation frosts which is the loss of stored heat from the soil. The conditions that most characterise this event are little or no cloud cover, and low humidity.
Fruit crops in the undulating Mallee areas can also experience radiation frosts but also cold air invasion. This is when cold air flows down slopes until it pools, causing damage to crops. Barriers that may cause pooling are fence lines, channels, soil banks and wind breaks. | A peach that may have been affected by frost. |
Northern Victorian fruit growers can immediately reduce risk of frost by undertaking a number of low cost activities. Options ready for immediate adoption are managing soil surfaces, air movement, irrigation and orchard hygiene. Many of these activities are low cost compared to their potential benefits.
Affects of freezing temperatures can vary widely by species and growth stage. But as a general rule young active growth and flowers are most susceptible. Other factors affecting frost sensitivity of fruit crops are general health, nutritional imbalances and water content of plant tissues or soil.
Reducing frost impacts.
Remove vegetation
Removing or minimising cover crops leading up to the spring danger period is beneficial. Vegetation reduces heat entry to and loss from the soil. Standing vegetation absorbs energy from the sun and mulch blankets the soil. Removing vegetation from the soil surface allows the sun to penetrate and warm a larger volume of soil allowing it to radiate heat during the night. Moist soils improve the ability of the soil to store daytime heat for release at night.
Irrigation
If water is available it can be a source of heat in the orchard. In scientific terms, it has a mass specific heat capacity of around 1,000 calories/kg for every degree in temperature. Irrigation water can be 7°C. Overhead irrigation systems can alter temperatures by six degrees Celsius. Risks with this approach include limb breakage from ice formation, water logging of soils and unavailability of water. Drip irrigation does little during the frost event and is better utilised to improve the conductivity of heat by ensuring a moist soil. Pulsing under tree micro jets can be used to protect crops by releasing the heat stored in water, but it is important to note that a complex relationship between temperature and humidity means that initial short term air temperature response can be a cooling one.
Frost Fans
Although not really a low cost option, frost fans are becoming increasingly common. Stationary frost fans work by mixing air within the first ten or fifteen metres of the surface for a higher average temperature near the ground. They may protect from 4-6 Ha. Helicopters can also be used and work on the same principles as frost fans by mixing warmer air from above the inversion layer with the colder air closer to the surface. The effectiveness of these techniques depends heavily on the individual situation and the presence of an inversion layer.
Frost warnings
Frost warnings are of limited use unless growers have prepared in advance with increased soil moisture, irrigation systems or frost fan installations, very little can be done to protect crops with a few hours notice. Farm specific warning devices may be more effective than area wide warnings as variations in temperatures can be quite localised. Some growers have automated irrigation systems that are set to turn on automatically.
Chemical Frost Protection
There is little independent scientific research data on chemical frost protectants and anecdotal evidence fails to consistently demonstrate adequate protection. There is however some data suggesting that improved tree health can influence plant sensitivity to frost.
Orchard Hygiene
Damage can occur to leaves from ice forming on the surface and growing in via stomata, but laboratory results indicate ice won't form without a nucleating device such as bacteria until temperatures reach – 40°C. Bacteria that cause bacterial canker or blossom blast can be the source of ice nucleation as well as being common in Victorian orchards. Paying careful attention to orchard hygiene principles may be advantageous.
Barriers
Careful examination of the farm lay out and monitoring temperatures will indicate potential danger zones. Hopefully these zones are not planted with early varieties. Barriers can be an effective means to protect crops from cold air invasion. They can be constructed from plants, shade cloth or solid fences. Bales of straw are often a fast alternative. Take care not to block air, just steer it past your danger zones, but not into your neighbours’ crop.
For more information visit www.dpi.vic.gov.au/fruitcheque or call DPI Swan Hill on 0350331290
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