Cover Crop Archives - Tzabaco Rancho Vineyards

Passive Frost Protection

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!

Timeline:

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.

Weather Watching

As we are in the middle of bud break, we are also watching the weather for several important reasons.

Frost – If it freezes for long enough in our cozy section of Dry Creek Valley (locally known as the Banana Belt), shoots and clusters will wilt and die. We rely on passive frost protection which worked for 50 years until the Merlot was frozen in Spring 2008. Pretty good odds so we continue to rely on these methods.
Cover crop management – Passive frost protection is based on the exposure of soil to the warming rays of the sun. But not just any soil – the best is dark, wet, firm, exposed soil that can absorb that heat and radiate it back at night. Keeping our cover crop mowed low is crucial. But we do need to keep the cover crop viable to help dry out the soil as the rains recede. We will eventually make mow vs disk decisions depending on rainfall in the next month and the need for competition within each individual block of grapes.
Mildew pressure – We need to exit this weather pattern to cool, windy days to help remove humidity. Mildew spores are everywhere and just need the right combination of warmth and humidity to grow. We started the season with dormant sulfur sprays and will continue to fight this ever present threat this Spring with Stylet oil – a great product approved for use in organic vineyards.

It can sometimes seem like a juggling act with decisions needed for each varietal block but we have been through this many times before and, while we can’t control the weather, we feel confident we can deal with most issues, with frost excluded!