March 26, 2010 (4:40 pm)
Topics covered in this advisory
1. What precautions have you taken so far?
a) row covers applied yesterday?
b) moist cover from rains going into the early morning?
2. Minimum temperatures
**row covers should be more than adequate
**watering on covers not needed
3. Winds
** low likelihood of frost formation tomorrow morning with such sustained winds > 5 mph
** need to watch for frost in Central NC and protected areas
** all growers using sprinkling should plan on a long night – you still have to monitor winds and temperatures on an hourly basis in the field tonight until sunrise
** because of evaporative cooling issues with sprinkling, plan to turn on sprinklers at wet bulb of 30-31 F, or if using digital thermometer, turn water on with reading of 30-31 F blossom temp in coldest area of field
4. AWIS forecast for ENC, Central NC and Sandhills
5. See summary points at bottom of advisory
Dear Growers and Agents,
Whew! I just got caught up on all special requests to look at weather in specific zip codes around NC, VA, DE and the region. I apologize for not having this advisory out sooner, but with all the call volume its been a challenge to analyze all the different weather products, talk and write! But, I feel that most growers have done a good job of preparing and if anything, we may be over-prepared.
As of yesterday, almost all strawberry growers either took steps to apply row covers in anticipation of a cold event tomorrow morning, or they went ahead and got their irrigation systems ready (and hopefully calibrated all their field thermometers). Some larger growers applied covers to a portion of their crop, and then plan to irrigate if needed in the morning for the balance of their acreage. Other growers are planning to go strictly with watering, if needed?
All growers seem to be doing a good job of checking their forecasts, and for producers who intend to irrigate (if necessary), the one bit of critical weather information YOU NEED TO GET FOR TONIGHT IS AN HOURLY WET BULB TEMPERATURE ( a second advisory will be sent for those growers who are planning to irrigate). But, let me say now that WET BULB information is needed to help you decide when to start the irrigation up. We recommend starting up at a wet bulb temperature of 30 F, and it may be that you will reach a wet bulb of 30 F several hours ahead of when the air temperature in the plant canopy is 30 F. If you have a digital thermometer with thermocouple that can be stuck in the bloom, you should plan to start watering when the bloom temperature is 30/31 F. By the way, you should “stick” the blooms soon this late afternoon (if not done already) – always better to do this in daylight!
Row covers applied by most growers yesterday
I am finding that the majority of growers have applied row covers yesterday (as we did at Clayton and Salisbury). And, with such light rains last night, I suspect row covers could have been pulled today, but we simply weren’t sure yesterday about how much rainfall to expect, so pulling on Thursday was a good decision. Some of us have been a little worried about high winds today and “flapping” of the the covers against the plants. I just spoke to Rocco at Clayton and they made a point to pull the covers a little tighter yesterday so that we would not get quite so much flapping today and early tonight – this can actually be a big concern when winds get into upper 20s and 30 mph range.
Moist cover worry? (Not with these winds!)
As we may get a shower at around 4 pm today at Clayton (very light), some growers have called to ask if moist covers could possibly cause a problem tomorrow morning with frost? But, I don’t think the covers will remain moist overnight with so much wind blowing and the rainfall accumulations being so slight today and this early evening. We determined at Salisbury that a rain of 0.04 inch only just moistens the cover, and the areas I have studied today around the state will not be getting too much more rain than this amount. My belief is that the covers will be good and dry by early morning – when we want them to be dry. If the covers were going to be soaking wet going into tomorrow morning, then there would perhaps need to discuss some extra precautions, but let’s drop that issue for now and talk about minimum temperatures and winds.
2. Minimum temperatures
If you go to this website right now, http://envirsion.com/strawberry/ you can get an instant look at the minimum temperature for your region and if you plug in your zip code, you can get some localized hourly information on temperatures, winds speeds and dew points (I recommend you try this out). I also recommend that compare this newer product from Envirsion with the existing weather service(s) you use. I am actually attaching the AWIS minimum temperature map within this advisory (just below), so you can compare the 2 products. The Envirsion forecast was just run at about 3:00 pm today, and Mark Brooks has noted that “The temperatures increased a bit. And less rainfall expected.”
