Comments on Types of Spring Cold Events in North Carolina (Friday, Jan. 27, 2017)
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I am actually going to be speaking to the NC Muscadine Grape Association tomorrow morning (1/28/17) in Elizabethtown, NC, and I am posting here a paper I wrote for HortScience in October 2008, Spring Cold Injury to Winegrapes an Protection Strategies and Methods:
What is particularly interesting to all fruit growers about this paper is the introductory section where I discuss the importance of having a clear understanding of what exactly do we mean by the words frost and freeze, as these two terms are often used interchangeably! But, from a cold management standpoint, it is very important to refer to frosts and freezes as distinctly different phenomena!
Essentially, a FROST is caused by radiational cooling and occurs under clear skies with calm wind conditions. The calm winds can allow what is called an inversion to develop with the coldest temperatures right at the ground level (where a strawberry plant is located), and this is why in “Strawberry World” we are so very focused on weather forecasts such as the one provided by SkyBit of what the minimum temperature may be at ground level vs. the temperature at the weather shelter height of 2 meters (~6’6″). Often, the ground level strawberry canopy forecast minimum can be significantly LOWER than the temperature forecast for 2 meters. Just take a look at the SkyBit for Central Crops Research Station tomorrow morning (Fig. 1), and note that the minimum temperature in the strawberry canopy (ground level) could be as low as 26 F, but at the weather shelter height (2 m), it will likely be 4 F warmer, or 30 F — just look at “Min Air Temp” for local area outside the field (this is weather shelter height temperature).
Lets assume it’s the time of year when muscadines are breaking buds and strawberries may be in bloom, I would not be that concerned about a minimum temperature forecast of 30 F because the muscadine cordon is actually right up around 5-6′ and newly emerging grape shoots should be able to “handle” a low of 30 F. But, if you are a strawberry grower, your crop could be “toast” if you don’t “do something” to keep those strawberry blossoms from reaching a minimum of 26 F. Its a dead certainty that you will lose open strawberry blossoms to a minimum of 26 F.
If you examine the very bottom table (Hourly Forecast) you will note that winds will be very calm tomorrow morning in Clayton NC, and may only be 1-2 mph. So, unless things change rather dramatically from this morning’s SkyBit forecast at 7 a.m., and this evenings forecast at 7 p.m. (I will be driving to Elizabethtown this evening from Williamsburg, VA, so I may not see that update), I would anticipate a potentially damaging radiational frost tomorrow morning at Clayton for strawberries, but not muscadines.
FREEZE EVENT – a very very important distinction between a frost and freeze event has to do with winds. When a grower receives a weather advisory or warning from the NWS, it is very helpful to know that a freeze event may include a significant amount of wind. And, the issuance of a freeze warning (with sub-freezing temperatures and winds exceeding 10 mph), also TELLS the fruit grower that there is going to be almost NO OPPORTUNITY to apply cold protection measures (like sprinkling). Of course, in strawberries it is possible to throw on a row cover and pretty much neutralize the wind effect and I have seen strawberry crops in full bloom survive freeze events, such as the Easter Freeze of 2007, by applying a row cover and irrigating continuously on top of the cover through the entire event. But, to my knowledge, applying row covers is not a common practice in commercial vineyards in North Carolina!
The good news for muscadine grape growers in North Carolina is that odds of encountering a serious windborne freeze during the budburst period (late March/early April), are very very low. But, because strawberries can bloom “so darn early,” strawberry growers must always be ready to deal with this sort event, and about the only way to manage it is to use SPRINKLING + ROW COVERS. When I got started at NC State back in the 1980s, we simply did not know what to do in the event of true windborne freeze during strawberry bloom. Thankfully, due to some fine work by John Earp, my former research technician in horticulture, and Wallace Baker, former superintendent at Central Crops, we came up with the sprinkling and row cover idea in the early 1990s that is effective for protecting strawberry blooms into the low teens.
FROST/FREEZE. A third type of meteorological phenomenon that I wish to mention here before I begin my travel down to the grape meeting in Elizabethtown tomorrow, is the term frost/freeze. As defined by the NWS, a frost/freeze event combines characteristics of both a radiational frost as well as advection (heat loss by wind). Of significance to fruit growers is that there may be potential to modify environmental conditions in an orchard, vineyard, or small fruit planting (e.g. blueberries, blackberries and strawberries) with active protection methods that are UNAFFECTED by winds greater than 5 mph, but less than 10 mph. In the mountains of NC, there are numerous tree fruit and vineyard sites that have BOTH radiation frost problems in the spring after budbreak as well as difficulties with frost/freeze events. For these locations, overhead sprinkler irrigation may be worth looking into. Crop losses from a frost/freeze event can be quite extensive because these subfreezing events are typically very long in duration (often greater than 10 hours), and can occur over 2-3 days in a row!
In speaking yesterday with an irrigation engineer from California, Michael Illia, A.E., Senior Product & Solutions Manager, Netafim USA, I learned of a sprinkling system that could be of some possible interest to muscadine growers who have experienced costly losses to radiation frost events in recent years, and this type of system could be effective in frost/freeze events as well.
I asked Michael to simply list out all the components that would be needed for a 10 acre vineyard system, including mainlines, laterals, sprinklers, sprinkler connections, etc. I told him to assume a Carlos muscadine vineyard spacing of 10 feet across and 20 ft in the row. And, he recommended a 40 x 40 spacing of the irrigation nozzles. The particular nozzle needed for this set-up is what they call the “gray” nozzle that does 2.2 gpm. We also assumed the vineyard would need 60 gpm/acre, or 600 gpm/10 acres. What is of particular interest about this system is that the main lines and laterals roll out! These are called “Flexible Pipes” that are for popular in seasonal crops in California. You can actually view these roll out pipes at this link:
This is a technical sheet that I will be sharing from Michael at the Muscadine meeting tomorrow: Background Notes for Muscadine Meeting Talk – 10 acre vineyard.
I am looking forward to a good discussion tomorrow at the Muscadine meeting about the possibilities of using sprinkler irrigation for both frost and frost/freeze management. I am already aware of at least one muscadine vineyard manager who has been closely studying sprinkler systems used in blueberries, and given the fact that a single frost event last April 6th (2016) caused a 3 ton reduction/acre in yields of mature Carlos vines, this could be something that is really worth looking into!