Frost Forecast – What Now?

Friday, May 19, 2023

With the potential for spring frost not yet over and some cultivars showing signs of growth, the following is a chart outlining hardiness of different stages of growth for grapes.  This data has been accumulated form observations across Canada and the eastern US.

Growth stage

Critical Temperatures

Suggested temperatures for start-up of wind machines

Dormant bud

Minus 5 C

Minus 2 to Minus 3C

Dormant swollen bud

Minus 3 C

Minus 1 to 0 C

Bud Burst

Minus 2 C

0 C to Plus 1 C

One leaf unfolded

Minus 1.5 C

Plus 1 C to Plus 2 C

Two or more leaves unfolded

Minus 1 to 0 C

Plus 1 C to Plus 2 C

Information below outlines some possible steps to deal with frost and pokes some holes in the myth of burning things in or near the vineyard for frost control!

Mowing cover crops down to 3-4” tall (or less) can help cold air settle closer to the ground, further away from the tender shoots in the canopy, and also promote any air movement at that level as well to help mix warmer air with the cold. Some people may suggest applying products out there that they promote as helping to prevent frost injury to newly emerged shoots. There is no evidence that they actually have any kind of impact on reducing frost injury, so I wouldn’t recommend spending time and money on them.

Burning off bales of straw or hay or other products to warm the vineyard have not proven effective in Canada.  The concept of burning is not to warm but to create a dense cloud layer of smoke to trap warmth near the ground if present, it is not warming the air. Unless it is absolutely calm you are more likely just going to create smoke drift and smell versus helping. Secondly, if you start a ranging brush pile burning you can actually punch a hole through any potential inversion layer and actually draw more colder air down into your vineyard. The bottom line - smudge pots, burning bales or brush piles don’t really help much plus they can create a fire risk to nearby fields and bush areas with small trees and long dry overwintering grasses!

Contributed by: Dr. Kevin Ker