I had a great email conversation with our county viticultural advisor, Rhonda Smith, this week about passive frost protection. She is a co-author on a narrated slide presentation called “Passive Frost Protection“. The University of California Cooperative Extension also has several other slide shows on Active Frost Protection, but I was more interested in the passive methods as we live in the Dry Creek Valley “banana belt” and have never used active frost protection methods.
Due to the frost losses that we, along with so many others, experienced in 2008 due to freaky La Nina related Spring frosts, I thought I would revisit this topic as we are in another La Nina weather pattern.
The most intriguing aspect of freezing events is the way air flows from higher to lower areas just like a stream of water. This river of air will flow down hills into valleys, moving around slightly higher valley floor areas, pooling in lower valley floor basins and stopping if impeded by a row of trees, a raised berm, levee or fence. (Image courtesy of Mercy Olmstead, PhD, University of Florida-IFAS)
So, what can one do passively to decrease the chance of freezing injury?
1) Mow the cover crop as low as possible to allow sunlight to reach the ground and warm it during the day. That warmth will be released back into the air at night.
2) Provide as much undisturbed bare earth around the vines as possible. Do not cultivate or disc just before a potential frost. If cultivation is needed follow the disc with a roller then wet thoroughly. No sprinkler system? Then do this when rain is forecast so that the large air spaces that are formed during cultivation are reduced. No compaction (large air spaces) or dry soil decreases the ability of the soil to hold the warmth of the sun.
3) The most interesting thing that I learned this Spring is that 99% of ice nucleation between 23-32F is, in fact, facilitated by bacteria! One could possibly reduce the population of ice-nucleating bacteria before a freeze and reduce the formation of frost and accompanying frost damage by using copper or zinc. It is also possible to introduce a non-ice-nucleating bacteria to out compete the ice-nucleating bacteria. Cover crops are often reservoirs of ice-nucleating bacteria so mowing the cover crop and reducing the vegetation can be helpful.
I will update this blog entry as we move through April to see how our passive frost efforts work!
Week of April 3-9 – Possible cold weather forecasted for the end of the week so all cover crops are mown. Chard, Petite sirah and Petit verdot given their first fungicide application that includes the micronutrient zinc on Friday, the day before a significant frost is forecast.
Week of April 10-16 – a few days in the high 30’s but no frost here!
Week of April 17-23 – no frost forecast due to unsettled, cloudy weather.