Mark Bailey: Garden freeze damage and recovery
We recently experienced several nights in a row of freezing temperatures that left few cold-sensitive plants untouched. Now that the damage has been done, the next step is to determine what should happen now and for the rest of the winter.
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 that can occur in cold weather. The first is simple chilling damage that 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 often is 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.
If freeze damage has occurred and the plant looks dead, a natural reaction might be to immediately prune away the cold-damaged vegetation. Removal of the dead vegetation could make the problem even worse for a plant that’s already hanging onto life by a thread. Dead vegetation can help to provide some cold protection to still-living tissue lower down the plant. While it may not look aesthetically pleasing, it’s generally best to leave cold-damaged plants alone until warmer weather arrives. Once the threat of freezing temperatures has passed, that’s when a careful assessment of freeze damage should happen. Dead leaves will be notably wilted or curled, or have already dropped off if they are dead. Branches and stems do not show freeze damage as readily as leaves, so wait till new growth occurs or a simple “scratch test.” Use a fingernail to scratch off a small spot of bark: living tissue underneath will be bright green, while dead tissue will range from tan to black. At the point where living and dead tissue meet, the dead branches can be pruned back to living tissue.
Fortunately, there are many plants that can be grown in even the coldest Florida winters. Cold-tolerant fruit trees include mulberries, peaches, plums, persimmons, muscadine grapes and loquats. Most leafy green garden plants, such as kale, broccoli, turnips, spinach, etc., are well adapted to winter conditions.
For more information, call the Marion County Extension office at 671-8400 or email ironhill@ufl.edu.
— Mark Bailey is the Sustainable Agriculture and Food Systems Extension Agent for UF/IFAS Extension Marion County. For more information, contact the Marion County office at 671-8400. The Extension Service is located at 2232 NE Jacksonville Road, Ocala, FL 34470.
This article originally appeared on The Gainesville Sun: Mark Bailey: Garden freeze damage and recovery
