First frost
/ˌfɝst ˈfrɔst/
Definition
First frost refers to the first occurrence in a season when air at ground level drops to 32°F or below long enough for ice crystals to form on exposed surfaces. In gardening and plant biology, it marks the start of frost risk for tender plants, unlike a light freeze, which may involve slightly lower temperatures and broader tissue damage. Timing varies by local climate, elevation, and site conditions.
Effects on Plants
- Damage to tender growth: After first frost, cells in leaves and soft stems can rupture as ice forms, causing water-soaked tissue, blackening, and collapse in frost-sensitive annuals and tropical plants.
- Signal for dormancy: In many temperate perennials and woody plants, first frost often coincides with leaf drop and the shift into winter dormancy, when growth slows and energy is stored in roots, bulbs, or stems.
- Harvest timing: Gardeners use first frost as a practical cutoff for harvesting warm-season crops and protecting houseplants moved outdoors, since tomatoes, basil, and many ornamentals are easily injured by low temperatures.
- Not equal to a hard freeze: Light frost may only mark exposed surfaces, while colder events can kill deeper tissues and roots. Plant survival depends on species, plant maturity, site exposure, and whether temperatures stay low for long.
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