This illustrated report describes a new technique developed by some "veterans" of strawberry freeze protection in North Carolina for protecting strawberry blossoms in windborne freeze conditions. Going back to the mid-1980's when strawberry plasticulture was first introduced to growers the mid-South, we knew that there was one major problem with this system that forces a strawberry crop into a picking window that is two weeks earlier than matted row. Right! We can encounter terrible windborne freezes. Regrettably, these freezes in the first and second week of March are becoming more and more frequent, and the commercial strawberry plasticulture growers in North Carolina and the surrounding states will either adopt this new system of freeze protection, or continue to face staggering crop losses. The windborne freeze of March 8, 1996, reduced the North Carolina strawberry crop by 50 to 60%. That year we recorded temperatures in the mid-teens at our NCSU research station facility in Clayton, NC (about 12 miles southeast of Raleigh) and winds speeds as high as 25 mph. Our 'Chandler' plants had an average of 2 exposed blossoms and many more "tight buds" that were outside the crown. So, how did we manage a 90% crop of marketable 'Chandler' fruit at the Research Station in spring 1996 when all of the growers in the area had their crop reduced 50 to 60% by that awful windborne freeze of March 8, 1996? The answer we found had to do with a new freeze protection approach of irrigating on top of row covers: Spunbonded row covers that come in widths wide enough to cover 5 strawberry rows (50 ft) and lengths of 200 ft or more. A medium weight row cover (0.9 oz./sq yard) will typically cost $1200 per acre. But, if you buy them a few days before one of these March freeze events, expect to pay more! 

What you need - row cover, digital thermometer, toughness, and a better berry price! 

There are two critical items that you must have to utilize our technique: 1) the row cover itself (Fig. 1 - All images will be opened in a new browser window), and a digital thermometer (Fig. 2). You will also need to be a hardy individual, like Rocco Schiavone, Res. Tech. III, Dept. of Hort. Science, NCSU (Fig. 3), who is the Research Technician at NC State who stays out for as many as 10-14 nights in March and early April with the strawberry crop at Central Crops Research Station! And, if you are going to go to this extra trouble and expense to help guarantee your customers strawberries every year, you need to go up on your price! You must figure the cost for materials (around $1200/acre - not including the hooks for holding the covers down) and the labor (at least 3 workers are needed to unfurl the covers in a no wind situation - the "no win" situation is when you try to do this in the wind!). 

Why do I need to spend another $160 to $200 on a digital thermometer?

For too many years we have been "guesstimating" strawberry blossom temperatures with traditional mercury or alcohol type thermometers that we lay on the ground (matted row) or on top of the plastic bed at the height of the strawberry plant. But, these thermometers are for measuring air temperatures, not plant tissue temperature. They are not always one and the same. Plant tissue temperature can be a degree or two colder at times! Plus, how do you tell the actual temperature of a strawberry blossom when it is covered in an inch of ice at 4 in the morning? Easy! With a digital thermometer, thermocouple connectors and thermocouple grade wire, you can know the actual blossom temperature with the flick of a switch. The blossom temperature will tell you when you need to turn the irrigation on at night (before the blossom temperatures start dropping below 31 F), and when it is safe to turn the irrigation off in the morning (usually by 8:30 to 9:00 am it is safe) But don't guess, use your digital thermometer to tell you when the blossoms are actually climbing up to 31-32 F to start cutting-off. If you are monitoring temperatures with a digital thermometer and thermocouple (as you should be) you will see this rise in temperature and feel comfortable knowing that they are above the critical temperature of 30 F. On windy mornings, you can literally make ice all day long! If you wait for the "ice to melt" with winds from the NW at 20 mph you can pump until 3 PM (we like to reserve our days for sleeping). ** The key idea here, as explained by Dr. Katie Perry, Ag. Meteorologist in the Feb. 1998 "The Strawberry Grower", is that air temperatures will warm more slowly than the blossoms after sunrise because the air is warmed indirectly (i.e. radiant heat from the sun heats the surface and solid objects which in turn transfer heat to the air by conduction and convection). Normally, by 8:30 to 9:00 am there is adequate radiant heat from the sun to take the blossom temps above 32 F. 

Step-by-step 

The first step is to cover your field with the spunbonded row cover "blanket" as soon as you hear a forecast that even mentions "a cold arctic air mass up in the Midwest" that is possibly headed our way! Figure 4 shows a portion of our research field at Clayton covered with a medium weight cover (0.9 oz/sq. yd). To our knowledge, no one has determined how heavy a row cover is needed for this technique where we irrigate on top of the row cover material to form an "ice blanket" over the crop. We do know that a true ice blanket does not work! You need the "floating" row cover on top of the crop for this to work. You do not need to support the row cover with hoops over the plant rows. The row cover-ice blanket can lay directly on the plants with very minimal injury. It is possible that even lighter weigh covers would work, but most growers wish to get at least two to three years of use out of a row cover, and for that reason you should not purchase materials lighter than 0.9 to 1.0 oz/sq. yd. Materials that are 0.6 oz may only have one season of usability - the lighter weight materials easily tear. Dr. Gina Fernandez is currently evaluating strawberry freeze protection and row cover thickness in her trials at the Vernon James Research And Extension Center, Plymouth email:Gina_Fernandez@ncsu.edu, also check out http://www.ces.ncsu.edu/depts/hort/hil/berry.html

