Frost Protection Calculator

Frost can damage or kill plants when temperatures drop below 32°F (0°C). Different plants have different cold tolerances, and different protection methods offer varying degrees of warmth.

Frost severity levels:

• Light frost (29-32°F / -2 to 0°C): Tender plants damaged. Hardy plants usually fine.
• Moderate frost (25-28°F / -4 to -2°C): Most garden plants damaged. Hardy plants may survive.
• Hard freeze (below 25°F / -4°C): Severe damage to most plants. Even hardy plants can be injured.

Protection methods and their temperature gain:

• Row cover (lightweight, 0.5 oz): +2-4°F (+1-2°C)
• Row cover (medium, 1.5 oz): +4-8°F (+2-4°C)
• Row cover (heavy, 2.0+ oz): +6-10°F (+3-5°C)
• Plastic sheeting: +4-6°F (+2-3°C) — remove during day to prevent overheating
• Mulch (3-4 inches): +3-5°F (+2-3°C) for root zone protection
• Water jugs / thermal mass: +2-4°F (+1-2°C) — water releases heat as it cools
• String lights (incandescent): +3-5°F (+2-3°C) — the heat from bulbs warms nearby plants
• Cold frame: +10-20°F (+5-11°C) — a small greenhouse structure
• Heated greenhouse: Full protection at any temperature

Plant cold hardiness categories:

• Tender: Damaged below 32°F (0°C) — tomatoes, peppers, basil, squash
• Semi-hardy: Tolerate light frost to 28°F (-2°C) — lettuce, peas, beets
• Hardy: Tolerate hard frost to 20°F (-6°C) — kale, spinach, broccoli, garlic
• Very hardy: Survive below 20°F (-6°C) — parsnips, leeks, some onions

Important tips:

• Apply protection before sunset. Plants lose heat rapidly once the sun goes down.
• Water the soil well before a frost. Moist soil holds more heat than dry soil.
• Remove covers during the day to allow sunlight, air circulation, and pollinator access.