A frost warning doesn’t have to mean losing a season’s worth of work. Most frost damage is preventable if you act in the hours before temperatures drop, not after you see the first white coating on your plants in the morning.

Northern Hemisphere? This applies roughly December–February. Southern Hemisphere (Australia, New Zealand, South Africa, southern South America)? Your winter falls in June–August instead — the same advice applies, just shifted six months.

Know your frost threshold

“Frost” isn’t one temperature — it’s a range, and different plants tolerate it differently:

• Light frost (29–32°F / -2 to 0°C): Tender annuals (basil, tomatoes, peppers, squash) are damaged or killed. Hardy vegetables (kale, spinach, carrots) are unaffected.
• Hard/killing frost (25–28°F / -4 to -2°C): Most semi-hardy plants die back. Even hardy perennials can show leaf damage.
• Severe freeze (below 25°F / -4°C): Only truly cold-hardy plants and dormant perennials survive without protection.

Check your local forecast for the overnight low, not just the daytime high — frost forms when temperatures near the ground drop below freezing, which often happens even when the air a few feet up reads a couple degrees warmer.

The night before: what to do

1. Water the soil (not the leaves) in the afternoon. Moist soil holds heat better than dry soil and releases it slowly overnight, raising the temperature immediately around your plants by a degree or two. Do this hours before sunset so leaves have time to dry — wet foliage going into a freeze is worse than dry foliage.

2. Cover plants before sunset. Heat radiates out fastest right after the sun goes down, so covering after dark is often too late. Good covers:

• Frost cloth / row cover — purpose-made, breathable, and can stay on for days if needed. The standard choice for garden beds.
• Old bedsheets or towels — a solid backup for a single cold night. Avoid plastic sheeting touching the foliage directly; where it contacts leaves, it conducts cold straight through and can cause worse damage than no cover at all.
• Cardboard boxes or overturned buckets — fine for individual small plants.

Anchor the edges of any cover to the ground with rocks, bricks, or landscape pins so it traps ground heat underneath instead of just blocking wind. For taller or more delicate plants, use stakes to keep the material from resting directly on the foliage.

3. Remove covers the next morning once temperatures rise back above freezing, ideally by mid-morning. Leaving covers on through a sunny day can overheat and smother plants.

Container and indoor plants

Potted plants are more vulnerable than plants in the ground because their roots aren’t insulated by surrounding soil mass. If frost is forecast:

• Move containers against a south-facing wall or under a covered porch — even a few degrees of shelter from open sky makes a difference, since clear skies at night lose the most radiant heat.
• Cluster pots together; they buffer each other.
• For genuinely tender plants (citrus, tropicals), bring them inside or into an unheated garage, which typically stays several degrees warmer than open air overnight.

What doesn’t actually help

• Fertilizing before a freeze — this doesn’t increase cold tolerance and can push tender new growth that’s even more vulnerable.
• Pruning frost-damaged growth immediately — wait until the danger of further frost has passed. Damaged growth can actually help insulate the healthy growth beneath it, and pruning too early risks stimulating new growth right before another cold snap.
• Spraying plants with water expecting an “ice insulation” effect — this is a real commercial-orchard technique, but it requires continuous spraying all night to work and will do more harm than good if you stop partway through, since the water refreezing without a fresh coating draws heat back out of the plant.

Quick reference

Forecast low	Action
33–36°F (1–2°C)	Watch closely; water soil in the afternoon as a precaution
29–32°F (-2–0°C)	Cover tender plants before sunset; move containers to shelter
Below 29°F (-2°C)	Cover everything tender, bring in what you can, expect losses on unprotected tender plants

The plants worth this effort most are the ones you can’t easily replace mid-season — established annuals in full production, or anything you started from seed months ago. For genuinely hardy vegetables and perennials rated for your zone, a light frost usually isn’t worth the trouble of covering at all.