At Eureka Nursery we have found that for plants in the open, the most practical method of frost protection that really works when carried out correctly is "Pulsed overhead irrigation".
What is it:-
Frost protection by pulsed irrigation involves the use of an irrigation controller to turn on overhead sprinklers for a set time duration at a set repeat interval, e.g. on for 20 seconds every 2 minutes. This must be started immediately the air temperature drops below zero and continued until it rises back above zero, which means that an irrigation controller with a frost sensor input is required.
Making it work:-
While this method is highly successful when carried out correctly, it often fails because of some common misconceptions and implementation problems. Here are some common reasons for failure:-
- It is often said that "watering the frost off" plants before the sun gets on them will prevent frost damage.
Keeping foliage moist or spraying with an anti transpiration preparation may reduce wilt from the sun or a drying breeze after a cold night and be of some limited use. However, real frost damage is done by the low air temperature freezing fluids in the plant, which means that the irrigation pulsing must be started as soon as the air temperature drops below zero and continued until it again rises above zero.
- Water freezes and blocks irrigation lines.
This is another reason that irrigation must be started as soon as the air temperature drops below zero. Likewise any water feeds to the irrigation must also be started at this time. As long as the water is moved at regular intervals in the the pipes it will not freeze.
- Sprinkler heads freeze.
Knocker sprinklers, particularly those with plastic knockers are prone to ice build up. Gear head sprinklers have never given this problem in any frosts we have experienced here.
- Irrigating too heavily.
It is a misconception that ice must be watered off plants for efficient frost protection. In actual fact it is the process of the ice forming on the plants that provides the thermal energy required to stop the temperature of the plant tissue falling below zero. This is known as latent heat of fussion and no amount of heavy irrigation can match this. Over irrigation can damage plants and just cause a quagmire, particularly with repetitive frosts night after night.
At Eureka Nursery we use Hunter MP3000 sprinklers at 8m square spacings in a 2 row opposing 180Deg. arc configuration and a pressure of 40psi at the sprinkler heads. This will give a worst case precipitation rate of roughly 20ml/Hr with continual irrigation. The irrigation controller is custom built and is set to start pulsing the sprinklers for 15 seconds at 2 minute intervals when the air temperature drops below zero. This interval will progressively reduce as the temperature falls, till it is pulsing at 15 seconds every minute when the temperature reaches -5Deg. C. giving an approximate average precipitation rate of just 5ml/Hr
This progressive reduction in the pulsing interval is not essential and most irrigation controllers with a frost sensor input will do an adequate job provided they are able to be set with the required intervals.
Sprinklers start pulsing as soon as the air temperature drops below zero. Progressive build up of ice on the foliage through the night is what actually protects the plants.
Only when air temperature rises above zero again do the sprinklers stop. This means they may be pulsing from well before midnight till well after the sun rises. When the sprinklers stop, the ice will simply thaw in the sun.
Younger plants may bend over and suffer some breakage due to weight of ice but damage is relatively minor.
More advanced plants handle the weight of the ice better.