Frost Protection for Tender Plants: A Practical Guide

Frost damage is often sudden and disappointing: a plant that looked healthy yesterday is blackened and limp this morning. The good news is that much of it is preventable with simple, cheap measures, applied at the right time. Frost protection is not about wrapping every plant in the yard. It is about knowing which plants are tender, which nights are risky, and how to give those plants a few extra degrees of protection when it matters.

Frost forms on clear, still nights, when heat radiates away from the ground and air near the surface cools below freezing. A common guideline is that tender plants can be damaged at around 0 degrees Celsius, and more severely below about -2 degrees Celsius, although exact tolerance varies by plant and by how long the cold lasts. A short, light frost is very different from several hours below freezing with a cold wind.

Which plants need protection

Tender plants are those that cannot survive frost, or that are damaged by it even if they recover later. Examples include many summer bedding plants, tender herbs, young vegetable seedlings, and plants grown in pots that are normally kept indoors or in a greenhouse. Hardy plants, by contrast, are adapted to cold and usually need no protection, although their flowers or new growth can still be damaged by a late spring frost.

New growth is especially vulnerable. Soft, fresh shoots in spring have little protection, even on otherwise hardy plants. That is why a late frost after a mild spell can be more damaging than a cold snap in midwinter, when plants are dormant. Check plants after a mild, wet period: if they have put on soft new growth, they are more at risk.

Pots need extra attention. Soil in a pot freezes faster than soil in the ground, because it is exposed on all sides and has less mass to hold heat. A plant that would survive in a border may die in a pot in the same frost. Move vulnerable pots first, and protect the pot itself, not just the leaves.

Covers that work, and covers that do not

Fleece is the most useful frost cover for most gardeners. It is light, lets some air and water through, and can be draped over plants or supported on canes so it does not crush soft growth. Use two layers on very cold nights, and secure the edges with bricks or stones so wind does not lift it. Remove fleece in the morning once temperatures rise, so plants get light and air.

Cloches, cold frames, and upturned buckets or pots can also protect individual plants, as long as they do not trap damp air for days. Ventilate on mild days, and remove covers when frost has passed. Plastic sheeting directly on leaves is less helpful, because it conducts cold and can stick to wet leaves, causing damage where it touches. If you use plastic, support it above the plant so it does not rest on foliage.

Mulch helps protect roots, but it does not protect leaves from frost. A layer of mulch typically 5 to 7 centimetres thick, kept away from stems, insulates soil and reduces temperature swings at root level. It is useful for plants in the ground, especially after planting in autumn. For material choices, see mulch types compared.

Old blankets, towels, or hessian can work in an emergency, but they become heavy when wet and can crush plants. Use them only for one night, supported above the plant if possible, and remove them in the morning. Never use a cover that stays wet and heavy for days, because it can cause rot and break stems.

Pots, watering and position

Position matters as much as covering. Cold air pools in low spots, dips, and enclosed corners, so plants in a hollow may frost before plants on higher ground. Move pots to a sheltered spot near a wall, where stored heat radiates overnight, and out of cold wind. A south-facing wall is often warmest, but any sheltered wall is better than an exposed, windy corner.

Wrap pots with hessian, bubble wrap, or fleece, and raise them on feet so water drains. A pot sitting in a saucer of water in frost is at risk of both root rot and cracking. Check drainage holes are clear before winter, and empty saucers after rain.

Watering timing matters in frost. Water soil in the morning on a frost-free day, so plants are hydrated but leaves are dry by nightfall. Wet leaves freeze more readily and are more likely to be damaged. Do not water tender plants in the evening before a forecast frost, and do not water frozen soil, because the water cannot drain and roots cannot use it. If you use irrigation, check it before cold weather: see drip irrigation basics and flush or drain lines that could freeze.

What to do after a frost

Do not prune frost-damaged growth immediately. Blackened leaves and stems may look dead, but they can protect living growth below from further frost. Wait until the risk of frost has passed and new growth appears, then cut back to healthy, green tissue. Pruning too early can expose tender inner growth to the next frost.

Check plants gently, without pulling or bending frozen stems. Frozen stems are brittle and snap easily. Wait until they have thawed before handling them. Water lightly if soil is dry and no further frost is forecast that night, but do not flood a plant that is still recovering.

Some plants recover from the base even if top growth is lost. Others do not. Give a plant a few weeks in spring before deciding it is dead, especially perennials and woody plants that may shoot from below ground. Scraping a small area of bark on a woody stem can show green tissue underneath if the plant is alive, but do this sparingly and only on a small area.

Common mistakes

• Covering plants too late, after frost has already settled on leaves.
• Using plastic directly on leaves, which can cause damage where it touches.
• Leaving covers on for days, which traps damp and encourages disease.
• Protecting leaves but forgetting the pot, so roots freeze.
• Watering in the evening before a frost, leaving leaves wet overnight.
• Pruning frost damage immediately, exposing tender growth to the next frost.
• Handling frozen plants, which snaps brittle stems.
• Assuming a plant is dead in early spring, before it has had time to regrow from the base.

A cold-night checklist

When frost is forecast, work through this short list in the afternoon:

• Move vulnerable pots to a sheltered spot, wrap pots, and raise them on feet.
• Drape fleece over tender plants, supported above soft growth, and secure the edges.
• Cover cloches and cold frames, and close vents before nightfall.
• Remove saucers of standing water from under pots.
• Water soil in the morning only, and leave leaves dry by evening.
• Bring very tender plants indoors or into a frost-free greenhouse if possible.
• Check the forecast again in the evening, because a clear, still night can be colder than predicted.

Frost protection does not need to be elaborate or expensive. A roll of fleece, a few canes, some pot feet, and a habit of checking the forecast on clear evenings will protect most tender plants most of the time. The plants that survive are usually the ones that were moved, covered, and left alone until the cold passed, not the ones that were fussed over in the morning after the damage was done.