Frost protection for fruit crops Part 1 An integrated approach By Dr Terry Mabbett

Periods of low temperature at and below 0°C are responsible for frost and frost damage in fruit crops.

    Many areas designated frost free including some with sub tropical climates can be hit out of the blue by freak cold ‘snaps’. These may cause complete loss of a season’s crop and sometimes longer-lasting damage or even mortality of the trees themselves. Many important citrus growing areas of the world experience such problems.

    Top fruit and stone fruit crops in temperate regions regularly receive and cope with a few degrees of frost. But they can be hit by prolonged periods of extreme low temperature with which traditional frost protection measures may be unable to cope.

Crop differences

The earlier flowering stone fruit crops like plums, damsons and peaches are generally more vulnerable than later flowering stone fruit like cherries and top fruit (apples and pears) simply because they are more likely to experience frost.

    Late frost is often thought to be the most damaging to fruit trees but frost timing per se is less important than how it affects specific crops at their most susceptible stages of bud and blossom growth and development.

    Fruit trees and buds in the dormant winter condition have a high degree of natural and inherent frost protection, but this all changes once bud swelling and bud burst gets underway.

   The temperature at which fruit buds sustain frost injury primarily depends on stage of development. As flowers begin to swell and expand from buds into full blossom they become more susceptible to frost injury, and critical temperatures at which frost injury occurs will rise accordingly.

    For instance it takes temperatures as low as -17°C to cause 90% kill of apple buds at the ‘silver tip’ stage. But as apple buds swell, break and develop through green tip, tight cluster and first bloom stages the same level of damage (90% kill) is sustained at progressively higher temperatures of –12°C, – 6°C and – 4°C.  Peaches show the same gradation of temperature for 90% kill [first swelling (-17°C), green calyx (-15°C), first pink (-9°C) and first bloom (-6°C)].

   There are clearly differences in resistance to freeze injury between different crops and varieties, but also within and between trees in the same orchard. For instance, buds that develop more slowly tend to be more resistant to freezing injury, so position in the orchard or even on the tree may have an effect. Growers should give due consideration to conditions preceding as well as during spells of cold night weather, because prolonged cool weather hardens bud resistance to frost during the early stages of development.

    Two types of frost situation may occur. The first is called an “advection frost” that occurs when freezing air is blown into the orchard to displace the warmer air that was present prior to frost occurrence. Advection frosts are characterised by moderate-to-strong winds, no temperature inversion, cloudy or clear skies, and freezing temperatures even during daylight hours.

    The second and most common frost situation is called “radiation frost”, caused by heat rapidly lost by radiation from the earth and crop at night into a clear sky. Radiation frosts are characterised by light winds, temperature inversions, clear skies and daytime temperatures above freezing point (0°C).

An integrated approach

Frost protection like pest control should be an integrated programme combining every benefit from all available measures, and should not rely on single-measure strategies. Frost protection measures are designed to minimise exposure of crop plants to frost or enhance resistance to low temperature effects on organs, tissues and cells and therefore the physiology, growth and reproduction of the crop plant.

    Frost protection measures are usually defined as ‘passive’ or ‘active’. Passive frost protection is generally taken to describe good management practice that will:

• Minimise the occurrence of temperatures at or around the buds and blossoms that are sufficiently low to cause tissue damage or
• Maximise resistance and tolerance of fruit buds to frost damage.

    Active frost protection measures are those taken specifically to create or enhance protection against an impending frost situation. But there are grey areas surrounding use of such definitions.

    The planting of cold susceptible crops away from frost prone areas or the use of fruit varieties with enhanced frost resistance is clearly a passive measure. Use of covers at night is less clear and will depend on whether they are put in place as a matter of course or in response to specific information relating to an impending frost situation. Use of wind machines or sprinklers triggered by frost alarms is indisputably an active measure.  Strictly speaking ‘Active Frost Protection’ should be reserved to describe decision-led measures operated in response to frost warnings or triggered by sensor-activated frost alarms.

    Other grey areas may arise when passive measures like planting position are used. For instance, general recommendation is that frost susceptible trees or varieties be planted on the warmer west or south facing slopes. But other measures aimed at delayed blooming and therefore less risk of encountering a frost situation may recommend planting on a north-facing slope.     Similarly, soil surface management including the elimination of any ground cover plants is widely recommended especially when trees are bare of leaves. But when trees come into the leaf and the coldest point in the crop profile shifts from the soil surface to the top of the canopy, soil and ground cover management becomes less important