Understanding Plant Hardiness and Cold Stress

By Erin Marissa Russell

It’s likely that cold damage can be so catastrophic for plants because of the wildly transformative effect cold stress has on a plant’s cells. The plant’s vital metabolism is disrupted and the system must adapt, with some processes making a speedy recovery while others take longer to heal. Each cell experiences upheaval within its interior and in every molecule that the cell contains.

What temperature will cause damage to my plants from cold stress ?

Cold stress is the result of a plant being exposed to temperatures below the level of cold it can withstand, and this threshold is not set in stone—it varies from one plant to another. Not all plants are subject to cold damage. For example, plants native to very cold climates are likely to be extremely cold hardy, with the ability to withstand frosts and freezes without any resulting stress or damage to their cells. However, more tropical plants may start to show signs of stress if the temperature drops lower than 50 degrees Fahrenheit (10 degrees Celsius) for too long. 

Despite the variation in how plants respond to cold, there are some rules of thumb that gardeners can apply to plants in general. Temperate fruit plants are susceptible to cold stress damage in the temperature range from 32 degrees Fahrenheit (0 degrees Celsius) to 39.2 degrees Fahrenheit (4 degrees Celsius). Subtropical fruit plants can experience damage once the temperature drops to 46.6 degrees Fahrenheit (8 degrees Celsius). Tropical fruit plants start to see damage at 53.6 degrees Fahrenheit (12 degrees Celsius).

Types of Freezing Damage: Intracellular Versus Extracellular Freezing

Freezing damage can cause either intracellular or extracellular freezing. In intracellular freezing, ice crystals form and, once they grow large enough to be seen with a microscope, the crystals kill plant cells by shattering the structure of the cell’s interior. In extracellular freezing, water inside a plant cell is hijacked and used instead to form ice crystals within the cells, creating a water-vapor deficit that dehydrates the plant. Water inside of a plant can sometimes resist forming solid ice even at temperatures as low as -52.6 degrees Fahrenheit (-47 degrees Celsius).

Categorizing Cold Stress: Chilling Injury Versus Freezing Injury

Plants can sustain two types of damage to their tissues as a result of cold stress. Temperatures that range from freezing, or 32 degrees Fahrenheit (0 degrees Celsius), up to 68 degrees Fahrenheit (20 degrees Celsius) inflict chilling injury. Plants that experience chilling injury may exhibit problems with seed germination, issues developing flowers or fruit, low yield, or shortened shelf life in storage. 

When damage is minor and the temperatures a plant survived were not lethal, the plants can usually recover from chilling damage. Plants tend to sustain low damage or no damage at all when temperatures descend slowly and gradually, as the smoother transition allows the plant to become somewhat accustomed to the cold, developing a thick skin against cold damage.

When the temperature dips underneath the freezing point (32 degrees Fahrenheit/0 degrees Celsius), plants can sustain freezing injury. The severity of a plant’s chilling injury can tell you a great deal about how it will handle a freezing injury. Plants that are especially sensitive to temperatures in the chilling injury range may die as a result of the slightest touch of frost. Three categories can be used to classify the severity of a plant’s sensitivity to cold stress: frost tender, frost resistant, and cold hardy. 

How much cold can my plant withstand? An Explanation of the Cold Hardiness Categorization of Plants

Frost tender plants are the most susceptible to cold damage, and short instances of freezing temperatures can prove fatal for them. Plants with this high sensitivity to cold tend to develop the water-soaked appearance and floppy texture we mentioned earlier, and they dry out quickly when exposed to warmer weather, resulting in a speedy death. Plants including beans, corn, maize, rice, and tomatoes fall into this category.

Frost resistant plants are moderately sensitive to cold temperatures. However, if the transition to cold weather is eased with a gradual period of cold acclimation (also called hardening off) for plants in this second category, they can survive freezing weather and ice in their cells. Plants including broad-leafed summer annuals (which can die after exposure to temperatures just under freezing) and perennial grasses (which survive exposure to temperatures down to -40 degrees Fahrenheit/-40 degrees Celsius) can be described as frost resistant. 

The lower temperatures dip, the more intracellular water moves out of the cells of frost resistant plants and joins the extracellular ice, and the more dehydrated the plant becomes. Stress resulting from this lack of hydration eventually damages the plasma membrane of cells in frost resistant plants beyond repair. If the intracellular water does not freeze into ice once the temperature has dropped between 26.6 degrees Fahrenheit (-3 degrees Celsius) and 23 degrees Fahrenheit (-5 degrees Celsius), once the supercooled water does get cold enough to form intracellular ice, there is danger of the crystals materializing in an instant, killing the cells.

Truly cold hardy plants tend to be woody temperate varieties. Many of these plants survive temperatures that would be fatal to frost resistant or frost tender species because their intracellular water can go through deep supercooling without forming the ice crystals that destroy cells and dehydrate plants. That said, the plants with the highest resistance to cold damage can tolerate temperatures down to an astonishing -320.8 degrees Fahrenheit (-196 degrees Celsius), and they do not use supercooling of intracellular water to survive.

The lowest temperature a plant will be able to withstand depends on the plant’s genetic makeup as well as factors like whether the plant’s transition to freezing weather was eased with cold acclimation, how acclimated to the cold a plant has been before the freeze, how cold it gets and how quickly the temperature drops. Other factors that influence a plant’s ability to survive cold weather include the age of the plant, the climate so far in the growing season, and how well the plant fared as a seedling. 

To know for sure the temperature at which your plants will begin to sustain cold damage, you should search for this information about the specific varieties of plants you are growing in your garden. If you’re looking for ideas about how to prevent cold damage and protect your plants from chilly temperatures, there are a few articles listed below that you should take a look at next.

Learn More About Plant Cold Hardiness and Cold Stress

Hardening Off Plants: Common Reader Questions and Answers

How to Protect Plants in the Winter

How to Start Seeds Indoors in the Winter

How to Store Plants in the Winter

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