Bean plants are warm-season crops, originating from tropical and subtropical regions, which makes them highly intolerant of low temperatures. They are extremely sensitive to any cold exposure, particularly temperatures near or below freezing. This vulnerability defines the growing season for beans, as their ability to thrive is directly tied to consistently warm conditions.
Temperature Thresholds by Growth Stage
The bean plant’s tolerance for cold changes significantly throughout its life cycle, with the earliest stages being the most susceptible to injury. Germination requires soil temperatures that are relatively high, ideally 15°C (59°F) or above. Cooler soil can significantly delay emergence and result in poor stand establishment. Planting into cold, damp soil subjects the seed and emerging root to stress that can compromise the plant before it even breaks the surface.
The seedling stage, when the plant has just emerged, is arguably the most vulnerable period due to its exposed growing point. While a brief period of temperatures between 29°F and 32°F (–1.7°C to 0°C) might be tolerated, any hard freeze is typically lethal. Studies on pinto and white bean seedlings have shown that a temperature drop to -3.25°C (about 26°F) can result in the death of 50% of the plants. If the primary growing point below the cotyledons is damaged, the plant cannot recover and will die.
Mature bean plants remain tender and are easily killed by the first hard frost of the season. The tissues do not possess the physiological mechanisms to withstand the formation of ice crystals within their cells. Temperatures dropping to 32°F (0°C) or slightly below will cause cell wall rupture and result in the death of the above-ground foliage.
Recognizing Chilling and Frost Damage
Two distinct types of cold stress affect bean plants: chilling injury and frost injury, each with unique symptoms and causes. Chilling injury occurs at low but non-freezing temperatures, generally when air temperatures fall below 10°C (50°F) for an extended period. This damage is a physiological disorder that involves the loss of integrity in the plant’s cell membranes.
Visually, chilling injury manifests as general stunting, a notable slowing of growth, and sometimes a yellowing or bleaching of the leaves. The cold stress impairs the plant’s ability to perform necessary metabolic functions, which may also leave it more susceptible to disease. These symptoms are often delayed and become more apparent after the plant has returned to warmer conditions, indicating damage to the internal systems rather than immediate tissue death.
Frost injury, in contrast, results from temperatures at or below 32°F (0°C), where water inside the plant tissues freezes and forms ice crystals. Immediately after exposure, the affected foliage will appear dark and water-soaked, quickly progressing to wilting and browning or blackening of the leaves and stems. This type of damage is permanent, and any tissue that has turned black will not recover.
Strategies for Cold Protection
Protecting bean plants from cold requires proactive planning. The most effective strategy is to ensure seeds are planted only after all danger of frost has passed and the soil has sufficiently warmed, exceeding the 15°C (59°F) threshold for optimum germination. Waiting until the average nighttime temperatures are consistently warm reduces the risk of early-season cold stress.
For unforeseen late-season frosts, physical barriers are the most reliable immediate defense. Covering plants with horticultural fleece or row covers traps the radiant heat rising from the soil, preventing the formation of ice crystals on the foliage. Sheets or blankets can also be used as a temporary measure, draped loosely over the plants and secured at the edges to prevent cold air from entering. It is important to ensure the covering material does not rest directly on the leaves, as this can transfer the cold and cause injury where the two surfaces meet.
Cultural practices can also provide a small buffer against cold damage. Applying a thick mulch, such as wood shavings, around the base of the stems can insulate the core of the plant and stabilize the soil temperature. Additionally, lightly watering the soil in the late afternoon before a predicted frost can be beneficial. Moist soil retains heat better than dry soil, which can slightly warm the air immediately surrounding the plants overnight.