Calhoun County, AL – Horticulturalists at the Extension Office answer many calls every year when a late frost is expected about how to protect plants or blooms from the cold. A grower needs to think about frost protection long before the planting process.
Site selection plays an important role in frost protection. Cold air settles in low laying areas, and planting on a site higher in elevation than the surrounding areas will be beneficial to reduce/avoid the risk of cold damage to the plants. Planting on steep slopes may be hard to manage, but gently rolling hills work well. Sometimes a grower does not have a choice, but we often get asked if a northern or southern facing slope is best. Fruit may bloom earlier on a south facing slope as it warms up faster, but the blooms can also be protected from the north wind on a south facing slope. In contrast, plants on a north facing slope may be slow to begin blooming in the spring, but can get damaged by late spring cold temperatures.
Pay attention to the trees and hedges around the field. A fence row of hedges can trap cold air and not allow it to drain to a lower area which can cause a frost pocket to form. If hedges and woods are creating frost pockets, cutting wide swaths in the woods or hedges will allow the cold air to drain away. Keep in mind that while trees planted on the downhill side of an orchard can trap cold air in the orchard, trees such as pine, planted/growing/located on the north or northwest side of an orchard can block cold air entering an orchard.
Chill hours, or the plant’s winter chilling requirement, play a critical role in frost protection and should be considered before purchasing plants. We have a publication that explains winter chilling in detail on our web site at www.aces.edu. For example, every peach variety does not share the same chilling requirement. A grower needs to plant the fruit crop varieties that match the chilling accumulation in their location. It is common for a low chilling variety to bloom early in higher chilling locations, leading to cold damaged blooms and fruit loss.
Pruning early in the year can stimulate plant growth, delaying pruning until later in the spring can sometimes save a crop. If time allows, pruning fruit crops while they are in bloom will not hurt the plant and can save a crop.
Overhead irrigation can be used on crops to protect blooms from cold damage. The sprinklers need to be turned on before the temperature reaches a certain point and turned off after the ice melts. Heat is released upon freezing, maintaining plant temperatures at 32 °F under a protective ice layer. It is effective for radiation frosts, typically down to 23-27oF provided application is consistent and continuous, which could last for days. A grower would need a good supply of water in order for this to work, and it is not effective when temperatures drop below a certain point or during windy conditions. Overhead irrigation has been used for freeze protection by many different fruit growers, such as strawberry and blueberry producers. This is not something a home producer would typically do, but some farmers practice this method.
While overhead irrigation was very commonly used by strawberry growers in the past, it is recently more common to see the use of row covers. Row covers come in a variety of widths and thicknesses and are used to cover crops such as strawberries. Row covers can be used for more than one season, but they can tear as growers cover and uncover a crop. Row covers are not commonly used on tree fruits and blueberry crops on a large scale because the woody stems will tear holes in the cover. However, smaller growers have made frames over crops such as blueberry and citrus to hold the row cover off of the woody stems with success. On a small scale, old blankets or sheets can be used to protect plants. Some growers may try to use clear plastic or a tarp to protect plants. Plastic and tarps can heat up quick in the morning sun, and those high temperatures under the plastic can damage plants.
Low tunnels and high tunnels can be used for cold protection as well. A tunnel is a structure covered in plastic. High tunnels are made to walk in or even drive a tractor in, while low tunnels are typically much smaller and cover a row of plants. Tunnels can get too hot, and they also can get cold on certain nights. There are management options a grower can use to get the heat out, and things can be done to protect plants further in a high tunnel.
As with anything, the different levels of frost protection have varying associated costs. The high tunnel offers very good frost protection but comes at a higher price, while choosing a peach variety with a chill hour requirement that matches the growing location will not cost any additional money. If you need additional information on frost protection, give us a call at your local Extension Office.
