Damaging frosts typically occur after a cold, cloudy day followed by a still night with clear skies. The earth quickly radiates any stored heat after sunset and an ‘inversion’ layer occurs with the air being coldest near the ground with a warmer layer of air above it.
The degree of frost injury to kiwifruit depends on the minimum temperature reached, the rate at which the temperature falls and the amount of time below a critical temperature.
Critical temperature is defined by Young (1966) and Johnson and Howell (1981) as “the lowest temperature a bud, or shoot, can endure for 30 minutes or less without injury”.
However, it’s not all about avoiding catastrophic loss. The aim is also to protect against cold injury to increase a grower’s current yields and improve the quality of the fruit, while ensuring continuity of supply.
Frost fans are a well proven technology for mitigating the devastating losses that can occur from a significant frost event. The frost fans will start automatically to prevent damage from cold events that may only last a few hours in the middle of the night or early morning.
Frost Fans work by utilising warmer and drier air from the ‘inversion’ layer to create air movement at the fruiting or flowering height in orchards on still cold nights preventing damage to flowers, soft tissue and fruit.
Many growers have moved away from using water sprinkling in preference for frost fans. Their reasons include the amount and cost of water, maintenance before and during frosts, and increased risk of disease.
Anecdotal evidence from growers has shown while the use of frost fans can save a crop from total loss in a severe frost, they can play a significant role in improving the yields and quality of fruit that can slowly be eroded by multiple previously unrecognised cold events.
Frost fans are prevalent in New Zealand and Australia’s growing areas, with FrostBoss fans being used in vineyards and orchards managed by large corporates through to independently-owned, family-run businesses.