Frost and Freeze Impacts on Plants

Freeze

Figure 1. Freeze damage on older cabbage foliage in a field. (Courtesy: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org)

A freeze occurs when air temperatures drop to 32 degrees Fahrenheit or below. At this point, water inside plant cells can solidify, expand, and rupture the cells, which leads to tissue damage. A “hard freeze” or “killing freeze” is typically further defined as air temperatures of 28 degrees Fahrenheit or lower and can kill most non‑dormant plants.

Plants affected by freezing temperatures can show varying symptoms. This includes water‑soaked or translucent tissues in leaves, stems, or fruit (Figure 1); wilting or limp growth after thawing; and blackened or browned (necrotic) areas caused by cell rupture and tissue breakdown (Figure 2). Death or survival of annual flowers and vegetable crops depends on species hardiness, variety, plant age, the specific plant parts damaged, and duration of freezing temperature exposure. For example, some fruit tree species that are in bloom and experience freezing air temperatures can suffer damage to their delicate floral tissues, killing blossoms outright, preventing pollination, and eliminating that season’s fruit crop (Figure 3).

Frost

Figure 4. Frost as visible ice crystals on a tulip at a western South Dakota cut flower farm. (Courtesy: Sarah Schwede, Joyful Blooms Flower Farm)

Frost forms when air temperatures fall close to freezing (loosely defined within 33 to 36 degrees Fahrenheit) while the surface temperature of plants, soil, turfgrass, vehicles, or other objects drops to 32 degrees Fahrenheit or below. When this happens, moisture in the air condenses into liquid dew or directly forms ice crystals. Surfaces cool more quickly than the surrounding air, which is why frost can develop even when the recorded air temperature may not reach 32 degrees Fahrenheit.

The dew point is an important factor in frost risk; it is the temperature at which air becomes fully saturated with moisture. When the dew point is above 32 degrees Fahrenheit, frost formation is less likely because moisture condenses as liquid rather than freezing. When the dew point is below 32 degrees Fahrenheit, frost is more likely, especially in dry air, which cools faster than humid air. In open fields with bare soil, radiative frost can occur at the height of the plant canopy during clear, still nights as heat is lost from the soil surface, increasing the chance of cold injury.

Similar to freeze, frost damage is dependent on species hardiness, variety, plant age, the specific plant parts damaged, and duration of surface freezing temperature exposure. Frost injury tends to be more superficial, localized, and less severe than that of freeze, with symptoms like light bronzing or bleaching on foliage, marginal burns, and small scattered necrotic spots (Figure 6). A few minutes of exposure may cause little harm, while several hours can lead to significant injury. Beginning in May, specialty crop growers and hobby gardeners should monitor daily and weekly forecasts, paying attention to dew points and expected early‑morning lows to guide decisions about planting tender crops or providing frost protection when needed.