Frost appears as a delicate dusting of white ice crystals on outdoor surfaces near the ground. This phenomenon occurs when the temperature of an object’s surface drops below the freezing point of water, causing water vapor from the air to condense and freeze directly onto that surface. While visually appealing, this weather event signals danger for many garden plants.
The Conditions Required for a Light Frost
A light frost typically occurs when the air temperature measured at standard height dips to between 33°F and 36°F, while the temperature directly at the ground surface falls to 32°F (0°C) or slightly below. This surface temperature range is cold enough to kill tender plants. This event is far less severe than a hard or killing freeze, which involves temperatures dropping to 28°F (-2.2°C) or lower for several hours, causing widespread damage to most vegetation.
The formation of frost relies heavily on radiative cooling, which is most efficient on clear, calm nights. Without cloud cover to trap heat, the ground and plant surfaces rapidly radiate stored warmth directly into the atmosphere after sunset. This cooling effect causes the surface temperature to fall faster than the air temperature measured higher up. Calm winds are also necessary because strong winds would mix warmer air aloft with the colder air near the ground, preventing the surface from reaching freezing temperatures.
This mechanism explains why frost can appear even when a weather forecast predicts an overnight low slightly above freezing. The surface of the plant or the ground is the actual site of the temperature drop, which can be several degrees colder than the official air temperature reading. The presence of sufficient atmospheric moisture is also a factor, as water vapor must be available to form the ice crystals when the surface temperature reaches the dew point.
Protecting Vulnerable Plants
Protection is necessary for plants that cannot tolerate freezing water in their tissues, particularly tender annuals, tropical species, and new spring growth. Warm-season vegetables like tomatoes, peppers, and basil are highly susceptible to damage, often showing blackened or distorted foliage immediately afterward. Conversely, hardy plants, such as established perennials and cool-season vegetables like kale and spinach, are generally safe and can survive minor cold events without intervention.
One effective method for safeguarding vulnerable plants is covering them with a physical barrier. Lightweight materials like old bed sheets, blankets, or horticultural fleece are recommended because they trap warmth radiating from the soil overnight. These covers should be draped loosely over the plants and extended all the way to the ground to trap heat, often supported by stakes to prevent the material from touching the foliage. Place these coverings before sunset to trap the day’s warmth and remove them the following morning once temperatures have risen.
Another simple but effective technique is to water the garden soil deeply in the late afternoon before a frost is expected. Moist soil holds significantly more heat than dry soil, a concept known as thermal mass, and this stored heat slowly radiates upward during the night. The release of this warmth can slightly raise the air temperature immediately surrounding the plants, often providing just enough insulation to prevent freezing.
For container gardens, the easiest and most reliable solution is to move the potted plants to a sheltered location. A garage, covered porch, or placing them against a warm exterior wall of a building provides enough thermal protection to prevent damage from a light frost. This allows sensitive plants to continue their growing season after the brief cold snap has passed.