Last and first frost dates in Helena

When is the last frost in Helena?

The last frost in Helena usually comes around 19 May: that is the median of 2016–2025, so in about half of those years the last night at or below 0 °C came later. Hardy crops start before it and frost-tender ones only after it: in the table below arugula can go into open ground from 28 April, while corn waits until 26 May. The first autumn frost follows around 3 October, about 137 days later: harvest or cover frost-tender crops before then.

When to sow and plant

Planting out: a week after the last frost once the soil is warm enough for the crop; frost-hardy crops three weeks before it; an unheated greenhouse three weeks earlier. Seedlings are sown so they are ready on that day. Each window starts on the Greenly calendar date and ends on the last day that still leaves the crop’s days to harvest (from its card) before the average first autumn frost, plus six weeks of picking for crops harvested over weeks (tomatoes, peppers, aubergines, cucumbers, courgettes, pumpkins, watermelons, beans) — our rule; earlier in the window is safer.

When the soil warms up in Helena

The day the top 7 cm of soil first averages each temperature over a week, median of 2016–2025 (ERA5 via Open-Meteo). The soil reaches 10 °C 9 days before the last spring frost and 15 °C 18 days after it.

Frost dates near Helena

Of the nearest cities on our list, the last spring frost ends earliest in Boise (15 April) and latest in Missoula (18 May).

Seeds, seedlings and markets in Helena

Within 15 km of Helena OpenStreetMap lists 2 farm-supply shops and 6 plant nurseries (collected 28 October 2026). The nearest plant nursery to the city centre is Keeping It Green, 1 km away.

• Keeping It Green — plant nursery
  Columbia Avenue 2005, 59601 Helena · 1 km · keeping-it-green-helena.com

• Where to buy seeds and seedlings in Helena
• Selling your own harvest in the United States

Places: © OpenStreetMap contributors, ODbL.

Weather in Helena by month

Average weather in Helena over 2016–2025: the warmest month is July. Months that average 20 °C or more by day: June–September (21.1–27.3 °C). Nights average below 0 °C in November–April (-11.5–-2.6 °C). About 491 mm of rain falls in a year, on 102 days; May is the wettest month (76 mm) and July the driest (23 mm).

Average of the daily highs and lows, rain per month and days with at least 1 mm (ERA5 via Open-Meteo, the same grid cell as the frost dates).

Planted in autumn

From a month before the first autumn frost, so the roots grow before winter, to the late-planting limit on each crop’s page (about two weeks before the ground freezes, by ten years of weather).