by Sally Colby
On many farms, strawberries are the first fruit crop for U-pick or markets. For full crop value, frost protection is critical.
Dr. Mark Hoffmann, small fruit Extension specialist at North Carolina State University, has some pointers for growers. “Cold protection depends a lot on the physiology of the plant,” he said, adding that dormant plants are more cold hardy because the bud is still inside the crown. “As soon as the plant transitions to bloom, the temperature of tissue is getting higher until bloom, which is the most vulnerable stage …You can get damage if temperature at tissue level drops below 30º.” One challenge in providing protection is that a single plant can have several different stages of development.
Hoffmann described the differences between freeze and frost events. Freeze events are more common in winter, are windborne and occur when a thick layer of cold air moves into an area. It’s usually windy before and after freeze events. Hard freezes usually occur over several days and often alternate with warmer temperatures.
Frost events are often based on inversion – a layer of cold air at 30 to 200 feet with warmer air above – and usually threaten crops from late winter through late spring. Such events are usually due to radiation. “Radiation of heat is emitted from plant tissue,” said Hoffmann. “That lowers the temperature of the plant tissue and the plant can become damaged. Frost events happen at around or even above 32º, and the dew point is important.” Hoffmann added that although frost events can devastate many crops, strawberries can be protected fairly well.
Hoar frost is associated with high humidity and identified by ice crystals visible on plants. Black frost happens in low humidity when temperatures are around 32º, with no signs of typical frost such as visible ice crystals. Forecast tools are very important, and Hoffmann advised growers to look at forecasts 10 days out.
With freeze events, there’s a lot of cold air moving for several days, usually at higher wind speeds. Freezes can be extremely dangerous for hard to protect plants, but are not common. “What is common is frost-freeze events – a combination of radiational heat and weather changes,” said Hoffmann. “They are usually not as long but can be devastating, so they need to be forecasted by a weather tool for your region. Monitoring the weather forecast at this point is extremely important.”
Hoffman described the critical tissue temperatures during various development stages. For tight bud, the critical temperature is 22º. Popcorn state is 26.5º and pen blossom is 30º, which is when damage can occur through radiation heat loss during a frost event. “That can happen very quickly,” said Hoffmann, “even if air temperatures are way above freezing. Often we get frost events at temperatures at 38º or 39º.” On a clear night there can be radiational heat loss.
Crowns should be protected from freeze damage through winter in temperatures down to 10º to 15º. Row covers should be used if temperatures drop below that level. Between March and May, blossom protection from frost events is necessary when temperatures are between 32º and 40º, especially when there’s little or no cloud cover and low wind speed. “It’s also possible in spring to have frost-freeze events and rarely late spring freezes,” said Hoffmann. “They can be very devastating, so you have to monitor for those.”
In order to provide protection at the correct time, growers must know what kind of cold event will occur. University-based weather advisories for the area are a reliable tool for weather prediction. “Always look at minimum temperatures,” said Hoffmann. “We also often talk about wind speeds and dew points. Check for humidity and other things that might impact – especially for frost events – whether you will see a hoar or a black frost.”
Hoffman advised growers to first look at the air temperature – is it higher or lower than 32º? If temperatures are lower, look at wind speeds. High wind speeds and low temps are a problem because the combination could signal a frost-freeze, or advective freeze event. Higher air temperatures might not be a problem unless the sky is clear, and in that case, monitor for hoar frost. Low dew point and low humidity can signal impending black frost.
Row covers and sprinkler systems are the two main frost protection tools. Each has pros and cons, and Hoffmann said it’s best to have both because there’s no way of knowing what kind of cold event will occur.
The goal of row covers in lethal temperatures is to protect the crown from cold or to protect flowers from frost and freezes. Hoffmann pointed out that knowing minimum temperatures will not always be sufficient to predict weather and adequately protect plants.
Row covers have advantages: They can be used in freeze and frost, improve GDD, don’t require night shifts and don’t require water. However, row covers are labor intensive, can invite insect and disease buildup underneath, don’t work well in wet weather, often don’t hold up in wind and can contribute to poor pollination.
Hoffmann recommended monitoring tissue temperatures under row covers. “If you don’t do that, you’re going to have to look at dew point and humidity to protect plants when they need to be protected,” he said. “Row covers are usually put on in the afternoon before the sun goes down and left on until the frost or freeze event is over.”
Sprinkler systems are another protection method, but Hoffman said it’s critical to have good, consistent water coverage on bud tissue for adequate protection. The goal is to have consistent water that’s continually building a new ice layer on leaf bud tissue because this is the source of energy providing heat for bud protection. “If you lose the thin water layer on top of the ice, you’re going to lose the ability to protect the bud tissue from freezing, and will actually freeze the tissue,” he said. “If that isn’t happening you will cause more harm than good.” Accumulated ice should be clear, not white.
It’s important to have a decent rotation time to get a constant coverage on plant tissue. “You want uniformity in distribution and droplet size, and a system that is freeze-tolerant to protect from ice build-up,” said Hoffmann. “Otherwise you’ll have to shake the sprinklers to knock the ice off, and during that time, you aren’t providing coverage for plants.”
Hoffman cautioned growers to understand that application inches per acre depends on nozzle size and not water pressure. The more inches per acre, the more gallons required, and nozzle size is critical in determining whether the sprinkler will deliver good coverage.