Freezing Conditions During Spring Grapevine Deacclimation

For grape farmers, spring is an especially uncertain time when Mother Nature can be benevolent or violent. So far, after a mild February with temperatures in the high forties with a few fifty degree days, spring seemed to be just around the corner. However, with a bomb cyclone on March 2nd and a repeat performance on March 7th, Mother Nature is loudly proclaiming that we are still in the throes of winter.
As much as we would like to welcome warmer weather, we need these two months in March and April to prune our vines before budbreak, which in our region is typically around the last week in April in warm years, and the first week in May, in cooler years.
During the spring, while the vines are deacclimating and coming out of dormancy, they can be subject to freezing conditions. There are two types of freezing conditions known as radiation and advective freezes. The following table lists the conditions that occur under each of the freezing conditions.¹

Radiation Freeze	Advective Freeze
Radiation freeze occurs when the warm air which is normally closer to the ground is replaced by colder air, pushing the warm air above the cooler air mass (an inversion)	Advective freezes occur when a cold air mass moves into the area with subfreezing temperatures
Winds less than 5 mph	Winds higher than 5 mph
Clear sky	Sky may be cloudy
Cold air mass 30 to 200 feet thick	Cold air mass 450 to 3,000 feet thick
Inversion develops	No inversion
Cold air in the low spots
White or black frost damage
Easier to protect	Difficult to protect

There are a few methods to protect vines against spring freeze damage. These include:²

Passive Protection Methods	Active Protection Methods
Site selection	Heaters
Cultivar selection	Over-vine sprinkling
Cultural practices	Wind machines/helicopters

One of the more impressive photos of Active Protection Methods that I've seen is this from the Girl and the Grape and author, Alison Crowe kindly gave me permission to reproduce it here:³ She saw this scene when she was zipping along Hwy 121 between the towns of Sonoma and Napa, in Carneros.
It's so counterintuitive to imagine that sprinkling vines with water to create a coating of ice is an effective protection against freeze damage. This method works because when water goes from a liquid to solid state, turning into ice, there is a release of energy in the form of heat. As long as this coating of ice is kept wet, the temperature will remain at 32 degrees F and act as protection to the young buds. If the ice dries out, the water will begin to evaporate and the ice will get colder than the air temperature. Therefore, over-vine sprinkling must continue until the temperatures exceed 32 degrees F and the ice coating begins to melt. In addition, over-vine sprinkling will not work when winds are in excess of 10 mph and temperatures are lower than 24 degrees F.
While we as farmers have a few tricks up our sleeves to protect our crops, I'm always reminded of what our friend, Charles Massoud (the wise one) says:
“We are in a partnership with Mother Nature, but she is the senior partner.”
References:
1. Mark Longstroth, Michigan State University Extension, What is the difference between a frost and a freeze?, April 21, 2015.
2. Workshop Proceedings: Understanding and Preventing Freeze Damage in Vineyards, University of Missouri-Columbia, December 5-6, 2007.
3. Alison Crowe, Girl and the Grape.