Plants cannot seek shelter when temperatures drop, forcing them to rely on complex internal and external mechanisms to withstand the cold. A plant’s survival is determined by its species, stage of growth, and ability to acclimate to temperature changes. Cold tolerance is a dynamic state, meaning the same plant can be more susceptible to damage at one time of year than another. Understanding these biological limits is the foundation for successfully growing flowers and plants.
How Freezing Temperatures Affect Plant Cells
When temperatures fall below the freezing point, plant cells face two primary threats: mechanical damage from ice and severe dehydration. Water inside the cells contains dissolved solutes, giving it a slightly lower freezing point than the purer water found between cells. Ice crystals typically form first in these intercellular spaces. This extracellular ice creates a low water potential, drawing liquid water out of the cells in a process called freeze dehydration. This desiccation causes the cells to shrink and can damage the cell membranes.
If the temperature drops too rapidly, or if the plant cannot draw water out fast enough, ice can form directly inside the cell’s protoplasm, known as intracellular freezing. The sharp edges of these internal ice crystals physically rupture the cell membranes, which is almost always fatal. This is why a sudden, hard freeze is often more destructive than a gradual temperature decline.
A distinct form of damage, known as chilling injury, affects many tropical plants at temperatures well above freezing, typically between 32°F and 55°F (0°C to 13°C). This injury is caused by metabolic dysfunction when membranes lose their fluidity in the cold, not by ice. Symptoms often include wilting, water-soaked lesions, or a failure of the roots to absorb water, which can lead to death if low temperatures persist.
Classifications of Cold Tolerance
Horticulturalists commonly group plants into three categories based on the lowest temperatures they can tolerate without significant injury. This classification is a general guide for gardeners in determining when to plant or when to intervene with protection.
Tender Plants
The most vulnerable group is known as tender plants, which are sensitive to any frost. They sustain damage or die when temperatures drop to 32°F or slightly below. Most annuals originating from tropical climates, such as impatiens, begonias, and tomatoes, fall into this category.
Half-Hardy Plants
Plants classified as half-hardy possess a limited ability to withstand cold and can generally survive a light, short-duration frost. They tolerate temperatures dropping to approximately 28°F to 25°F, though a hard freeze below this range will cause serious damage or death. Examples include pansies, snapdragons, and petunias, which thrive in the cool weather of early spring and late autumn. They usually require protection or indoor shelter if a prolonged, deep freeze is predicted.
Hardy Plants
The most resilient group is called hardy plants, which can survive prolonged freezing conditions, especially if they have had time to cold-acclimate. Many perennials, shrubs, and trees can withstand temperatures well below 20°F, sometimes surviving conditions as low as -15°F or colder. These plants have evolved mechanisms to allow water to freeze in the intercellular spaces while protecting cell contents from dehydration and internal ice formation. This ability allows many plants to successfully overwinter in temperate climates.
Immediate Strategies for Frost Protection
When an unexpected cold snap is forecasted, gardeners can employ several immediate strategies to help vulnerable plants survive the temperature dip. One effective intervention is to water the garden deeply the day before the freeze event. Moist soil absorbs and holds significantly more heat during the day than dry soil. This stored heat is then slowly released overnight, raising the temperature immediately around the foliage.
Covering plants is another effective defense, as it traps radiant heat rising from the soil and blocks frost formation. Protective materials, such as old blankets, burlap, or commercial frost cloth, should be draped over the plants. Ensure the covering extends all the way to the ground to maximize the heat-trapping effect. If plastic sheeting is used, ensure it does not touch the foliage, as cold can transfer directly through the plastic and damage the plant tissue.
Potted plants are more vulnerable because their roots lack ground insulation and should be moved to a sheltered location. A garage, shed, or placement against the warm wall of a house can offer several degrees of protection from freezing air temperatures. Any protective coverings placed over plants should be removed promptly in the morning once temperatures rise above freezing to prevent overheating and allow for air circulation.