How Much Cold Will Kill Plants?
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How much cold required to kill a plant is dependent on many factors. Some of the factors involved in cold damage include the actual temperatures, the length of time that the plant is exposed and the growth stage of the plant. Also important is the cold hardiness of the plant, the environmental zone where it lives and the specific growing location.
For a plant to survive and thrive for many years, it must tolerate the year-round conditions in the area it is grown. These conditions include the lowest and highest temperatures as well as the amount and distribution of rainfall. Plant hardiness zone maps were originated by the USDA and overseen by the National Arboretum to indicate where various plants are best adapted to grow. The map divides North America into 11 separate zone with each zone being 10 degrees F warmer or colder in an average winter than the adjacent zone. Fine tuning of the map includes further division into “a” and “b” areas of the regions. USDA hardiness zones are a good tool to indicate where various permanent landscape plants can adapt. The mapping system cannot take into account record breaking cold temperatures, the amount of snow cover or soil drainage.
Careful examination of stems now can help the grower determine the current damage. Darkened or blackened stems and leaves indicate cell damage. Stems and leaves will often have different zones or varying degrees of damage. You may be surprised by what you find. I wanted to cut some gardenia foliage for a flower arrangement after our record breaking cold temperatures. The foliage and the end of the stems on the outside edge of the large canopy were mainly clear. There was some damage to the tiny immature leaves in the top growth at the tips of the stems. (Tender and immature growth is more likely to be damaged by extremes in temperatures.) The leaves turning black on the back were located 6 inches down into the canopy. Both the opposite leaves were beginning to drop off the plant. The snow lightly covering the top of the stems probably insulated them like a blanket from the record breaking cold temperatures.
Hard frost and low temperatures freeze water in the plant cells. This causes dehydration and damage to cell walls. If the sun is shining, cell walls are more likely to be damaged killing leaves and stems. The damage is often described as appearing water soaked or blackened.
Pruning too early to remove damaged leaves and stems opens the plant up for non-damaged leaves and stems to be injured by another cold spell. Wait until early spring to prune damaged leaves and stems. Different plants will need pruning in different ways. Take into consideration the location of the damaged material in the plant and the degree of the damage. Individual branches can be selectively removed if that is all that is needed. Whole shrubs may be cut to the ground if the damage to the majority of the stems is severe. I suspect the entire above ground portions of some shrubs have been severely damaged. The good news is that if all the plant material above ground did not weather the cold there is a good chance it will regrow from the root system. I have seen this for larger older plants including crape myrtle and camellias.
If you need to replant use the USDA hardiness zones to choose plant materials that are included in zones 7b to 8a.