The arrival of colder weather signals a significant transition for gardeners and property owners, marking the end of the warm growing season. This change is defined by the “first frost,” an event that serves as a natural cue to prepare for winter conditions. The first frost determines the fate of countless tender plants that cannot survive freezing temperatures. This seasonal marker requires immediate action to safeguard property and maximize the final harvest of the year.
What Constitutes the First Frost
Frost formation is a process of deposition, where water vapor in the air directly turns into a solid ice crystal on surfaces. This event requires specific atmospheric conditions, including clear skies, calm winds, and adequate moisture near the ground. Clear nights allow heat to radiate quickly away from the earth’s surface, causing temperatures to drop rapidly, a process called radiational cooling. Frost can form even when the air temperature measured by forecasts is slightly above freezing, often in the 33°F to 36°F range. This is because the ground and plant surfaces cool faster than the surrounding air, leading to a “ground frost” where the surface temperature dips below 32°F.
The Crucial Difference Between Frost Types
Cold weather events are categorized by severity to predict the level of damage to vegetation.
A “light frost” occurs when temperatures fall briefly into the 29°F to 32°F range, damaging sensitive annual plants such as tomatoes, peppers, and beans. This initial frost often appears as a white, crystalline layer, and hardier plants can recover quickly.
A “moderate freeze” is defined by temperatures falling to the 25°F to 28°F range, which severely damages or kills most seasonal flowers and vegetables. When temperatures drop to 24°F or lower, this is considered a “hard freeze,” which destroys nearly all unprotected vegetation and can even harm the roots of some perennial plants.
A “black frost” is not visible as white ice crystals because the air is too dry, but the temperature is low enough to freeze and kill plant cells. In a black frost, the internal plant tissues freeze and die, often turning black and wilted the following day.
Preparing for the End of the Growing Season
Once a frost prediction is issued, the first priority is to harvest all heat-loving crops that cannot tolerate freezing temperatures. This includes picking tomatoes, squash, cucumbers, and peppers, even if the fruit is still green, as they will be ruined by the cold. Green tomatoes and some squashes can still ripen indoors once they are removed from the vine before the freeze. Sensitive container plants and houseplants that spent the summer outside should be moved into a garage, shed, or indoors to protect them from the cold.
For annuals or late-season vegetables that you wish to save, covering them with a frost cloth, burlap, or old sheets can provide a few degrees of protection. It is important that the covering extends all the way to the ground to trap heat radiating from the soil, creating a warmer microclimate around the plant. Property maintenance is also necessary, requiring the draining of irrigation systems, disconnecting garden hoses from outdoor spigots, and storing them to prevent pipes from freezing and bursting. Finally, pruning indeterminate tomato plants by topping them off encourages the remaining energy to ripen the last fruit before the season ends entirely.