Search this and you get one number back, with impressive consistency: bring them in when nights reach about 50 °F, which is 10 °C. An extension service says it, a garden design site says it, and the summary at the top of the page says it.
That number is also the one our own cold advisory uses, and we did not get it from them. It was set at 4 °C first and moved up to 10 °C during a horticultural review, on the grounds that tropical foliage starts suffering around 10 to 12 °C and a threshold of 4 arrives far too late to be a warning.
Not the best fit right now
The plant's fine, this spot just isn't a match.
Just the right amount of light
Too cold here
Humidity suits it
Airflow suits it
Temperature is a deal-breaker here. The other three cannot make up for it.
Cold is one of five
Temperature is the event people are warned about. It is not the only one that reaches a plant standing outside, and the app watches five, each with an advisory level and a more serious one:
| Event | Raised as an advisory when | Raised as a warning at | Runs for a plant indoors? |
|---|---|---|---|
| Heat | Day's highest reaches 35 °C | 40 °C | Yes |
| Cold | Night's lowest drops to 10 °C | 0 °C | Yes |
| Heavy rain | 10 mm in three hours, or 25 mm across the day | 50 mm across the day | No |
| High wind | Gusts reach 10.8 m/s, force 6 on the Beaufort scale | 17.2 m/s, force 8 | No |
| Heavy snow | 3 mm of water equivalent in three hours, or 10 mm across the day | 25 mm across the day | No |
The last column is the part worth carrying away. Three of those five stop at the wall, so a plant that has come inside has left three of its five weather risks outside with the balcony. Heat and cold follow it in, because a wall does much less to a cold snap than it does to rain. Carrying a plant the other way switches those three back on, and the light it walks into is the bigger change.
The alert lands on the first night, not the worst one
Each kind of event is raised once for a forecast run, on the earliest day that crosses the threshold. That is the right behaviour for a prompt to act, and it means the alert you see is not a summary of the week. Here is one declared five-day forecast run through the check twice, once as a balcony plant and once as the same plant indoors:
| Forecast day | High / low | On the balcony | Inside the room |
|---|---|---|---|
| Day 1 | 18 °C / 14 °C | No alert | No alert |
| Day 2 | 16 °C / 8 °C | cold (advisory) | cold (advisory) |
| Day 3 | 15 °C / 9 °C | No alert | No alert |
| Day 4 | 12 °C / -1 °C | high wind (advisory) | No alert |
| Day 5 | 11 °C / 2 °C | No alert | No alert |
Day 4 has a night at −1 °C, which is a frost and would be raised at the more serious level on its own. What you get is the Day 2 advisory, because Day 2 crossed the line first. Read an advisory as "act now", not as "this is how cold it gets". A plant arriving from a shop rather than from the garden meets the same set of checks, and three of the four can answer on its first day.
What an alert changes besides the plant's address
An active heat or cold alert does more than suggest moving the pot. Two other pieces of care stand down while it runs, and one carries on unchanged:
| Care decision | No alert running | Heat or cold alert running |
|---|---|---|
| Repot hint | Can be raised | Held back until it clears |
| Feeding | Runs while the plant is growing | Paused, whatever the season says |
| Watering | Follows evaporation demand | Follows evaporation demand, with no separate override |
Watering is the one that does not need an override, because a heatwave already arrives inside the evaporation figures it reads. Repotting and feeding are both requests for the plant to spend something, and an alert is precisely when it has nothing spare. The reasoning is worked through on the repotting page and the feeding one.
Bringing it in is a spot decision too
A plant that has spent the summer outdoors is about to lose most of its light in one afternoon. The place you put it decides whether it merely sulks or drops half its leaves, and the two spots people reach for first, the empty corner and the shelf away from the window, are usually the worst two in the room.
Take a reading of the place you are planning to put it before you carry it in, with the free spot checker if you would rather not install anything. Cold at the glass is worth checking too, since the comfortable minimum varies by six degrees across ordinary houseplants, and a leaf touching a winter window is colder than the room it is in.
Thresholds and severities on this page are read out of the alert engine rather than restated from a forecast service, and the wind levels are the Beaufort scale, force 6 and force 8. The indoor and outdoor columns come from running each event twice rather than from our notes about it. Whether an alert reaches you also depends on the plant being marked as standing outside.