The Bureau of Meteorology said yesterday's low of -5.7 degrees Celsius at Renmark in the state's Riverland was the equal-second coldest temperature on record for the town.

Meteorologist Matthew Bass said it was the region's sixth consecutive day below zero.

"Those sort of temperatures are not what we see very frequently in South Australia ... it's a very significant, severe low temperature."
Frost form when the dewpoint temperature has dropped below zero and the air is condensed to form ice crystals which then land on a surface.

Each year, frosts destroy millions of dollars worth of crops in Australia.

"If it's a very dry air mass, sometimes the temperature can drop below zero without that white frost forming, but obviously that can still do damage to plants and things that freeze," Mr Bass said.

Farmers fear impact of frost

Citrus grower Mark Doecke has had four consecutive frosts at his 30-hectare property at Sunlands about 200 kilometres north-east of Adelaide.

He has delayed picking his fruit because of the frost.

"You might have a frost or two, that's okay, but when you get four in a row and not very warm days it sort of accumulates the affect," he said.

It can damage the skin on citrus or it can freeze cells inside the orange so then it dries out when it thaws out—one's cosmetic and one's internal quality.

"When it comes to selling your fruit you'll have a dry patch on top of the orange, so you cut your nice looking orange up and it's got this big dry bit in the middle of it."

Fighting frost a sweet job
There are strategies to counteract the most severe damage from frost, including the use of frost fans and keeping the soil moist, but Mr Doecke is trying a different method.

He has turned to using the thick, sugary syrup molasses.

"Virtually molasses is just sugar product anyway so you coat the whole tree with sugar and it reduces the freezing point of the leaf and the fruit.

Nervous wait for avocado growers
At nearby Ramco on the banks of the Murray River, avocado growers were waiting anxiously to see what, if any, damage the frosts would have.

Kym Thiel, a grower and Avocados Australia tri-state director, said the tropical fruit could be severely impacted by frost.

"Avocados do hate frost, there's no doubt about that. 

"At the moment we do have a very nice crop hanging out there so it is a nervous time of year for growers to get through this frosty period and come out the other side, hopefully with some fruit hanging there."

He said any damage to the crop would usually be evident within weeks, but in some cases it could be much sooner.

"In severe cases or isolated patches where there are low-lying trees you certainly see some leaf damage that afternoon or the next day."

The Bureau of Meteorology said there was likely to be a reprieve from frosts for the rest of the week, but that was little consolation this early in the season.

"We've got two more months of winter to go — it's likely that we will see some more runs of significant frost, but hopefully not as cold as we've seen."