When is it safe to plant tomatoes out? Ten springs of night temperatures in ten cities

Tomatoes want warm nights, not just frost-free weather. In all ten cities we checked, the last frost came 3 to 11 weeks (20 to 75 days) before nights reliably reached 10 °C (50 °F) [1]. That is the night temperature Cooperative Extension recommends before tomatoes go outside [2].

Why “after the last frost” is too early for tomatoes

The classic advice is to plant tender crops like tomatoes after the last frost. For tomatoes, this is not enough: their growth stalls, and chill damage can still occur, when nighttime temperatures drop below 10 °C (50 °F) [2].

Cooperative Extension guidance is clear: wait until night temperatures are steadily at or above 10 °C (50 °F) before setting out tomatoes [2]. This is because tomatoes’ metabolism, root growth, and fruit set slow down in cool conditions. Setting them out too soon risks weeks of poor growth, purple-tinged leaves, or even stunting.

In many cities, the gap between the last frost and the first warm week is not a few days—it can stretch for months [1]. Planting on the last frost date, as many guides suggest, puts tomatoes into weeks of cold, slow growth.

Table: How long is the wait between last frost and tomato-safe nights?

Our own ten-year computation (2015–2024) of weather records in ten cities shows how last frost and the first week of tomato-safe nights rarely line up [1]. The table below gives the median dates for each, and the number of days between:

In London, for instance, the last frost usually falls around 4 April, but the first week of steady 10 °C nights doesn’t arrive until 9 June—a 66-day wait [1]. Seattle and Edinburgh are similar, with waits of 75 and 72 days [1]. By contrast, Atlanta and Denver’s gaps are much shorter (20 and 27 days) [1].

A worked example: London’s two-month tomato wait

Suppose you garden in London and follow the classic advice: plant tomatoes when the last frost is past. In 2015–2024, the median last frost came on 4 April [1]. But the first week when every night stayed above 10 °C didn’t arrive until 9 June [1]. That means your tomatoes would wait 66 days outdoors for their first week of warm nights [1].

A looser rule helps less than you would think. Waiting for six of seven nights at or above 10 °C brings planting forward only to 26 May, still 53 days after the last frost [1]. The first tomato-safe week had begun by 9 June in six of the ten years, and by 21 June in nine of the ten [1].

This gap is not unique to London. Seattle and Edinburgh show the same pattern: frost ends, but spring nights stay too cold for another two months [1].

What about cities with shorter gaps?

Not every city faces such a long wait. In Atlanta, the last frost is around 17 March, and tomato-safe nights arrive by 6 April—a gap of just 20 days [1]. Denver and Minneapolis have a little longer to wait (27 days), but that’s still much shorter than in the UK or Pacific Northwest [1].

What’s going on? Cities with drier, more “continental” climates have frost later in spring, but warm up more quickly once it’s over [1]. Places with oceanic or maritime climates—like London, Seattle, or Edinburgh—lose frost early but take months to reach warm spring nights.

In practice, this means a grower in Atlanta could plant tomatoes less than three weeks after the last frost. In London, waiting only three weeks after the last frost would leave plants shivering through cold May nights, growing slowly or suffering damage [1].

What you can do if nights are still cold

If you want to plant tomatoes out before nights are reliably warm, you’ll need to offer protection. Cloches, fleece or a cold frame shelter plants from the coldest nights. How much warmer they keep them depends on the setup. A thermometer inside tells you.

Indoors, on a balcony, or in an unheated greenhouse, conditions may be a little warmer than the city average—but not always. Urban heat can help, but frost pockets and exposed spots may run colder than the city’s average grid cell [1].

For most small-space growers, the most reliable method is to check a local weather forecast for the seven days ahead. Wait until every night is due to stay at or above 10 °C (50 °F), and then set out your tomatoes [2]. If a cold snap is forecast, delay planting or be ready to cover plants overnight.

Try this tonight

Check your 7-day forecast for nighttime lows. If every night is at or above 10 °C (50 °F), your tomatoes are ready to move outdoors.

Questions readers ask

What if I have a greenhouse or use fleece?

Protection like fleece or a cold frame can help; how much it warms the plants depends on the setup, so measure it. Use a thermometer to check actual minimums.

Can I risk planting tomatoes out a bit early?

Planting before nights are reliably warm means slower growth and possible leaf damage. In a mild spell, you might get away with it—especially if you’re ready to cover plants at short notice. But most years, the first truly warm nights arrive weeks after the last frost [1].

Do peppers and other tender crops follow the same rule?

Yes: peppers and most other heat-loving seedlings also want nighttime temperatures at or above 10 °C (50 °F) before you plant them out [2].

How the numbers work

These figures were computed for this article, not estimated by its writer. Inputs come from the numbered sources or are assumptions, labelled as such; change an assumption and the results change with it.

Sources

1. Our own data, 30 Oct 2026: Last frost and first warm nights in ten cities, 2015-2024 (Open-Meteo ERA5)
2. Tomato Time? - Ask Extension