Buds snug in ice rugs
Every time I see a photograph of a budding grapevine covered in icicles, it amazes me that the buds are not frozen solid and the coming year's grape harvest has not been destroyed before it even gets started. But the fact is the ice forms a little protective coating around the bud and stops this from happening.
How does it do that? Ice is cold.
For the answer, I went to Dr Glen Creasy, senior lecturer in viticulture at Lincoln University.
There are several ways to prevent frost damage – for instance, vineyard heaters that warm the freezing air before it has a chance to settle and turn to ice, or helicopters and frost fans that circulate the air, mixing the cold air close to the ground with warmer air sitting in a layer some metres up.
The danger here is that if it is a prolonged frost, there is a danger of running out of warmer air to mix. Then the buds freeze, anyway. And then there is the noise problem associated with this method.
I spoke to a Waipara resident who lives several kilometres away from a field of frost fans, and her comment was: "It's not too bad for us, but I pity the poor buggers who live nearby. They will be really suffering."
The safest frost-damage prevention method is to use water. The important requirement is a good water storage system, just in case there is a series of frosts to be overcome.
So when you see icicle-covered vines, what is happening?
When the air temperature drops to 0.3°C, water sprinklers are set to automatically turn on, and they stay on until the air temperature rises to 2°C.
The way it works is a bit like the reverse of a jug boiling. Add heat to water and it releases energy to become steam, but in this case, water turning to ice releases energy in the form of heat.
It sounds totally bizarre, but Creasy assures me that as long as water continues to flow over the vine, the icicles will continue to freeze and release energy. The temperature will always remain at 0C, and will not damage the crop. Stop applying water, and the ice would cool to minus 3°C and damage would occur.
If you don't believe it, try this simple test. Put a pot of cold water on the stove and fill it with ice cubes, turn on the heat and take the water temperature. It will remain at 0°C until all the ice has melted.
Fortunately, there has been little need for any frost protection in the region so far with mild temperatures pointing (optimistically) to a bumper season.
With September warmer than average, there is already plenty of healthy-looking growth on the vines, and hot weather equates to happy plants and lots of growing activity.
Creasy measures the season's fluctuating temperatures in growing degree days (GDD). Temperatures are taken daily over the entire growing season, and every time the temperature rises 1°C above 10°C, it is awarded a GDD. For instance, a 15°C day would earn 5 GDDs.
This becomes particularly important to anyone wanting to set up a vineyard. Knowing how much quality sunlight an area is going to get influences what grape varieties are planted.
Lincoln University's vineyard gets on average 950 GDDs, while another just down the road at Tai Tapu registers 1150 GDDs, and the cabernet sauvignon region of Hawke's Bay gets 1400 GDDs.
I was in Invercargill last week. On the outskirts of town was a large billboard and on it an image of local rugby identity and one of my first bosses in the liquor industry, Les McFadden. It was enough to drive me to drink, so . . . cheers, Les.
Wine of the week
Villa Maria Cellar Selection Merlot/ Cabernet Sauvignon 2006
From the Hawke's Bay Gimblett Gravels region, this is a great example of the wonderful marriage of two great grape varieties. The merlot provides soft, lush, ripe plumy flavours, while the cabernet gives it extra intensity and depth by adding flavours of spice, blackcurrants and is that black olives? There is a firm backbone of tannin, which suggests, although this wine is drinkable now, that it has the potential to improve further over the next few years. A deliciously full-flavoured wine.