Perfect Conditions for FROST on Tuesday Morning (5:30pm, Mon, 3/21/16)
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Fig. 1. It will definitely be cold enough for frost in most areas of NC tomorrow morning – with so much of the crop in tender bloom stage, this could be a serious cold event if growers don’t take precautions! A majority of growers have already applied row covers. But, some growers may be using sprinkler irrigation for frost protection tomorrow morning, and this can also be very effective. This advisory contains some review information about correct usage of sprinkling for a radiation frost.
Dear Agents, Agronomists and Growers,
Tonight’s conditions are perfect for a Radiational frost. This type of cold event is caused by longwave radiational losses of heat from the ground and solid objects (like a strawberry blossom) and occurs when clear skies and calm winds allow an inversion to develop and temperatures near the surface drop below 32 F. Also, dewpoint (DP) temperatures tonight are near or above the freezing point (0 F), and this indicates the lower atmosphere is relatively moist, and if you are using sprinkler irrigation, you will need to pay very close attention to the start of ‘‘ice crystal’’ formation on strawberry plant tissues. Any surface moisture, including ice crystals, can serve as an important extrinsic agent on the plant surface along with Ice-Nucleic Active (INA) bacteria. Surface nucleators may trigger ice formation inside plant tissues, and whenever ice forms in the plant tissue, there will be damage regardless of how long it took to reach that point.
For growers using sprinkler irrigation, the sprinkling should be initiated at the first appearance of frost ice crystals to avoid damage to open blossoms. If you have already applied row covers to prevent frost damage tonight, there is nothing else to be done. But, if you are using sprinkling, you must plan on visiting your strawberry planting on a frequent basis during the night to observe with a flash light when ice crystals start forming on vegetation near the ground level, or even on top of row covers that be rolled to the side. As soon as you detect any frost forming on exposed strawberry plant tissues, turn the water on!
If you are in the Raleigh-Durham area, AWIS weather services is forecasting that the first signs of FROST might be around 5 a.m. tomorrow morning (table below). Frankly, this is only a guess! It could be that FROST could start forming much earlier in the morning than 5 a.m., and I am aware of situations when AWIS called for frost at say 5 a.m., but then I would later learn from someone in the area that they saw ice crystals as early as say 2 a.m.!
So, the AWIS forecast is best utilized to get an idea about whether conditions might favor a frost (and they do in many areas tonight), BUT never count on them to Time the Frost for you! If you are using sprinkling, it is your job to monitor field conditions yourself to see when ice crystals start forming.
Table 1. Please note how winds will die down this evening (Mon) and by Tue morning will be “0 mph” at 5 a.m. In addition to no winds, skies will be clear in RDU area, and dewpoints are in the perfect range for frost formation on Tuesday morning (27-29)…there is plenty of moisture in the air for frost to form. Though the air temp minimum for RDU will be 35 F, that is definitely cold enough for frost formation at canopy level where it will drop below 32 F. AWIS is saying the FROST will occur at 5 a.m., but it could be MUCH EARLIER IN THE NIGHT! Growers using sprinkling are 100% responsible for monitoring their crop during the night to see when ice crystals begin to form.
When to shut down sprinkling?
Wet bulb temperature can be used to safely determine when sprinkling can be
shutdown in the morning – water can be safely turned off when the wet bulb
temperature is above 32oF. In the Raleigh Durham area, please note wet bulb will be at 35 F at 7 a.m. – that is when it “should be” safe to shut down. But, I would wait a little longer and make sure the ice is nicely melting. Waiting for all of the ice to melt is an older method that can also be effective tomorrow morning for knowing when to safely shut down under the kinds of conditions expected tomorrow morning.
You can look at the bottom of the SkyBit forecast below to see an hourly table that shows when the wet bulb will be above 32 F at Clayton tomorrow morning (answer is sometime between 7 a.m. and 8 a.m.)
Table 2. Clayton is about 20 miles southeast of Raleigh, and at this location the minimum at canopy level is going to be 31 F. That is cold enough for good frost formation! We cannot know exactly when ice crystals may begin to form at the canopy level, so a grower would need to monitor the crop directly. Most growers using sprinkling will set a FROST ALARM at a specific temperature, and it might be well to consider setting it around 37-38 F for tonight (to be safe).
• If you wish to become a real expert on Sprinkler Irrigation for Strawberry FROST and FREEZE protection, you may wish to download this document: StrawberryPlasticulureAdvisoryApril1505
Here is the latest update from AWIS for our region:
Detailed forecasts for MD, VA, NC, SC and GA
Dr. Barclay Poling,
Retired Strawberry and Muscadine Grape Specialist
p.s. If you are using sprinkling, remember the saying, “Those who snooze, lose.” Be sure to set that frost alarm!