Frost alerts for home gardeners
Know before dark whether tonight will freeze your garden.
Your weather app forecasts the air two metres up at the airport. Frost forms on leaves. Plants Won't Freeze works out the low where your plants grow and sends one calm note each afternoon: cover the tomatoes, bring the pots in, or relax.
Tonight · Burlington, VT
Frost likely tonight.
Your garden
Weather app
Cover tomatoes, peppers and basil before sunset at 6:21 pm.
The garden low
Your garden runs colder than the forecast.
On clear, still nights the ground and leaves lose heat to the open sky and end up several degrees below the air the forecast measures. A garden in a dip runs colder still. That's why frost shows up on nights the forecast called 37°.
Weather app
Your garden
The app starts from Apple Weather's hourly forecast, then adjusts for cloud, wind, dew point and the kind of spot you garden in. Both numbers stay on screen, so you can see why it disagrees with your weather app.
One note, before dark
Told in time to do something about it.
Covers need to go on before sunset to trap the day's warmth. So the note arrives in the afternoon, at a time you pick, and names the plants at risk and the time of sunset. On a safe night it says so, so you know the app is still watching.
Frost likely tonight in Burlington: garden low ~33°F (forecast 37°F). Cover tomatoes, peppers and basil before sunset at 6:21 pm.
No frost tonight: low 45°F, cloudy. Nothing to do.
Update: the forecast dropped to 31°F. Frost now likely. Cover before you turn in.
No plant library to fill in
Tell it what you grow, in one tap.
Tomatoes die at the first frost. Kale shrugs off 28°. Pots of citrus want to come in at 50°. Pick the groups you grow and the verdict covers the one most at risk, and says which ones are fine.
Tender crops & flowers
Tomatoes, peppers, basil, beans, squash, dahlias
Alert at 34°F / 1°C
Pots & tropicals
Citrus, monstera, succulents summering outside
Bring in at 50°F / 10°C
Half-hardy
Lettuce, chard, carrots, beets, peas
Alert at 29°F / −2°C
Hardy
Kale, cabbage, broccoli, spinach, garlic
Alert at 25°F / −4°C
Hoses, barrels & taps
Drain and disconnect before a hard freeze
Alert at 28°F / −2°C for 3 hours
Questions
Good to know
Where does the forecast come from?
From Apple Weather: the hourly forecast for your spot, official frost and freeze warnings where your country issues them, and long-term averages for your usual frost dates. The garden low is worked out on your phone from that forecast.
Can it promise my plants won't freeze?
No forecast can. The app tells you how sure it is in words ("likely", "borderline") and, with Pro, checks the next morning how the night really went. The name is what it says on a quiet night, and the goal is to give you time to act on the others.
Does it work outside the US?
Yes. It works anywhere Apple Weather has a forecast, in °F or °C, including Canada, the UK and Ireland, Europe, Australia and New Zealand, where there often isn't an official frost warning at all.
Do I need an account?
No. There's no sign-up and no server of ours. Your garden and your settings stay on your phone, and the notes are scheduled on the phone too.
Will it nag me all year?
No. The daily note runs only during your frost seasons, autumn and spring, worked out from your location. Outside them you hear from the app only if a frost actually threatens. Nothing arrives between 9 pm and 7 am.
My garden is in a cold spot. Can I adjust it?
Yes. Pick the kind of spot you garden in (a hollow, a hillside, a city yard) during setup, and change it any time. With Pro you can set your own offset from a thermometer in the garden.
Enjoy the evening. We'll watch the sky.
Plants Won't Freeze is coming to iPhone this autumn, in time for first frost.