Two different kinds of cold damage
Most overwintering advice assumes the enemy is frost. For half of these plants it is not, and that is why they are lost in September by gardeners watching the wrong number.
Freeze damage happens at or below 0 °C. Ice forms inside plant cells, ruptures the cell walls, and the tissue dies. It is sudden, visible the next morning, and it is what a frost warning is about.
Chilling injury happens well above freezing. Tropical and subtropical plants — peppers, citrus, most houseplants that summer outside — suffer real physiological damage somewhere below 10 to 12 °C. Membranes stop working properly, the plant cannot take up water or nutrients efficiently, and it responds by dropping leaves, pitting, blackening or simply stopping.
There is no ice and nothing dramatic to see on the night, which is exactly the problem. A pepper plant left out through a fortnight of 6 °C nights in September is often beyond rescue before the first frost arrives.
Thresholds by plant
Bring the plant in before its threshold, not on it.
| Plant | Act below | Damage type |
|---|---|---|
| Peppers and chillies | 10 °C (50 °F) | Chilling |
| Citrus in pots | 10 °C (50 °F) | Chilling |
| Houseplants summering outside | 12 °C (54 °F) | Chilling |
| Begonias, tuberous | 10 °C (50 °F) | Chilling |
| Basil | 10 °C (50 °F) | Chilling |
| Tomatoes | 4 °C (39 °F) | Chilling, then freeze |
| Geraniums (pelargoniums) | 4 °C (39 °F) | Freeze |
| Fuchsias, tender | 2 °C (36 °F) | Freeze |
| Dahlia tubers | After the foliage blackens | Freeze |
| Canna and gladiolus | 0 °C (32 °F) | Freeze |
| Rosemary, bay, olive | −5 °C (23 °F) | Freeze |
| Hardy perennials | Leave out | — |
Two patterns worth noticing. Everything in the chilling group has a threshold at or above 10 °C, which in most temperate climates arrives weeks before the first frost. And a plant in a pot is always more exposed than the same plant in the ground, because there is no soil mass around the roots to buffer the swing.
Bringing a plant in properly
The move itself kills more plants than the cold, usually through pests or overwatering.
- Inspect for pests, thoroughly. Undersides of leaves, leaf joints, the soil surface. Aphids, whitefly, spider mite and mealybug all breed explosively in a warm dry room with no predators. Treat before the plant comes in, not after.
- Cut back by a third to a half. Less top growth to support with less light, and less leaf area for pests. Skip this for succulents.
- Repot into fresh compost if the plant has been in the ground or the pot is full of roots. Shake off garden soil, which brings its own passengers.
- Move it in stages if you can. A few days in a porch or a shaded spot before the final move lets it adjust to lower light rather than dropping half its leaves at once.
- Cut watering right back. This is the big one — see below.
Light, warmth and water are a trade-off
The mistake is to treat "indoors" as one place. What the plant needs is its minimum temperature and the most light available, and those often pull in opposite directions.
A warm dark room — the classic failure. A pepper on a shelf in a heated hallway with no window keeps trying to grow, cannot photosynthesise enough to support it, and exhausts itself.
A cool bright place is what most tender perennials want: an unheated porch, a frost-free conservatory, a bright windowsill in an unheated room. Cool enough that the plant slows down, bright enough that what growth it makes is paid for.
A dark cool place works only for plants going genuinely dormant — dahlia tubers, canna rhizomes, gladiolus corms. Lift, dry, and store in a frost-free garage or cellar in barely damp compost. Do not put a leafy plant there.
In low light and cool temperatures a plant is barely transpiring, so compost stays wet for weeks and roots rot. Water only when the top of the compost has dried out, water less than feels right, and never leave a pot standing in water. A slightly dry overwintered plant recovers; a waterlogged one does not.
Putting them back out
Reverse the process, and be slower about it than about bringing them in.
- Wait until nights are reliably above the plant's threshold, not just days. A 15 °C afternoon with a 6 °C night is not the moment for a pepper.
- Harden off over one to two weeks. A few hours outside in shade at first, building up. A plant that has spent winter indoors has thin, soft leaves that scorch in direct sun.
- Watch the forecast, not the calendar. A late frost after everything is out is the classic loss, and the first frost dates reasoning applies just as much in spring: the last-frost date is a median, and half of all years frost later than it.
- Keep fleece to hand for a fortnight after they go out — when to cover your plants covers what is worth what.