by Sally Colby
Strawberries are a valuable crop, and although protecting them from late winter freezes can be tricky, understanding the nuances of frost protection gives growers an advantage. Dr. Barclay Poling, Department of Horticultural Science at North Carolina State University, described advances in frost protection.
“Sprinkler irrigation was our main means of protecting strawberries,” said Poling of early frost protection. “We experimented with row covers in the early 1980s, and by the 1990s, we figured out how to adapt row covers to strawberry plasticulture system.” More growers are now relying on row covers for frost protection rather than worrying about setting up sprinklers at the right time.
In trials aimed at finding out how strawberries respond to row covers, researchers set up equipment to measure wind speed, rainfall, air temperature, humidity, dew point, soil temperature and sensors for light measurement under the covers. Three different row covers – 0.75 ounce, 1 ounce and 1.25 ounces – were tested.
Poling said with an air temperature of 22º at ground level, there were notable differences under various row covers. “For the one ounce cover, the average temperature was 27.4º,” he said. “That’s 1.7º higher than with the 1.25-ounce cover. The 0.75-ounce product gave us 26.3º, which wasn’t too different from the 1.25-ounce cover, but it was interesting that the lightest weight cover could give that much protection and slightly better than 1.25-ounce cover.”
Depending on location, many plants are emerging from dormancy – even those that appear to be dormant. “In a totally dormant state, crops can survive shots of cold even below 10º without damage to the crown,” said Poling. “However, as plants transition to the new leaf stage where flower buds start to emerge, we are entering the part of the season where the rules change. These stages of development put us into a much tougher situation about how much cold the buds can handle without economic damage.”
At this point in the season, good forecast information is critical. Growers who rely on covers for their cold protection program without any irrigation backup need precise forecasting information to do the best job managing the crop. “Weak or mediocre weather information is just generating a lot of data,” he said, “and I’d be reluctant to rely on some of these forecasts in terms of scheduling covers on or off.” Poling stressed the fact that every year is different and growers should have access to a reliable two-week forecast.
The most critical juncture for the crop is when the sun angle changes and the crop begins to respond. “Don’t be fooled by how dormant the plant looks,” he said. “It’s no longer a dormant plant when you can see the young trifoliates pushing through.” Poling suggested growers inspect plants for trifoliates and branch crowns.
“I cut them open to see the condition,” said Poling. “Hopefully they’re a lovely ivory color with no brown discoloration. I also look at the main crown, where the most advanced flowers are formed, and hope the first primary flower is still alive. When the flower bud emerges from the crown, we’re in a different ballgame, and we haven’t sorted this out. The hardiness of emerged flower buds might be better than what older literature reported.”
The use of row covers in autumn for growth enhancement may negatively affect the crop the following year because covers interfere with plant hardening. “One of the biggest mistakes a grower can make is leaving the cover on in late fall when the plant needs exposure to multiple nights of colder temperatures to become hardened for the winter season ahead,” he said.
For short day varieties, Poling said it’s clear that during autumn and early winter they are setting up flowers during shortening days. Once a row cover is on a crop in late winter, the timing for removal can be difficult. If a row cover becomes soaked in rainy or icy weather, it has no protective value.
Poling said black frost conditions are especially dangerous. “We’ve learned from experience and research that strawberries have a definite critical temperature of 27º,” he said. “If there are no frost crystals, a good percentage of blooms subjected to 27º will survive and others do not. But with a temperature lower than 27º in the absence of frost the entire bloom will be lost. Even if the dry air won’t make frost, be ready with protective measures and don’t let the blooms drop below 28º. Row covers of one ounce or 1.25 ounces provide fantastic protection against that type of frost.”
While he’s careful not to predict what may occur in various growing regions, Poling wants growers to be prepared to act quickly. “We started off with excellent growth in fall but since the beginning of the year, we’ve had cold air,” he said. “Sometimes you’re better off without covers, but always be on the alert for a clipper and have row covers ready.”
Ideally, covers should be removed at popcorn stage, and definitely before bloom, to ensure adequate pollination. However, Poling warned that rapidly changing weather conditions mean swift decisions are often necessary. For example, should covers be removed on a sunny winter day if the temperature is 58º at noon? Row covers applied when they aren’t truly needed can deharden the crop on warmer days in late winter, then if severe cold comes through, there’s danger of plant injury. “Row covers are a wonderful defensive tool to deal with specific cold events,” he said, “but be cautious about using them for extended periods.”
Poling advised growers to focus on the three Cs, beginning with the cold event itself – how cold will be it tomorrow, in a week and in two weeks? “This is where we need excellent forecasting,” he said. “Also looks at crop stage – see how far the crop has advanced, and capability to protect – you can’t protect with a wet row cover, so how will you counteract the cold event? Do you have the right covers, the right weight, and extra covers if it’s brutally cold and you don’t have irrigation as back up?”
Protecting strawberries during the most vulnerable stage means being prepared to deal with cold, sub-freezing temperatures combined with wind, frost/freezes and winds higher than 5 mph. “Now is the time to take advantage of and optimize plant hardiness,” said Poling. “Do that best by keeping covers off and put them on only prior to a devastating freeze.”