Again, get your covers out and pinned down at least one day ahead of the cold front. We pin our covers down with soil mounds (or annual grass rye clumps we pull up from the aisles) every 2-3 ft on the edge of the covers. For holding the covers down you can also use tobacco bed hooks, feed grain bags filled with sand, or even coarse nylon bags filled with gravel! Do some experimenting! 

Next, check out your irrigation system. You don't want to just barely get the row cover down minutes before the front hits, and then find that your irrigation system has a malfunction. Check out your irrigation system by the last week in February (for North Carolina). Because of high winds you will need to use some type of Vaseline or grease product on the nozzles to keep them from freezing up so easily. One product that we use is Cycle spray, a white grease product with a Teflon ingredient (Fig. 5). We need about 4 cans/acre/night. Others have suggested lithium grease. Go ahead and spray the grease on the irrigation sprinkler heads as shown in Figure 6 (spray to get good penetration of the springs). Re-spray each nozzle head after "beating the ice off". You still have to walk the lines and knock ice off your sprinkler heads - the grease products reduce the frequency of this unpleasant task! Instead of knocking ice off every 10-15 minutes, the sprinklers may keep turning for 30 minutes with the re-application of the spray. 

The optimum sprinkler rate for this technique has not been worked out. What has worked well for us is an irrigation rate of 0.175 inch/hr (7/64" nozzle, RB29 JH) for winds of 5-8 mph and minimum temperature of 24 F. With colder temperatures and higher winds, we are not exactly sure what nozzle size, precipitation rate and pressure is best. Over 10 years ago we did an experiment at Clayton with three nozzle sizes, and found that the 7/64" nozzle (applying 0.175 inch/hr @ 70 psi) did a better job on high wind nights than the 9/64" (applying 0.290 inch/hr @ 70 psi) which were more difficult to keep turning. Even though the irrigation pattern is not uniform on high wind nights (a sprinkler spacing of 40ft x 40 ft is definitely recommended), the overall field looks like a fog bank when we are irrigating in the high winds. We know that we are getting a finer droplet when we run up the pressure, but perhaps this is working to our benefit in terms of creating the fog bank? It would be interesting to investigate these questions further in the future as to the ideal droplet size. Anyway, under higher winds we still use our intermediate size nozzle (7/64" nozzle, RB29 JH), but we will run at 80 psi on nights with wind speed in excess of 10 mph. The higher pressure also allows us to apply at the rate of 0.20 inch/hr instead of 0.175 inch/hr at 70 psi. 

Start irrigating on top of the row covers when the blossom temperatures AS MEASURED WITH THE DIGITAL THERMOMETER AND THERMOCOUPLE drops to 32-33 F. One of the real niceties of this technique is that you don't have to start irrigating quite so early on nights of lower dewpoint temperatures. Take as an example how we ran our system at Clayton on March 10-11, 1998: 

Report from Clayton, 9:30 am - The crew at the research station did not turn on last night until 10:30 PM (outside temp 29 F; under cover blossom temp of 33 F). Winds were around 5-8 mph, irrigation rate of 0.175 inch/hr (7/64" nozzle, RB29 JH). The anticipated low for the night was about 22 F for this location (actual was 24 F) and average winds of 8 mph (actual were closer to 5 mph) . The system formed ice immediately after start up at 10:30 PM (growers in the area using traditional approach of no covers and irrigating had to begin sprinkling at 8 PM because of the extra low dewpoint temperature of 12 - these growers started at 35-36 F). (Note: this is one more of the benefits associated with row covers on low dewpoint nights - you can sometimes wait another 2 hours before having to cut-on vs. traditional irrigating). 

Soon after we cut on the water, the blossoms underneath the covers jumped 3-4F. The advantage of the row cover and irrigation is evident in the temperature differences between outside air temp and the blossom temp beneath the cover: 

Important: With irrigating on the row covers it is important to keep sprinkling on the covers as long as you can. Too many growers have the notion that you just make an ice cap and go to bed. Wrong! You will sustain much more injury with this approach. Final step - remove the covers! But, only after the front has passed through the area. We have not experienced major difficulties with pollination even with the row covers in place for up to 5-7 days. Hopefully, the covers will not be needed again until next year! Don't be fooled, these early March freeze events are real! We have had similar episodes March 11-13, 1992, March 8-11, 1996 and March 10-?, 1998! 

Acknowledgment: Most of the pioneering work on the row cover + irrigating was done with the cooperation of John B. Earp, retired Research Technician, Dept. of Hort. Sci., NCSU, in the late 1980's. Many thanks John!