50 degrees Fahrenheit (10 degrees Celsius) is a significant biological threshold for many plants. While this temperature is well above freezing, it falls within the range known as “chilling temperatures,” which can cause stress and injury to sensitive species. The actual impact depends entirely on a plant’s evolutionary background and its natural, adapted climate.
The Primary Factor: Plant Origin and Classification
For plants originating in tropical or warm-season climates, 50°F is too cold. This includes common garden vegetables like tomatoes, peppers, basil, and squash. Exposure to this temperature causes growth to slow or stop entirely. Their metabolic systems are optimized for much warmer conditions, and cell membranes begin to lose fluidity below 55°F, disrupting essential cellular functions.
Cool-season plants, by contrast, are suited for temperatures around 50°F. Vegetables like kale, spinach, cabbage, radishes, and parsnips thrive in this range. For these species, 50°F is often an optimal temperature that promotes sweet flavor development and crispness. Their cellular structure allows them to maintain normal function and tolerate brief drops slightly below freezing.
For hardy perennial plants, trees, and shrubs, 50°F causes no harm. Species like hostas, peonies, and many coneflowers are genetically programmed to withstand far colder temperatures. They use this chill to signal the onset of dormancy, which simply slows their growth until true winter conditions arrive.
Physiological Impact of Chilling Temperatures
When temperatures drop to 50°F, metabolic processes like photosynthesis and respiration slow dramatically below 55°F. This means the plant cannot produce or use energy efficiently for growth or repair. This metabolic slowdown causes sensitive warm-season plants to cease growing when temperatures remain low.
Chilling injury occurs above freezing, typically between 32°F and 55°F, and is distinct from freeze damage. Symptoms on sensitive plants can manifest as water-soaked spots, wilting, or discoloration of the foliage. This injury is caused by the physical stiffening of cell membranes, which impairs the transport of compounds across the cell barrier.
Cold soil profoundly impacts the root system, impeding its ability to absorb water and essential nutrients. Specifically, the uptake of macronutrients like phosphorus and potassium is significantly reduced. This can result in temporary nutrient deficiency symptoms appearing in the leaves. When roots cannot perform their function, the plant becomes dehydrated and nutrient-starved.
How Environmental Conditions Modify Cold Stress
Environmental variables can either intensify or alleviate the stress caused by 50°F. Cold, dry wind is extremely damaging because it rapidly accelerates water loss from the leaves, leading to desiccation and shriveling. Since the plant’s roots are already impaired in cold soil, it is unable to replace the moisture lost to the wind.
A few hours at 50°F is usually tolerable for most marginal plants. However, several consecutive days or nights at this temperature compound the metabolic stress. This prolonged exposure can lead to sustained growth inhibition or irreversible chilling injury.
Sunlight and cloud cover also modify cold stress. Cold, cloudy days are difficult because the lack of sunlight prevents the plant from generating enough energy through photosynthesis to cope with the low temperature. Conversely, a cold, clear night followed by an intense, sunny morning can cause photoinhibition. This occurs when the cold-stressed photosynthetic machinery is overwhelmed by high light intensity.
Practical Steps for Cold Weather Protection
Gardeners can take steps to protect vulnerable plants when cold weather approaches. A counterintuitive but effective measure is to thoroughly water the soil around sensitive plants before the temperature drops. Moist soil holds and radiates heat more effectively than dry soil, creating a slight thermal buffer that helps insulate the root zone.
Potted plants should be moved to a sheltered area, such as against the south-facing wall of a house or grouped tightly together. Masonry and concrete absorb heat throughout the day and slowly radiate that warmth back out at night, creating a localized microclimate that can be several degrees warmer.
For plants that cannot be moved, use temporary covers like frost blankets or sheets to trap residual ground heat. The covering material needs to extend all the way to the ground and be anchored down to prevent heat from escaping. Use stakes or hoops to ensure the material does not directly touch the foliage, which can cause moisture to condense and freeze.
Be aware of “frost pockets” in low-lying areas where cold air, which is denser than warm air, naturally settles. Plants placed in these topographical dips will experience temperatures significantly lower than the rest of the garden.