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Freeze taught strawberry growers hard lessons

The freeze that decimated many southern crops last Easter taught strawberry growers some hard lessons. But Barclay Poling, small fruits specialist for the North Carolina Extension Service, says the basic message is clear — growers need to be prepared to use row covers as well as irrigation to save their crops from freezing weather, and they need to do it with precision.

The latter means investing in an inexpensive piece of equipment and learning how to use it: A digital thermometer attached by wire to thermocouples that are themselves attached to the tissue of the plant. It will tell you exactly when plant temperatures go above or below the temperature at which you have to take action.

Poling says the digital thermometers proved their worth so emphatically in 2007 that he thinks they should be a part of the standard production equipment for a modern strawberry farmer.

“I am not going to be real happy next spring if a grower asks my advice on how to deal with weather events and he doesn’t have a thermometer to precisely monitor the temperature in the blossoms,” he says.

“The cost is relatively low, and there’s not a better payback for anything you can buy in strawberry production. Without one, you are in a dangerous guessing game.”

Irrigation continues to be the first line of defense when strawberries are in danger of a freeze. Sprinkling provides cold protection because latent heat is released when water changes from liquid to ice.

But that strategy falls short when the wind is blowing.

“When many of our growers tried to irrigate during the Easter freeze, they encountered overwhelming wind,” says Poling. “That led to evaporative cooling — when water goes into vapor, there is a cooling effect.

“Farmers who used only irrigation as a tool in the Easter freeze, especially from Winston-Salem west to the mountains, may have done more damage than good.”

Row covers can be the valuable second line of defense in this situation, as growers in Virginia found out at Easter.

“The strawberry growers here who had the most success were those who used row covers in combination with irrigation,” says Jeremy Pattison, Virginia Extension small fruit specialist. “They use covers mainly for temperature control, especially frost protection, and also to a degree for winter environmental control.”

In a freeze situation, covers can be expected to provide about eight degrees of protection, since they capture the heat in the ground to create a reservoir of heat to carry the berries through the night.

Also, row covers provide a service even when extreme weather is not expected, says Pattison. “If you keep them over the crop in the winter, the plant will be ready for a good start. Otherwise, strawberry plants tend to get beat up by winter conditions. The leaves may turn brown and become desiccated. By having them ready to go, you may get a four- to seven-day increase in early growth just from winter use of row covers.”

Pattison thinks more Virginia strawberry growers should use row covers, but there are obstacles.

“Labor is definitely a limiting factor, and another is that wind can be a problem when you are putting them out,” he says. “We need some new technology here, to come up with some efficient way to get the row cover out with fewer people. What I am envisioning is something that could be attached to a three-point hitch and extended with some kind of roll machine, either automated or hand cranked.”

As the cover rolls off, rock bags carried in a wagon are used to anchor the cover. When a particular use is finished, the farmer rolls up the cover on a reel and may leave it at the end of the row until the next use.

“Now, most of our growers cover about four or five rows at a time,” Pattison says. “If we get more automated, I think we might go to less width, maybe three to four rows at a time.”

Covers are affordable and durable, he points out, and they can be used several years in a row.

“They cost about $1,000 to $1,200 per acre and have huge economic impacts,” he says. “They help you get earlier production, they aid in frost protection, and they may reduce the need to frost protect a few nights a year, saving water and fuel.”

They are generally made of spun bonded polypropylene and are characterized by weight, with one to 1.2 ounces per square yard the typical weight for strawberries.

“If you are using one for frost protection, you might want to go heavier, maybe 1.5 ounces per square yard,” says Pattison. “The thicker they are, the more cold protection you get. But less light gets through, and a heavier cover might even slow the crop down a bit if you are using it to extend the fall growing season.”

Experience is a good teacher, but the lesson for strawberry growers was a harsh one this year, says Poling. “I have never seen a freeze of this magnitude at this late a juncture in the season, and the winds made it much worse. Still, in North Carolina, we had about an 80 percent strawberry crop, which was far better than any other fruit crop in the state.”

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