The question of when a plant freezes is deceptively complicated because the answer is not a single temperature like the 32°F (0°C) freezing point of pure water. Plant cells contain dissolved sugars and salts, which naturally lower the freezing point of the internal fluids. The actual temperature at which damage occurs depends on the plant’s species, its state of growth, and the speed at which the cold arrives. Understanding these biological mechanisms and specific plant tolerances is key to protecting sensitive vegetation.
The Science Behind Plant Freezing
The mechanism of cold damage relates less to ambient air temperature and more to how ice crystals interact with plant tissues. When temperatures drop, ice crystals typically form first in the extracellular spaces, the gaps between plant cells. This initial formation is called ice nucleation, sometimes triggered by specific bacteria or particles on the plant’s surface.
Many plants possess an adaptation called supercooling, which allows the water inside their cells to remain liquid even below 32°F (0°C). This ability is a primary defense against freezing, though it is limited, often failing once temperatures drop to around 23°F to 26.6°F (-5°C to -3°C).
The most common cause of lethal damage is cellular dehydration. As ice forms outside the cells, it draws liquid water out of the cells into the extracellular ice mass. This concentrates the cellular contents and causes the cell membranes to shrink and suffer damage. If the temperature drops too rapidly, ice crystals can form directly inside the cells, causing irreparable cellular rupture, which is lethal.
Categorizing Plant Vulnerability
Plant vulnerability to freezing is categorized into three groups based on their ability to withstand low temperatures.
Tender or tropical plants are the most susceptible, often showing signs of chilling injury before freezing occurs, with damage beginning below 50°F (10°C). For these sensitive varieties, critical damage occurs once the air temperature reaches 32°F (0°C), especially if the cold exposure is prolonged.
Semi-hardy plants, sometimes called half-hardy, have greater tolerance and can typically survive a light frost. These plants are usually critically damaged when temperatures fall into the mid-twenties, generally 25°F to 28°F (-4°C to -2°C). This mid-range group includes many annuals and common bedding plants.
Hardy or dormant plants, such as established perennial trees and shrubs, are adapted to handle much colder conditions. These plants undergo cold acclimation, or hardening, in the fall, which increases their ability to supercool and withstand cellular dehydration. Properly acclimated hardy plants can often survive temperatures well below 15°F (-9°C). The speed of the temperature drop is a factor, as a sudden freeze without prior acclimation can severely damage even hardy plants.
Strategies for Cold Protection
Proactive measures taken before a freeze warning significantly increase a plant’s chances of survival. One effective strategy is to thoroughly water the landscape a day or two before the cold arrives. Moist soil holds and re-radiates heat more efficiently than dry soil, providing a protective thermal blanket around the plant roots and base.
Physical covering traps ground heat and shields plants from cold air and wind. Covers, such as blankets, sheets, or commercial frost cloths, should extend to the ground to ensure heat remains trapped. The material should not directly touch the foliage, so using stakes or frames to create a tent-like structure is beneficial.
Potted plants should be relocated indoors to a sheltered space like a garage or basement where temperatures remain above freezing. For sensitive in-ground plants, applying a thick layer of organic mulch, such as straw or wood chips, around the base helps insulate the root zone.
Recognizing and Treating Freeze Damage
The immediate aftermath of a freeze shows characteristic signs of damage, often appearing as wilting, browning, or a dark, water-soaked, translucent look to the leaves. This appearance results from the ice formation and subsequent cellular damage within the foliage.
It is important to resist the urge to immediately prune away the damaged plant tissue. The dead material serves as a layer of natural insulation, protecting the lower stems and dormant buds from potential subsequent freezes. Removing the dead foliage too early can expose sensitive, living tissue to further damage and may stimulate tender new growth that will be killed by the next cold snap.
The correct time to prune is after the danger of the last frost has completely passed and new growth has started to emerge. Waiting until this point allows for an accurate assessment of where the living tissue remains, which can be confirmed by gently scraping the bark to check for green tissue underneath. If the soil is frozen following the event, check the plant’s water needs, as frozen ground prevents water uptake. A gentle watering can help defrost the soil and provide needed moisture for recovery.