Why did my plant go black after a cold night?

Frost damage is unmistakable once you know the sequence: healthy one evening, limp and glassy the next morning, blackening over a few days. It is also the plant problem where the standard instinct — tidy it up immediately — is most reliably wrong.

Step by step

1. Confirm it was frost.Damage that appeared within a day or two of a specific cold night, on the youngest and most exposed growth, on the top of the plant and the side facing the open sky rather than any compass direction, with tissue that went limp and water-soaked before turning brown or black. Growth lower down and inside the canopy is untouched because it was sheltered.
2. Understand why it happens where it does.Ice forms in the spaces between cells and draws water out of them, or forms inside them and ruptures the walls. Young, soft, sappy growth has thin cell walls and high water content, which is why it goes first, and why a plant pushed hard with nitrogen late in the season is more vulnerable than one that hardened off naturally.
3. Note that a clear still night is worse than a cold cloudy one.On a clear night heat radiates from plant surfaces to the open sky, and leaves can be several degrees colder than the air. Cloud, a tree canopy, a wall or a sheet of fleece all block that radiation, which is why a plant under a tree escapes and the one beside it does not.
4. Leave the damage in place.This is the important instruction. Dead outer growth insulates the live buds behind it from the next frost, and the true extent of the loss cannot be judged until the plant declares which shoots are alive. Cutting in February removes protection and often removes wood that would have shot.
5. Wait for new growth, then cut back to it.In mid to late spring, buds break along the live wood. Cut each stem back to the highest live bud, and only then. On evergreens damaged in winter this can be as late as June.
6. Check the roots on anything in a container.A pot freezes right through in a way that open ground does not, so container plants can lose their roots to frost while the top looks unharmed. A plant that fails to grow away in spring after a hard winter in a pot has usually lost its roots rather than its shoots.
7. Deal with the specific vulnerable cases.Magnolia and camellia flowers browned by frost, and camellia buds damaged when early morning sun thaws them too fast — which is why camellias are planted away from morning sun. Potatoes, dahlias and beans blackened by a late spring frost, which usually reshoot from below ground. Tender perennials such as fuchsias and salvias, which often shoot from the base even when the top is dead.
8. Protect properly next time, and remember the roots.Horticultural fleece over vulnerable plants on forecast frost nights, removed by day so light and air reach them. For containers, wrap the pot rather than the plant, or move it against a house wall. For borderline plants, plant in spring rather than autumn so they have a season to establish.
9. Watch the frost pockets.Cold air flows downhill and pools at the bottom of a slope and behind a solid fence or wall. That is why one part of a garden loses plants year after year. Move tender things uphill, and use a permeable hedge or an open fence rather than a solid barrier where cold air needs to drain away.

Tips

• Soil temperature lags air temperature by weeks. A warm April afternoon means nothing about whether it is safe to plant out — the last frost date for your area is the guide, and it is later than most people assume.
• Watering the soil, not the plant, on the afternoon before a frost genuinely helps a little, because moist soil holds and releases more heat overnight than dry soil.
• Fleece must not touch the foliage where you can avoid it, because contact points conduct cold. Use canes or hoops.
• Stop feeding tender and borderline plants with nitrogen from late summer. Soft late growth is what frost takes first.

Common mistakes

• Cutting off frost-damaged growth immediately — It removes the insulation protecting the live buds behind it and you cannot yet tell which wood is dead.
• Covering plants with polythene — It conducts cold where it touches and it holds condensation against the leaves. Fleece is breathable and is the right material.
• Leaving fleece on permanently — The plant needs light and air, and a plant kept soft under cover is more vulnerable when it is uncovered. Cover at night, uncover by day.
• Wrapping the plant and not the pot — In a container the roots are the vulnerable part. Insulating the pot matters more than insulating the leaves.

Questions people ask