I thought I would do a little science lesson for this newsletter as we often get asked about frost protection using overhead irrigation. People are amazed when they see images of icicles hanging from our plants and can’t understand why they are not burnt to a crisp. Some species that have been frozen by the frost will show no damage even though they look frozen solid.
These images are from last year as the frost has been light so far this winter (as I frantically look for wood to knock on).
While less confusing than Stephen Hawking’s theories on gravitational singularities, it can still bend the brain a bit.
Although I’m sure there are some science-minded people out there that can explain it better than I can, the basic principal is that when water freezes it releases latent heat – about 1 gram of water releases 80 calories of heat as it freezes. The mixture of ice and water exposed to below freezing point remains at zero degrees until all the water is frozen.
Most plants do not suffer frost damage until the temperature drops slightly below zero degrees because the freezing point of the plant tissue liquid is below that of water. The irrigation continues to layer water over the plants making ice until the air temperature rises above zero degrees Celsius.
The biggest danger is applying water faster than any evaporating or stopping before the temperature rises above zero degrees. Evaporating water absorbs about 600 calories of heat per 1 gram of water which is why we sweat to keep cool.
Our irrigation system has temperature sensors that will start the irrigation cycle running as the temperature hits just about zero degrees Celsius. Each area runs the irrigation system for about 3 minutes – roughly every 30 minutes, laying water over any ice forming which leaves beautiful icicles hanging from the plants. At the heart of the system is our massive pump station which can deliver water at 40 litres per second at 750kpa of pressure (a car tyre has about 100-140kpa of pressure in it).
I hope you have enjoyed today’s science lesson.