Cold stress causes damage ranging from minor leaf discoloration to tissue death. Every plant species has a unique critical temperature point, and exposure to cold temperatures triggers physiological responses that compromise cellular function. Recognizing these signs quickly is necessary to intervene and prevent irreversible harm. Understanding the difference between damage that occurs above freezing (chilling) and damage that happens below the freezing point (frost) is the first step in correctly diagnosing cold injury.
Symptoms of Chilling Injury
Chilling injury occurs when warm-weather or tropical plants are exposed to temperatures that are cold, but still remain above freezing, typically ranging from 32°F to about 50°F. This type of damage is often delayed and less immediately dramatic than frost damage, making it harder to identify right away. The initial signs may include a dull, water-soaked, or translucent appearance on the foliage, which happens because the cold disrupts the cell membranes and causes them to lose their ability to regulate water movement.
These cellular dysfunctions can also manifest as surface lesions, pitting, or bronzing on leaves and fruit several days after the cold exposure. Tropical fruits like bananas or bell peppers, for example, may develop a grayish-brown discoloration or sunken spots. Warm-season vegetables may also exhibit wilting, as the roots lose their ability to absorb and transport water efficiently, mimicking the symptoms of drought stress. Over time, these weakened tissues become more susceptible to decay and the eventual development of necrotic spots, especially if temperatures do not improve.
Visual Signs of Freezing and Frost Damage
When temperatures drop below 32°F, the damage inflicted is generally immediate and much more visually apparent than chilling injury. This freezing or frost damage is caused by ice crystals forming within the plant’s tissues, which physically rupture cell walls and disrupt the flow of water. The most recognizable sign is the sudden collapse of tender tissues, causing leaves and new shoots to become limp and wilted, sometimes resembling the look of severe drought.
Foliage and vulnerable growing tips often undergo a rapid color change, turning dark brown or black, which indicates irreparable cellular destruction. When the tissue thaws, the texture changes drastically; leaves may become translucent and mushy, similar to a frozen vegetable, or they may dry out and become brittle if the water loss is rapid.
Flower buds and developing fruits are particularly vulnerable to frost, often turning a dark color and failing to open or ripen. For example, the petals of frost-affected flowers may appear blackened, and succulents, which hold large amounts of water, may exhibit translucent patches where the cells have burst. These immediate visual cues are typically confined to the most exposed, above-ground parts of the plant, such as the leaves and young stems.
Assessing Structural and Delayed Cold Damage
Damage from cold exposure is not always restricted to the visible outer leaves and flowers; it can also affect the internal structure and trigger delayed reactions that appear weeks or months later. To determine the extent of damage in woody plants, a simple technique known as the “scratch test” can be performed. This involves gently scraping away a small section of the outer bark on a stem or branch with a fingernail or knife.
If the tissue immediately beneath the bark, the cambium layer, is bright green, the stem is alive and capable of transporting water and nutrients. Conversely, if the tissue is brown, black, or dry, that section of the plant is dead and should be traced back until healthy green tissue is found. Damage to the root system is harder to spot but is suspected if the plant fails to take up water after the soil has thawed, or if it suddenly drops leaves weeks after the cold event.
Other long-term cold effects include sunscald, where bark on the south or southwest side of a tree splits after rapid temperature fluctuations. A failure to bud out in the spring is a delayed sign that the plant’s internal systems were compromised by the winter cold. These structural assessments are necessary to determine if the entire plant is lost or if it can recover from the base or trunk.