Frost and Freeze Protection for Vegetables
Prepared by Chris Gunter, Extension Vegetable Production Specialist and Jonathan Schultheis, Extension Vegetable Production Specialist, Horticultural Science, NC State University
Weather predictions for this weekend include clear skies with minimal or calm winds, which are conducive to cold temperatures and could lead to freeze or frost conditions in North Carolina. Sunday morning, May 10, low temperatures are predicted to drop into the 30s with some temperatures possibly below freezing in low lying rural areas. Record low temperatures for May 10 are in the lower 30s at the Greensboro and Raleigh-Durham locations, and 39 ºF at Fayetteville. Not only are daily low temperature records threatened, but the latest spring freeze on record is May 10th for Raleigh (31 ºF) and May 8th for Greensboro (32 ºF).
As cold temperatures are threatening, agents and specialists are getting calls from concerned vegetable growers worried about potential damage on their crops. A frost occurs when temperatures dip to 32 ºF and water starts to freeze on low lying surfaces. If water within the plant cell or between the plant cells freeze, this can result in damage to plant tissue. The following vegetable crops can be grouped according to their cold tolerance based on their physiology. It is important to remember that cold damage results from the actual temperature and the duration of that temperature.
We usually think of frost as occurring at temperatures from 31-33 ºF and this will result in damage or killing the foliage of warm season plants like beans, corn, cantaloupe, cucumbers, eggplant, okra, southern peas, peppers, potatoes, sweet corn, sweetpotatoes, squash, tomatoes, and watermelon.
When temperatures dip below 26-31 ºF, this is a hard frost or freeze. There are cool season crops that will tolerate a temperature dip to these temperatures for a limited period of time. These include broccoli, cabbage, cauliflower, chard, English peas, lettuce, mustard, onion, radishes and turnips. They will likely show foliage damage due to the cold and this may result in a yield reduction later in the season, but the plant will survive.
In the event that the temperature drops below 26 ºF during a hard freeze for a longer period of time, there are some vegetables that will survive these temperatures. Cold season crops like Brussels sprouts, beets, collards, kale, parsley, and spinach should survive.
Charlotte Glen wrote a great article about seeds and seedlings surviving cold temperatures. We encourage you to check it out here:
Protecting from these cold temperatures can be a little more difficult to answer. Growers who have solid set irrigation in place and have experience with frost protection using overhead water applications may be planning to use this method. This requires large volumes of water and continuous applications throughout the cold event. For most vegetable growers, however, overhead solid set sprinkler irrigation is less common than drip irrigation or traveling guns and pivot irrigation systems.
Usually maintaining good soil moisture prior to the event can be helpful, as the water in the soil retains heat longer and releases it slowly during the cold event. For this reason, it is also recommended not to cultivate just prior to a frost or freeze, so that as much water can be retained in the soil as possible. The cultivation can damage plant roots and increase stress on the plants. In addition, cultivation opens additional spaces in the soil, allowing cool air to penetrate deeper into the soil profile.
Other methods for frost protection include using row covers, which come in various lengths and thicknesses depending upon the level of protection needed. Growers can also use waxed paper cups, to cover the transplants in the field, during an overnight cold period. These are labor-intensive methods, but may be an option if areas are small enough and the farm has sufficient labor to put on and remove the covers as temperatures warm back up. If coverings are used, it is important to monitor temperatures under the covers. Be prepared to remove the covers before temperatures under the cover rise too high and result in heat stress.
Growers can wait to transplant those crops that are most frost sensitive. In extreme cases, with recently transplanted Solanaceous crops like tomatoes and peppers, newly transplanted seedlings could be removed from the field until the cold event has passed and then retransplanted. Care must be taken during this process to minimize damage to the young roots of these plants. Place seedlings in a protected area under cover to prevent damage to those seedlings collected and placed back in trays.
Though much less common in vegetable crops, there are air fans that are used to move cold air away from low spots or frost pockets. In these low spots, cool air sinks and cannot drain away from the crop. These large air fans can either be permanently mounted in a field, or movable on a trailer or tractor mounted. They are designed to mix the air above which is warmer and prevent cool air from pooling around the crop. These methods are used more commonly in high-value orchard and vineyard crops. Care should be taken when evaluating the economics of any frost protection strategy.
Perhaps the conditions forecast will not be met. However, the ideas conveyed in this communication can be contemplated and potentially prepare, and act upon depending on your situation.