DIY frost alert: a connected sensor that tells you when to bring pots inside

Potted plants freeze from every side, so they’re the first casualties of the season’s first cold nights. French gardening guides (Le Conseil Malin, Jardiner Facile) use a simple rule that works anywhere, including the US: watch the overnight low.

The threshold

• Below 50 °F (10 °C): start watching the forecast.
• Around or below 41 °F (5 °C): bring frost-tender plants inside (geraniums, agapanthus, fuchsias, cannas, tender dahlias, tropicals, citrus, bougainvillea).
• Frost forecast: act right away.

Automate it with Home Assistant

1. An outdoor temperature sensor on the balcony or patio, shaded, at pot height. Zigbee, Tuya Wi-Fi or an ESP32 running ESPHome all work as long as it reports to Home Assistant.
2. An automation: numeric-state trigger below 5 °C / 41 °F, an evening/night time condition, and a phone notification: “Bring the pots in.”
3. An early warning: add a second trigger on your weather integration’s forecast low so you’re warned the day before, not at 3 a.m.

Your own sensor measures your balcony, which often runs colder than the nearest weather station on clear nights.

Once the alert fires

• Move tender plants to a cool, bright, frost-free room, ideally through a transition spot (sunroom, bright garage).
• Group hardier pots against a sheltered wall, raised off the ground, under breathable frost cloth (not sealed plastic).
• Water less and remove saucers: soggy soil that freezes damages roots and cracks pots.

Related: 7 houseplants to bring inside in September.

Sources: Le Conseil Malin, Jardiner Facile (in French).