According to the AWIS map (see top of advisory), our coldest spots in the Piedmont going to be in Vance County area (Pink = 30-32 F), and then west of Greensboro there is a pink band that runs on a diagonal from Stokes down to Henderson Cos. In the mountains you can see darker pink areas that also include Asheville (28-30 F).
If the Envirsion map (just generated) proves correct, then areas like Asheville could be between 32-36 F. But, even at the coldest temperatures being projected at this time, growers should remember that these minimums are not that serious compared to other years at this time in March. We have been through some major battles at this time of year with far more crop exposed at this time of year (we are running about 10 days -14 days late in crop development), and we have had temperatures in the low 20s and even teens to deal with! So, it is important to realize that we are not dealing with what I would call “seriously threatening temperatures”. In fact, in growth chamber work we’ve done, it is hard to kill any open blossoms at 28 F – we really need to go below 28 F to injure open blossoms in growth chambers. Where we can get into trouble with temperatures of 28-32 F, is with FROST! But, as you will read next, conditions are very unfavorable tonight and tomorrow morning for frost formation.
If you have applied a row cover already, I do not see the need for any additional watering on top! The temperatures in the forecast for tomorrow morning are simply too high to justify all that additional aggravation.
But, I can’t tell you how many emails and calls I have gotten today where the grower asked,
“Do I need to apply water over the top of the covers in the morning?”
Answer – very unlikely! Row cover swill handle these temperatures without supplemental watering. At Clayton, for example, we may get down to 29 F in the canopy and 32 F at weather shelter by around 6-7am. But, the row cover has already been applied (yesterday) and this cover 1.5 oz will provide complete protection down to temperatures in the low 20s. So, there should be no need for sprinkling on top. If you look around the entire state, you can find one area that is expected to have temperatures of 24-28 in upper mountains, but in this area the crop has barely broken dormancy and at the tight flower bud stage, the crop has excellent cold tolerance into the very low 20s. My point is that a row cover of 1, 1.2 or 1.5 oz should be more than adequate for what we are expecting tomorrow morning, and watering on top makes little sense with temperatures in the range of 26-32.
3. Winds
I am probably the last person on earth willing to trust a wind forecast, but it really does appear that winds will be sustained through the night and early morning at speeds that will prevent frost formation. Go ahead and check out Envirsion’s wind speed product right now (you will need to know that 1 m/s = 2.2 mph):
If you pull up the Envirsion wind map, notice that along the coast that winds of 9 m/s are expected in some locations – that’s nearly 20 mph! Then as you come inland from the coast, the winds drop off to 6 m/s range (13.2 mph). And, then look at most all of the CP and Central Piedmont, and you see generally bright yellow shading (4 m/s = 8.8 mph), with some bluish yellow areas – these are the areas with the least amount of wind for tomorrow morning, but even here, winds are expected to be in the range of 3 m/s (6.6 mph). You can see one of these areas in the western part of the state with smoky shading (blue/yellow). To get frost, you need to get below 5 mph (2.3 m/s), which is a darker shaded smoky-blue color. You can’t see any blue on the NC map!
So, if the winds really do stay up above 5 mph all night and into the early morning, then we should not see FROST in NC tomorrow morning.
