Avoidance of frost damage
In some seasons, considerable damage to flowers and young fruitlets is caused by frost. Desiccating winds at the time of flowering also serve to kill pollen and inhibit the activity of pollinating insects. Measures that can be taken to reduce potential damage from frosts:
- Avoid sites at high altitudes or the tops of exposed hills where wind frosts are a likely occurrence.
- Ensure flows of cold air down slopes and out of orchards in times of radiation frosts.
- Do not plant windbreaks, which impede this flow.
Orchard management aids to reducing frost damage
Management of the soil beneath trees can help alleviate frost damage:
- Most heat loss during radiation frosts (50%-80%) is from the soil surface.
- To help in avoiding damage from radiation frosts, keep soil surface free of weeds and grass, keep soil compact and moist.
- Although the use of canopies or polythene tunnels is widespread on other crops it has not proved economic yet on apples.
Installation of a frost protection system
Several methods of providing frost protection to orchard trees have been studied. All of these are used with varying degrees of success in different parts of the world. Only systems based on water sprinkling are currently recommended in the UK. Two types of sprinkler irrigation have been used: under tree systems with micro jets or over tree systems using impact spray nozzles.
Under tree sprinklers
- Low level, under tree micro sprinklers can reduce frost damage.
- In calm (no wind) conditions applications of 2 mm water/hour to compact and previously moist soils can raise orchard temperatures by 1 or 2 degrees 2 metres above the soil surface.
- Micro sprinklers cause no limb breakage, which is common following extended use of over tree sprinkler systems.
Over tree sprinklers
- Over tree sprinkler systems, using impact type nozzles applying 2-3 mm of water per hour during frosts, can provide useful protection to the flowers.
- The advantages of the systems are that they have low running costs and can be used for irrigation purposes as well as frost protection.
- The disadvantages are the high installation costs, the potential for limb breakage due to ice loads on the tree and damage to the soil structure by the large amounts of water applied.
- Water volumes applied can be reduced by use of a ‘pulsed’ system or by better targeting of the sprays, such that using mini sprinkler nozzles that hit only tree canopies and not the space between.