Each winter, before it gets too muddy, we load up the green frost fans onto our Bobcat, a somewhat precarious proposition, and then schlep them into place in the vineyard. There they sit through the winter, squatting silent in the rows, waiting for the chance to do their job come spring.
During dormancy, grapevines are amazingly frost tolerant, so much so that Spring frosts can come in one of two types. The most common is the radiative frost: on calm nights, the ground cools more quickly than the air above, trapping the cold air and creating an inversion layer. Like water, this cold air collects in low spots. The “top” of the inversion layer is usually around 20 feet off the ground, hence the propellers on tall poles you see in most vineyards. The idea is to break up the inversion by grabbing the warmer air above and mixing it with the colder air below. This will warm the vines only a degree or two, but it’s just enough to keep the buds from freezing and save the crop. Sprinklers are another control option: when timed just right, wetting the vines down will cause them to stay warm since water gives off heat as it freezes. But California’s severe drought has certainly curtailed sprinkler use.
The second type of frost is advective. This is when a mass of cold air moves into a region ahead of, say, a winter storm with origins in the Arctic. There is not much to be done when one of these types of frosts occur since the air is uniformly cold throughout. This is why the damage was so extensive in 2008 – a weather system moved in and the ambient air temp was 25º F. The future of an entire vintage burnt to a crisp in a couple of cold mornings. Dramatically damaging, advective frosts are fortunately rare in our California growing region.
Much work has been done on the technology of protecting young grapevine buds during radiative frosts. First and foremost is making sure that the vineyard has been mowed. This allows the cold air to settle to the ground, gaining a degree or two of temperature at the height of the cordon. Sheep really help by grazing down our vineyard cover crops when it’s too wet to get a tractor through. Historical control measures included cultivating, since bare soil will warm more during the day than that growing cover crop. But cultivating wet soils destroys the structure and causes erosion issues, among other things, so I prefer to use the sheep. Other control measures that have fallen out of fashion include smudge pots – ends up that effective, low-hanging fog of particulate emissions from the diesel fuel burning in them also causes some major pollution, not to mention breathing difficulties.
Purchasing and installing frost control measures, like fans, has always been an expensive gamble. It’s a lot of money for something you don’t always use or that might not work, climate change making it more so. The terrible frost year of 2008 was followed by two or three seasons when we barely turned the fans on. (Not necessarily a bad thing.) This type of risk has kept us from purchasing any more frost protection in the ensuing years – the fans we have work and where we might need more, we’ve never lost enough fruit to make it worth the large price tag.
This year has been no exception. The bone-dry winter months meant we started this season with a ridiculous amount of degree days. Bud break was early, the new growth at greater risk since frost is more likely earlier in the spring when the days are shorter. But the warm weather also meant warmer mornings, so we only turned on the fans for a week or so in March. Though we had some mild damage where we don’t have any frost fans, it wasn’t a bad frost year all in all.
So, one more year down, frost season survived and on to bloom and crop estimates and seeing what the harvest will bring.