Plant Health Alert – Protecting Plants From Severe Cold Weather
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Collapse ▲Cold injury, the damage caused by freezing temperatures, can occur on fruit, stems, leaves, trunk, and roots. Water inside these plant parts can freeze and expand, tearing cell walls causing them to leak. This damage may go unnoticed until the plant fails to come out of dormancy in the spring.
Temperature is the critical component in the severity and type of damage plants may suffer. Of course, tropical plants such as house plants or citrus trees will die at freezing temperatures. Temperatures at or just below 32 degrees may cause superficial damage on plants that are tender such as new spring leaves on Japanese maples. Temperatures at 28 degrees F or lower can severely damage tender plants killing tender smaller woody branches but leaving larger branches intact. Temperatures in the single digits especially if sustained over several days can kill even hardy plants to the ground. Often plant roots survive and the tree or shrub has to start over, regrowing from the roots.
This Japanese Maple was damaged by a late frost. Tender leaves near the top of the plant were ‘burned’ by the freezing temps. Non-native plants are not adapted to local weather extremes and are often killed by extreme cold or heat.
Types of Cold Injury
There are three types of cold injury. The first I call a burn. Oftentimes, cold-damaged plant parts will become mushy and turn brown or black. Over time, the damaged leaves or stems will dry out and appear to have been burned with a torch. Sunken areas may appear on branches and trunks and the bark may peel.
The second type of cold damage, desiccation, can be caused by winter winds. Cold air does not hold moisture like warm air (thus summer humidity and winter dry air) and can dehydrate plants if it is sustained for long periods of time. Leaves may dry up at the edges and eventually turn completely brown.
The third type of winter damage is wood splitting or frost cracks. This damage occurs on stems and branches. On particularly cold nights, water in the cells just beneath the bark of trees and shrubs freezes. When the sun hits these areas the next day, the water thaws quickly killing the cells and splitting the wood. Eventually, longitudinal (lengthwise) cracks may appear. Often these are not evident until the following summer.
Cold Hardiness
It has been a while since gardeners locally have had to consider cold hardiness in plants. Western North Carolina has not experienced a severe cold snap in a while. Gardeners have planted many plants that may not be cold hardy over the last decade. Crape myrtles, camellias, gardenias and other plants that prefer warmer climates could suffer. These plants may get frozen to the ground or even die due to severely cold weather.
Native plants are well adapted to cold. Rhododendron leaves curl tight to protect themselves during extreme cold.
Preventing Cold Damage
Preventing cold damage to plants begins by choosing plants that are native to our area or acclimated to the temperatures we experience. The USDA has created climatic zones based on average coldest temperatures for the regions of the US. Plants are categorized according to the areas in which they are hardy. Gardeners can choose plants that will perform in their area. For a map of the USDA hardiness zones they may visit the USDA website.
How to protect tender plants during the winter:
- Plant site selection can be crucial to winter hardiness of some plants. Planting under a tree canopy or near the southwest side of a home to maximize evening winter sun can protect plants. Planting on shady northern slopes can lead to frozen plants.
- Native plants are generally better adapted local weather extremes.
- Plant nutrition can play a minor role in freezing. Maintaining proper fertility levels can lower the temperatures a plant can tolerate by five degrees.
- Windbreaks can be constructed or planted to block frigid winter winds.
- Plants can be covered with fabric on cold nights. Be sure to remove the fabric eventually based on weather conditions.
- Finally, watering plants keeps them hydrated and prevents plants from drying due to cold air and frozen soil.