The temperature of 39 degrees Fahrenheit is a serious concern for many plants, even though it is above the 32°F freezing point. While 39°F (approximately 4°C) will not cause immediate freezing damage, it marks a threshold where cold-induced stress begins to occur. The potential for harm depends entirely on the plant’s natural history and its ability to tolerate low, non-freezing temperatures. Understanding this vulnerability requires examining the mechanisms of cold damage and recognizing which plants are most susceptible to chilling stress.
Why 39°F is a Critical Threshold for Plants
The temperature range around 39°F triggers physiological damage in plants native to tropical and subtropical climates. These species have not evolved the necessary cellular defenses to handle prolonged exposure below 50°F (10°C). At 39°F, the primary threat is chilling injury, which disrupts the plant’s internal functions rather than causing ice crystals.
Sustained exposure in the 35°F to 45°F range, especially overnight, slows metabolic processes. This cold stress impacts the plant’s ability to absorb water and nutrients, leading to wilting even when soil moisture is plentiful. If the air is windy or damp, the cooling effect on the plant tissue is exacerbated, intensifying the stress. For many garden vegetables and ornamental plants, 39°F represents a clear danger zone demanding protective action.
Understanding Cold Damage: Chilling vs. Freezing Injury
Plant damage caused by low temperatures is categorized into two distinct types, differentiated by temperature range and cellular mechanism. Chilling injury occurs above 32°F, typically between 32°F and 59°F, and affects tropical and subtropical species. At these temperatures, the lipid components of cell membranes begin to solidify. This loss of fluidity impairs the membrane’s function, preventing the normal exchange of water and nutrients and disrupting photosynthesis and respiration.
The physical symptoms of chilling injury often appear as water-soaked spots, surface pitting, discoloration, and a general collapse of the foliage. This damage is a slow, physiological disorder that may not be visible until days after the cold exposure. In contrast, freezing injury occurs below 32°F and involves the formation of ice crystals within the plant tissues, physically rupturing cell walls. Freezing damage is swift and often irreversible, causing the plant tissue to immediately turn black, brown, or mushy.
How Plant Type Determines Vulnerability
The vulnerability of a plant to 39°F depends entirely on its biological origin and its tolerance to cold temperatures. Plants are generally grouped into categories based on their sensitivity to these low, non-freezing conditions.
Tender or Tropical Plants
The most susceptible are tender or tropical plants, which originate in warm climates and suffer severe chilling injury at 39°F. Common examples include summer vegetables like tomatoes, peppers, and cucumbers, as well as bedding plants like basil and impatiens. These plants can be stunted or killed by exposure to temperatures below 45°F.
Semi-Hardy Plants
Semi-hardy plants exhibit a greater tolerance but still require attention when temperatures approach 39°F. This group includes cool-season crops such as lettuce, cabbage, and pansies. They can often withstand a brief dip into the mid-30s but will struggle with prolonged exposure or severe wind chill. Their growth will be significantly slowed, and their ability to recover from other stresses is reduced.
Hardy Plants
The most resilient are hardy plants, typically native to temperate zones, having evolved sophisticated mechanisms to handle cold. Established trees, shrubs, and winter annuals like calendula are generally unaffected by 39°F. They may even be in a process of cold acclimatization, or “hardening off,” which increases their resilience to future freezing events.
Immediate Steps to Protect Plants from Cold
When 39°F is forecasted, especially overnight, gardeners must take proactive steps to mitigate the risk of chilling injury for sensitive species.
- Move container plants susceptible to cold to a protected location, such as a garage, shed, or covered porch, where radiant heat offers a buffer. Placing them snugly against a warm, south-facing wall can also provide protection.
- For plants remaining in the ground or large containers, covering them is the most effective method to trap heat radiating from the soil.
- Use materials like old sheets, blankets, or commercial frost cloths, ensuring the cover extends all the way to the ground to completely enclose the plant and the soil surface.
- Avoid using plastic sheeting directly on foliage, as the cold can transfer through the plastic and damage the leaves where contact occurs.
- Thoroughly water the garden soil before sunset, as moist soil retains significantly more heat than dry soil, releasing this warmth slowly throughout the cold night.