It is no mystery why row covers are replacing overhead irrigation for spring frost protection of strawberries.

There are several reasons. Barclay Poling, recently retired North Carolina State University Extension horticulture specialist, says, one of the most convincing he ever heard came from a grower near Raleigh, N.C.

“His words were ‘I want to sleep!’” says Poling.

This grower had relied on overhead sprinkler irrigation for spring frost protection until he started experimenting with row covers in the late-1990s, he says. The all-night watering sessions required for frost protection with overhead irrigation were eliminated with row covers. 

Now, the only reason he has to use a sprinkler system might be for evaporative cooling and an occasional late-season windborne freeze during bloom, says Poling.  

“A floating row cover of 1.5 ounce-per-square-yard weight can do the same job as overhead sprinkler irrigation for frost protection,” he says.

You can also save a lot of money on the fuel bill for pumping water,” he adds. And by keeping the crop “dry” with covers, it isn’t necessary to spray nearly as often for botrytis during the bloom period.

The distinct disadvantage of overhead irrigation for frost protection is that it uses such a significant volume of water. It also causes erosion in the row middles and ends, and it washes pesticides from the strawberry and leaches fertilizers from the field. 

By comparison, strawberry row covers conserve water, reduce soil erosion, and reduce fertilizer leaching. They can also reduce spraying relative to conventional overhead sprinkler irrigation.

There are important limitations to using row covers for certain types of cold protection, especially when temperatures are expected in the low twenties or the teens during bloom.

Sprinkler irrigation has been the accepted practice for frost protection for many decades, says Poling. “If a grower did not have an adequate water supply for overhead sprinkler irrigation system, he or she was advised not to go into strawberry production.”  

The water requirement for an overhead sprinkler irrigation system is usually estimated on the basis of three consecutive frost or freeze nights, he says. For example, 5.4 acre-inches of water would be needed for sprinkling at the rate of 0.18 inch per hour for control down to 24 degrees F., for 10 continuous hours each night over three nights.  Or 1.8 inch per night for three nights equals 5.4 acre-inches. 

“An irrigation pond would need to hold about 150,000 gallons of water for each acre of plasticulture production under these conditions,” says Poling. “That’s a lot of water.”

Exciting development

The fact that small growers have demonstrated row covers can be used for late-season frost and frost/freeze protection without overhead irrigation is a fairly exciting development, says Poling.

“Farmers with relatively limited water supplies can now grow strawberries in the plasticulture system and achieve full crops in most seasons using the covers.”

For more information on strawberry row cover management, go to the North Carolina Strawberry Information Portal at www.ncsu.edu/enterprises/strawberries.

These row covers are known as spunbound or non-woven fabrics, says Victor Lilley, vice-president of Reddick Fumigants, Inc. “They are most commonly made of polypropylene or polyester,” he says. “They are manufactured by melting the appropriate plastic or combination of plastics and spraying fine filaments onto a belt.”

The belt conveys them to a bonding roller that presses and fuses them together where they touch.

Improvements in manufacturing over the last 20 years have made possible the availability of large covers that can be laid directly on plants. “Such covers can improve earliness, yield and quality,” Lilley says. “For plasticulture strawberry growers, the benefits are in the areas of over-wintering protection and late-season frost/freeze protection, and they also promote fall growth of plants in colder climates.”

Row covers give the greatest benefits when they are fitted to one’s individual strawberry production system, he says. “Geographical location, proper management of the covers with regards to application and removal timing and the management and use of covers with overhead irrigation for late-season freezes,” he says.

Strawberry growers in mild climates like the Coastal Plains and Piedmont typically use row covers for late-season frost or freeze protection. Most growers in these regions view row covers as insurance policies: They may not be needed, but are invaluable when they are.

When covers are used for only short periods such as this, their exposure to the elements is greatly reduced, so less expensive lightweight row covers can be utilized. The benefits of lightweight covers (0.5 ounce per yard) for freeze protection can be greatly enhanced if they are used in combination with overhead irrigation.

Growers in colder climates like the mountains should use row covers as part of their strawberry production system and should consider the heavier and better-quality materials due to the length of time these materials will be exposed to the elements.

 Some cold-weather growers apply covers as early as December, leaving them on until well into late February, thus extending their exposure to long periods of time.

Temperatures in these areas have been known to dip low enough to cause plant mortality if row covers are not available to protect and shield plant crowns.

For these reasons, medium to heavy covers should be considered, says Lilley. “But they should not be too heavy: Covers that block more than 50 percent of available light can actually slow growth and advancement of one’s strawberry crop.”

Lilley’s company, TriEst Ag Group was recently formed from a merger of Hendrix & Dail Inc. of Greenville, N.C.; Reddick Fumigants LLC of Williamston, N.C., and Hy-Yield Products Inc. of Rocky Point to service southern farmers’ fumigation needs. You can reach the TriEst main office at 800-637-9466.

chrisbickers@gmail.com