Light Frost: What It Is, Which Plants It Hurts, and How to Protect Them

A light frost — air below 32°F, ground unfrozen — seems minor, but it can devastate tender plants. Tropicals, subtropicals, and young seedlings are especially vulnerable. Ice crystals form inside cells; when they thaw, they rupture membranes, causing irreversible damage.

Signs of Light Frost Damage

• Pale, water-soaked, or browned leaf tissue (especially between veins)
• Blackened foliage on tropicals (cannas, elephant ears, basil)
• Blossom drop on fruit trees; scorch on young fruit
• Stem collapse in succulents and herbaceous annuals

Plants at High Risk

• Tropicals: hibiscus, bougainvillea, citrus, houseplants summering outdoors
• Warm-season vegetables: tomatoes, peppers, eggplant, potatoes, beans
• Annuals: impatiens, begonias, coleus, marigolds, petunias
• Early-emerging perennials: hostas, bleeding hearts, ferns

Protection

• Cover with frost cloth, sheets, cardboard boxes — remove by mid-morning.
• Move containers to garage/porch.
• Water soil well before nightfall (wet soil radiates heat).
• Use cloches or milk jugs (bottom cut out) for individual plants.

Monitor forecasts religiously in spring and fall. A 10-minute cover job saves months of growth.