North Central Florida will reliably experience a freezing event at some point during the winter months. As a consequence, some gardens can be more affected or even killed by freezing temperatures.
Freeze injury is generally caused by the formation of tiny ice crystals inside plant tissue. These ice crystals can result in damaged cell membranes and cell walls – this aggregate damage is responsible for killing or injuring a plant. The extent of damage or risk of plant death is determined by several key variables. Firstly, the susceptibility of a plant to freeze damage is the most important factor. As one might assume, plants originating from the tropics are notably more susceptible to freeze damage than plants originating from temperate climates. The extent of freeze damage also includes how extreme the cold was, how long the freezing temperatures lasted, maturity and size of the plants, and the degree of protection, if any, that they received during these freeze events. Plants that experienced gradual colder temperatures, or cold acclimation, are less likely to experience as severe freeze damage as plants that experience a sudden shift from warm weather to deep freezing temperatures.
Not all freeze events are the same nor how they affect cold sensitive plants. There are three scenarios which can occur in cold weather. The first is simple chilling damage which can be seen on tropical plants even before freezing temperatures have occurred. The second two are the most common: radiational and advective freezing. Radiational freezing is often associated with the occurrence of frost and is most likely to happen on very clear nights with little wind. Essentially, what happens is that heat from the plants and surrounding area radiates out into the night sky. Plants may freeze even if air temperatures are slightly above 32°F. In a radiational freeze event, plants can be protected by covering with fabric or placing plants under a roof or dense tree canopy. The last and most dangerous scenario to cold sensitive plants is the advective freeze event – also known as a hard freeze. This occurs when cold air rapidly blows in from the north, causing a quick drop in air temperature. Advective freezing is particularly deadly to cold sensitive plants because a plant is more likely to be completely killed compared to radiational freeze events. Complete protection from the freezing air is necessary to protect plants when advective freezing is expected. In this situation freeze protection of cold sensitive fruit trees and shrubs using sprinkler irrigation systems, if applicable, may be necessary.