There stands in our vineyard a powerful fan that most people assume is there for cooling. It’s not. It’s there for warming.
Should we get an overnight frost now that our buds have broken open, our strategy is to build a huge bonfire out of the piles of stumps we’ve accumulated in the vineyard and power up the fan to mix the warm air from the fire with the cold night air to hopefully boost the temperature enough to prevent the vines from freezing. A thirteen acre vineyard is a lot of ground to cover with just one bonfire and one fan, but our hope is it will be enough to keep the temperature from dropping to 29F, the point at which green tissue can be damaged.
We had to deploy our “stumps and fan” strategy twice last spring, on the nights of April 30th and May 1st. We were at least three weeks past bud break by that point and our vines were fully leafed out. We knew the forecast was calling for freezing temperatures overnight, so we set our alarm to go off every hour on the hour so we could check the weather. At 2am on both nights, the thermometer said 33, and we were out the door. The dogs had a blast. They thought we were camping! But for us humans, these were very tense nights. The temperature dropped by a full degree or more per hour! We knew we’d get dangerously close to the dreaded 29 degrees.
We took turns throughout the night driving the rows of the vineyard with a thermometer in hand to check the air temperature. The upper ten rows of the vines consistently were the coldest by two to three degrees. You always hear that it is darkest before dawn but in our vineyard I can tell you it’s always coldest right after dawn. As the sun came up we literally watched the frost form on the ground and spread like an oil spill through the coldest rows of vines.
It took two or three days for the damage to the vines to become apparent. And when it did, it was heartbreaking. The tips of all of our green shoots in the affected rows turned from green to grey and finally to black. Once the tip of a green shoot is damaged like that, the shoot can’t grow anymore and it will die along with the grapes it would have produced for our wine. About 15% of our vines suffered damage during those two frost nights last spring. But the rest of the vineyard did not frost. We like to believe it’s because we were up all night burning those stumps.
There are other strategies for fighting frost in the vineyard. Some grape growers use smudge pots that burn heating oil. They spread them throughout the vineyard and burn them like we do our bonfire to heat the night air. The advantage is there are lots of them, so you can spread them out. The disadvantage is there are lots of them, so you have to spread them out. And, we don’t like the idea of pots of heating oil possibly tipping over and getting into our soil. So we’re not doing that.
Some vineyards have an overhead sprinkler system for irrigation that can be turned on in cold weather to protect the vines by encasing them in ice. I know that sounds crazy, but if you think about it, water freezes at 32F and vines are damaged at 29F. So if the vines are inside a layer of ice, they are actually insulated from the colder air. Our irrigation system is a drip system that is low to the ground, so that won’t work for us.
But we do have one more trick we plan to try this year to protect the top rows in the vineyard. We noticed on those frosty mornings last year that the frost would creep into the vineyard from the higher ground of the horse pasture across the farm road from the vineyard fence and down into the vines. That showed us that the air flow pattern on our farm goes from the high ground to the low ground. So Bob had the idea to hang a plastic curtain along the vineyard fence when it frosts to stop the airflow. The idea is to block the cold air from the horse pasture from spilling into the vineyard to hopefully save the top ten rows from frostbite.
We don’t know if anyone has ever tried this before or if it will work but we could find out as soon as Tuesday night. The forecast calls for 33F overnight, and that’s cold enough for us to light the fire, crank up the fan and hang the curtain. Then, we’ll all spend a long cold, night around the bonfire, taking turns stoking the flames and driving the rows to check temperatures. We’ll also toast a few marshmallows and look at the stars. And I will spend at least some of the night in a huge dog pile on the ground with all of my dogs. Fighting frost is cold, exhausting work, but it’s also kind of fun. And it’s something fun to think about the next time you open a bottle of wine.