Frost…and why site selection is so important!

Misha looks at the recent frosts and why frost drained site selection is such an important consideration for a vineyard.

New buds and leaves are very sensitive to frosts.
On Friday 12th October, the Central Otago region received a frost forecast – two cold fronts were coming from the south and were due to spread across the country and to be slow moving for several days. We already had multiple snow and sleet showers in various parts of the region during the day. With cloud due to clear during the night, frost was forecast from midnight onwards and we were warned “frosts may be severe in sheltered places about Cromwell and Arrowtown”. With very low dew points recorded at midday on Friday, it was an early indication that temperatures could fall quickly under clearing skies and it was predicted this could be down to minus 4C. (Note; Low dew points mean that the air can heat and cool quickly due to the lack of water vapour in the atmosphere. This was a big factor in determining the forecast of the minimum temperatures).

News of this type gets the region in full frost-fighting mode especially when the forecasts indicate an 80% likelihood of the forecast being correct and so early in the growing season! All forms of frost-fighting equipment ie wind machines, various types of water sprinklers, and even frost pots were made ready across the region. Many of the bigger and more frost prone vineyards alerted helicopters to be on standby for the night.

The natural slopes of the vineyard drain the cold air down to the lake
At Misha’s Vineyard, we did nothing. We have no frost fighting procedure as we’ve never needed any to date. And with our vineyard’s slopes and location, it’s impractical for any sort of frost fighting across the vineyard. But we did choose a site that was naturally frost drained but we’ve learned with weather, “never say never”. Just because we haven’t had any real frost issues to date, doesn’t mean that will always be the case.

I must admit I didn’t sleep much that night. We could feel the temperature dropping as the cloud cover cleared. We don’t live on the vineyard but we’re not far away. The night was eerily still with absolutely no wind – all we could hear was the faint whir of wind machines and helicopters in the distance. We just crossed our fingers and hoped the vineyard would be OK.

Next morning, Aaron, our tractor driver who lives on-site at the vineyard, came into our Tasting Room early and reported that he had been up a lot of the night too, as he felt the temperatures drop and figured it was about 3.30am that our vineyard went below 0 C. He said that directly across Lake Dunstan from the vineyard, it looked and sounded like a war zone. Where it’s usually pitch black, he saw the flashing red lights of helicopters as they hovered above many of the vineyards hoping to stir up any inversion layer and various lights of vehicles moving around the landscape no doubt with worried vineyard managers trying to protect the vineyards. And he could definitely hear wind machines whirring at full speed as they tried to move the cold air around. This was a critical time for vines with bud burst just a few weeks back and now the vines with their first few delicate leaves.

Our trees at home had some impact from the frosts!
On the Saturday morning it was too early to really see the impact of the frost on the vineyard as it takes a couple of days for the initial signs of any impact to show. I walked around our garden at home and saw the impact of the frost on some of our plants and trees. I was feeling even more nervous about how our vineyard had fared. We heard reports during the day on the Saturday that some vineyards who fought the frost with water had to keep pouring water on until 11.30 in the morning as it kept freezing since it was very slow to warm up that day.

By the Monday, Tim, our Vineyard Manager, after two detailed post-frost inspections, gave us his report. He admitted he had been worried as the Bendigo sub-region certainly was hit hard by the frost, as were all the other sub-regions. Often some of the higher blocks at Bendigo manage to escape some of the frosts but it wasn’t the case this time.

Thankfully, the good news we’d hoped for was that most of our blocks at Misha’s Vineyard escaped damage pretty well with just some scarring around the edges of leaves on a few vines and the odd dead shoot randomly. Our Dress Circle where we grow our Pinot Gris showed some minor impact, as well as some of our Gewurztraminer in the Fruit Bowl block. However there are plenty of buds and shoots that are healthy to replace the few that are damaged – so it just means we don’t need to do as much shoot thinning on those vines. Our Pinot Noir across the vineyard came through well with very minor damage considering we had close to -2C for several hours. Our Riesling and Sauvignon Blanc on the steeper slopes were all absolutely fine. Overall Tim was confident there was nothing that would result in any reduction in yields.

Tim and Andy discussing the pruning that provided some replacement shoots in case of frost
We have now had over a week of warm weather and Tim has given the vines plenty of extra help in their recovery with some nutrients ie nitrogen and seaweed based. He’s very happy with how the vineyard has come through. We have since discovered, not all vineyards came through as well as ours did. It seems with poor inversion layers, the helicopters and wind machines only provided marginal benefit and even the frost fighting systems using water (ie flippers and sprinklers) had some problems because the protective ‘ice blankets’ that the water puts around the vines, stayed on for so long and it was hard to get the ice to melt. I remember our first viticultural consultant, Robin Dicey, always telling us frost damage is caused by degree and dose – basically it’s a combination of how cold it is, as well as long it stays that cold. Some reports around the region were that temperatures went down below minus 5C. From what we understand, all vineyards would have felt some effects of the frost but the percentage of the vineyard damaged will range from single digits to a large percentages and things are still being assessed as it takes longer to see damage on green tissue on the vines.

We have been reading a lot about frost in the past few days and New Zealand Winegrowers, our national body, has posted information about frost as many regions across New Zealand have experience frost over the last week. Notes from a Romeo Bragatao Conference workshop on frost from back in 2003 were re-posted and it has been interesting to read these and understand the impact of frost on vine development and to understand how to be better prepared for future frosts. We smiled when reading the last line in these notes : “Appropriate site selection is still considered the best means of frost protection”.

Andy and l looked for two years for a “frost free” site on which to establish Misha’s Vineyard. Our west-facing, steep slopes with plenty of rock to soak up afternoon sun, and a location on the west side of Lake Dunstan with the Dunstan Mountain range which isn’t as snow prone as the Pisa Range on the other side, are all things that keep us as safe as we can be. We don’t have wind machines on the vineyard (as our multi-level steep slopes aren’t suitable) and power lines would impact the ability for helicopters to fly around trying to find an inversion layer to push down. All I can say is ‘Phew!’. Thank goodness we spent the time on site selection” especially when we’re ‘on the edge’ in terms of grape growing!