4. AWIS - let’s see what AWIS has to say about FROST POTENTIAL in ENC, Central NC and Sandhills (note that only in North-Central NC do they mention a potential for FROST in Protected Areas)
AWIS Weather Services, Inc. North Carolina Frost/Freeze Forecast Produced at 525 AM CDT on Fri Mar 26 2010 ... Forecast for Tonight ... Northeast NC Coast Currituck/Camden/Pasquotank/Tyrrell Counties **** NEAR FREEZING COLD LOCATIONS **** **** LITTLE IF ANY FROST DUE TO WIND **** -------- East Central/SE NC Coast Carteret/Pamlico Counties **** NEAR FREEZING COLD LOCATIONS **** **** LITTLE IF ANY FROST DUE TO WIND **** -------- Southeast NC NWS FORECAST ZONES 87-90,96-101 **** FREEZE COLD LOCATIONS **** **** LITTLE IF ANY FROST DUE TO WIND **** -------- North-Central NC Franklin County Area **** FREEZE COLD LOCATIONS **** ** PATCHY FROST COLDEST PROTECTED AREAS ** -------- North/Central NC Alamance County Area **** FREEZE COLD LOCATIONS **** ** PATCHY FROST COLDEST PROTECTED AREAS ** -------- South/Central NC Richmond County Area **** NEAR FREEZING COLD LOCATIONS **** **** LITTLE IF ANY FROST DUE TO WIND ****
5. To Summarize:
1) If you applied row covers already, your main concern is to make sure that they are pinned down well as winds will be very high in some areas today and tonight — also, if you have a digital thermometer, now is the time to insert thermocouples in blossoms (before sunset)
2) Row covers of 1.0 oz/sq yd or higher will provide adequate protection against the minimum temperatures that are being projected for NC tomorrow morning – sprinkling on top should not be not needed
3) Most areas will have sustained winds tonight and tomorrow morning, and this will prevent frost formation on blossoms – but if you use sprinkling, you are not off the hook (read no. 5)
4) Temperatures at the canopy level and weather shelter height (6 1/2 ft) should be very similar due to sustained winds, and good air mixing
5) For sprinkler irrigation people this promises to be a “long night” as we can never be absolutely certain about wind speed forecasts, and growers who are depending on sprinkler irrigation MUST PLAN TO MONITOR WINDS AND TEMPERATURES THROUGHOUT THE ENTIRE NIGHT AND EARLY MORNING – IN AREAS WITH FORECAST FOR TEMPERATURES NEAR 32 F OR LOWER, YOU WILL NEED TO CHECK CONDITIONS BEFORE 11 PM, AND THEN EVERY HOUR FROM MIDNIGHT UNTIL SUNRISE – IF IT APPEARS THAT WINDS WILL LAY DOWN (< 5 MPH), AND THAT BLOSSOM TEMP COULD GO BELOW 30 F, THEN BE SURE TO START IRRIGATION UP BEFORE THE WET BULB GOES BELOW 30-31 F. YOU COULD END UP WASTING SOME WATER BY TURNING ON AT 30-31 WET BULB, BUT YOU WILL AVOID A “COLD JOLT” ISSUE AT START UP. THERE IS MORE RISK OF INJURY FROM A LATE START UP THAN MOST GROWERS APPRECIATE! DON’T VIOLATE THIS RULE. GROWERS WITH FORECAST MINIMUMS IN THE 34-36 F RANGE MUST BE SURE TO SET A FROST ALARM AT 36 F AND PLANT TO GO TO FIELD IMMEDIATELY IF ALARM RINGS TO CHECK WIND CONDITIONS AND TEMPS AT THE CANOPY LEVEL
A FOLLOW-UP ADVISORY FOR SPRINKLER IRRIGATION FOLKS WILL BE AVAILABLE BY ABOUT 6 PM
Start checking now to see if you can access wet bulb temperatures (for sprinkling only) – let me know if you have a problem and I can assist you.
I greatly appreciate Mark Brooks help all through today – the Envirsion forecast products have been very helpful!
Good luck to everyone!
Dr. E. Barclay Poling
Professor and Extension Specialist (Small Fruits)
Dept. of Horticultural Science
Campus Box 7609
NC State University
Raleigh, NC 27695-7609
919-515-1195
919-515-2505 fax
919-418-9687 cell
email: barclay_poling@ncsu.edu
