Frost protection is in a whirl

Press, 3 February 1984, Page 22

Frost protection is in a whirl

Late-season frosts can cause severe damage to horticultural crops, but Marlborough growers can look to the heavens when frosts are imminent.

Growers and researchers have worked out a method of frost protection using a helicopter flying above the endangered crops.

The down draught of the helicopter forces warm air

from an inversion layer down to ground level, resulting in a lift in temperature of the air around the crops.

The effect is similar to wind machines, which are used by many orchardists in Hawke’s Bay and one in Christchurch.

Trials using a helicopter have been under way since 1977 and researchers are confident they have now worked out an efficient system.

But Mr Don Cross, a farm advisory officer (agricultural engineering) with the Ministry of Agriculture and Fisheries at Blenheim, said there had only been one frost during the critical period for fruit this season, and the system had not been finally tested. The previous season there had been 17 frosts during the same period.

Extensive trials carrier out by the M.A.F., the Meteorological Service and local orchardists showed that temperature rises of between 2.sdeg and 4deg could be achieved by a single pass by a helicopter over the area.

However, the temperatures returned almost to their previous levels two to three minutes after reaching a maximum.

Temperature rises from a helicopter flying a pattern over the immediate area last longer than for single passes. A helicopter passing over an area at five minute intervals and in a swath of 50 metres can lift the temperature by about 2deg. The cost of using a helicopter is about $5OO an hour. A helicopter can operate effectively over 30ha an hour which gives a cost of about $l6 per hectare per hour, compared with an hourly burning rate for oil of about $4OO a hectare.

A thermolite, comprising a light which flashes and a horn which sounds when the temperature reaches Odeg, has been used in experiments. When the temperature climbs above freezing, the light and the horn switch off.

Mr Cross envisages thermolites being placed in the coldest parts of orchards and in positions where they would be visible from a helicopter. When the thermolites are activated, a helicopter could fly above the area, extinguishing the lights and returning each time the temperatures return to a danger level.

One of the best known Marlborough stone fruits is the sweet cherry which has a very short growing period — 8 to 10 weeks from flowering to harvest. The

critical period for the cherry occurs after the outer calix is dropped, leaving an exposed fruitlet. A frost severe enough to affect the exposed fruitlet is recognised as a problem which could occur once every five to seven years in the centre of the Wairau Plain. At Rapaura this can be once in three to four years.

Other stone fruits such as apricots, plums, peaches and nectarines are more vulnerable, flowering three to four weeks earlier than cherries. Mr Cross is also working on undertree systems which can also be used for irrigation. He hopes to carry out further testing of this method of frost protection next season.