Frosty starts as Marlborough's winter winds back the years

A spate of August frosts in Marlborough is a chilly wake-up call for many, but vineyards are welcoming the cool change.  

After Marlborough's unusually warm and wet July, the first 18 days of this month saw eight ground frosts and a chilly average temperature of 8.1 degrees Celsius.

Vineyard workers, such as Iosefo Kenner, have been braving the cold this month as temperatures drop below zero.
Thornhill
Vineyard workers, such as Iosefo Kenner, have been braving the cold this month as temperatures drop below zero.

Weather experts say frosts have become increasingly rare as Marlborough winters warm up, so the recent spate is a throwback.

Wine Marlborough general manager Marcus Pickins said the cold mornings were no cause for alarm in the wine industry, and many growers looked forward to a decent frost.  

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"It's probably a beneficial effect if anything," Pickins said. 

Upton Gardens co-owner Sue Monahan thought it was the hardest frost she'd seen all winter, snapping this photo on Sunday morning.
Sue Monahan
Upton Gardens co-owner Sue Monahan thought it was the hardest frost she'd seen all winter, snapping this photo on Sunday morning.

"It's good to get a couple of good hard winter frosts in, it kills off any pathogens, diseases or microbes leftover from previous growing seasons ... it's probably quite good for any creepy crawlies that aren't desirable."     

He said the frost would not affect budding in the vineyards, which would not begin for another five or six weeks. 

While they may be good for the vines, Thornhill Viticulture and Horticulture Labour Specialists South Island manager Alistair Mitchell said the frosty mornings could be tough on workers. 

Blenheim resident Hannah Lamb snapped a frosty start on Saturday morning at Waikakaho Valley, where it was 2C at 8am.
Hannah Lamb
Blenheim resident Hannah Lamb snapped a frosty start on Saturday morning at Waikakaho Valley, where it was 2C at 8am.

"With the frost early in the mornings it's quite difficult for the workers - it's cold, their hands are cold, it's not ideal conditions," Mitchell said.   

"But always following a frost is a beautiful day, so everyone waits for it to warm up."

Plant & Food research scientist Rob Agnew said August's heavy frosts had been noticeable after a warmer than average July.  

"The contrast has been between July and August because July was so warm - July's mean temperature was 9.7C and that was 1.8C above average."

"It's almost like July and August have been switched around temperature-wise."

Honor McKenzie, 6, and Chloe Dougall, 5, at netball on Saturday.
SCOTT HAMMOND/STUFF
Honor McKenzie, 6, and Chloe Dougall, 5, at netball on Saturday.

On Sunday morning there was an air temperature in Marlborough of minus 1.2C and a grass temperature of -4.7C, producing a heavy ground frost. Monday morning was slightly milder with an air temperature of 0.1C and grass of -4C. 

The heaviest frost was on August 6, with a ground temperature of -5.8C. 

A ground frost is measured 2.5 centimetres above a mown grass surface. The ground level is always colder than above, because after a sunny day the heat will escape from the soil and rise into the air. 

"When it's a really cool clear night, the temperatures closest to the ground is the coldest, hence any plants that are closest to the ground will suffer frost damage first," Agnew said.

Ground frosts were classified as anything below -1C, Agnew said. 

Blenheim man Kore Lawrence photographed Arapawa Island overlooking snake point early on Monday morning.
Kore Lawrence
Blenheim man Kore Lawrence photographed Arapawa Island overlooking snake point early on Monday morning.

Agnew said the frosts had been more noticeable because they followed a warmer July, but historically would have been far more common. 

"People who are in their 40s, 50s, 60s and older who were brought up in places like Blenheim, Christchurch would remember walking or biking to school, and it was very common to walk or bike through puddles and break the ice in winter. 

"It's a pretty rare morning in Blenheim now when there's frozen puddles, so that's just an indication of our winters getting warmer." 

There were 9.2 millimetres of rain over the weekend, bringing this month's total up to 37.8mm. The August average was 62mm.

Agnew said we could expect more rain later in the week. 

Blenheim resident Kore Lawrence, who was enjoying the view of Arapawa island in the Queen Charlotte Sound said the "polar blasts [were] definitely making their presence felt."

Hannah Lamb also snapped a frosty start on Saturday morning, when she was checking the cows at her family farm at Waikakaho Valley, where it was 2C at 8am. 

The Marlborough Express