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: Come through 10°F (-12°C), parsnips, leeks, some onions

Those four numbers are the thresholds this calculator works from. A parsnip left in the ground through a hard freeze is not merely surviving, it is improving: the cold converts stored starch to sugar and the root comes out sweeter in February than it went in in November.

Why the forecast is not the number that matters

The low you see on a weather app is measured at a standard height in an open, sheltered spot, usually at an airport. Your garden is not that place. On a still, clear night the ground radiates heat straight to the sky and the air an inch above the soil ends up several degrees colder than the air at chest height, which is where the thermometer sits. A dip in the lawn collects that cold air like water in a bowl.

Three to five degrees is the usual gap, and it runs the wrong way for you. A 33°F forecast is a frost warning for anything tender, not a reprieve. This is why the calculator tells you to cover plants that are technically still above their threshold.

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.
• Keep the cover off the leaves where you can. Anywhere fabric touches a leaf it conducts heat away, and you get a frost burn in the exact shape of the cover.