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Frost date calculator and sowing schedule
Enter your last spring frost date and get exact sowing, transplanting and harvest-deadline dates for 20 common crops. If you don't know your date, pick your zone below for an approximate one and refine it later.
| Crop | Start indoors | Plant out / sow | Method |
|---|
Your average last spring frost is the date after which frost becomes unlikely — everything tender is timed from it. Your average first autumn frost is when it returns, and it sets the deadline for anything still ripening. The gap between them is your growing season, which decides whether a 100-day crop can finish at all.
Approximate frost dates by USDA zone
Use this only as a starting point. Zones describe average winter minimum temperature, not frost timing, and two places in the same zone can have last frosts a month apart. Seattle and parts of northern Alabama are both zone 8.
| Zone | Last spring frost | First autumn frost | Season |
|---|---|---|---|
| 1 | 1–30 Jun | 1–31 Jul | ~30–60 days |
| 2 | 15 May – 15 Jun | 15 Aug – 15 Sep | ~60–110 days |
| 3 | 1–31 May | 1–30 Sep | ~110–130 days |
| 4 | 1–31 May | 1–30 Sep | ~120–150 days |
| 5 | 30 Mar – 30 Apr | 30 Sep – 30 Oct | ~150–180 days |
| 6 | 1–21 Apr | 30 Sep – 30 Oct | ~170–200 days |
| 7 | 15–30 Apr | 15 Oct – 15 Nov | ~180–220 days |
| 8 | 22 Feb – 30 Mar | 30 Oct – 30 Nov | ~230–270 days |
| 9 | 30 Jan – 28 Feb | 30 Nov – 30 Dec | ~280–330 days |
| 10 | 30 Jan or none | 30 Nov – 30 Dec | ~330–365 days |
| 11–13 | Frost-free | Frost-free | Year-round |
Outside the US
USDA zones are used internationally but were designed for North America, and much of the world uses different systems entirely. Rough guidance for temperate regions:
- UK: most of England and Wales sees a last frost in late April to mid May and a first frost in late October to November. Scotland and upland areas run several weeks later in spring and earlier in autumn. The RHS uses its own hardiness ratings (H1–H7).
- Ireland: similar to southern England, with milder coasts.
- Canada: Plant Hardiness Zones are a separate national system and do not map cleanly onto USDA numbers.
- Australia and New Zealand: seasons are inverted — last frost falls Aug–Sep and first frost Apr–Jun in cool regions, and much of coastal Australia is frost-free.
- Southern hemisphere generally: shift everything by six months, and put tall plants on the south side of the bed rather than the north.
Micro-climate beats the map
Your garden is not the weather station. These shift a real frost date by up to a fortnight:
- Cold air sinks. The bottom of a slope or a walled dip is a frost pocket and can be several degrees colder than ground twenty metres uphill.
- Masonry stores heat. A south-facing brick wall radiates warmth overnight and can hold off a light frost entirely.
- Water moderates. Coastal and lakeside gardens have later autumn frosts and milder springs than inland ones at the same latitude.
- Cities are warmer. The urban heat island effect is worth a zone in some places.
- Clear still nights are the dangerous ones. Cloud and wind both prevent radiative frost; a clear calm night at 2 °C can still produce ground frost.
Keep your own records. After two or three seasons of noting the actual last frost in your garden, your figures will beat any dataset. Until then, use the calculated date and keep fleece to hand.
Working backwards for autumn
The formula for the second half of the year: first frost date − (days to maturity + 14 days) = last safe sowing date. The extra fortnight allows for slower growth as daylight shortens, which catches people out — a crop that takes 50 days in June may take 65 in September.
Crops that suit autumn sowing: radish (25 days), rocket (30), spinach (40), lettuce (45), turnip (50), beetroot (55), kale (60). Kale, leeks and parsnips actually improve after a frost, so those deadlines are softer than the calculator suggests.
Related
- Planting calendar app — this schedule for every crop in your beds, automatically
- When to start seeds indoors — the month-by-month guide
- USDA hardiness zones explained
Want these dates for every plant in your beds?
Niwa builds the whole calendar from your location automatically. Free on iPhone.
Questions gardeners ask
How do I find my last frost date?
The most accurate source is your national meteorological service — NOAA in the US, the Met Office in the UK — which publish frost probability data by weather station. A local garden centre or allotment association will also know, and their figure accounts for local micro-climate better than any national map. Your USDA hardiness zone gives you only a rough range, because zone measures winter minimum temperature rather than frost timing.
Why zone is not the same as frost dateIs the last frost date a guarantee?
No. It is a statistical average, which means roughly a 50% chance of a frost occurring after it. For anything tender — tomatoes, peppers, basil, courgettes — most gardeners wait 7 to 14 days past the average date, or keep fleece to hand. A single late frost can end a tomato plant in one night.
How the app handles thisIs Niwa free to use?
Yes. Niwa is free to download from the App Store, and the core planner — the garden grid, the plant library and the planting calendar — works without paying. Optional in-app purchases unlock additional features.
See it on the App StoreHow many plants fit in one square foot?
It depends on the crop's spacing requirement. A plant needing 3 inches fits 16 per square foot, 4 inches fits 9, 6 inches fits 4, and 12 inches fits 1. Large plants such as vining tomatoes or zucchini need more than one square each. Niwa works this out for you and prints the count straight onto the grid cell.
Plants per square foot calculatorCan Niwa plan a raised bed of any size?
Yes. You set the bed dimensions — 4×4, 4×8, 2×8 or a custom size — and Niwa draws the square foot grid to match. You can keep several beds and several gardens in the app at once and switch between them.
Raised bed plannerHow does Niwa know when I should plant?
Niwa uses your location to find your average last spring frost and first autumn frost, then works backwards from those dates for every crop in your bed. Each plant gets its own sow-indoors, transplant and harvest window on a month-by-month timeline, so the dates are yours rather than a generic zone table.
How the planting calendar worksDoes Niwa warn me about bad companion plants?
Yes. As you place plants on the grid, Niwa puts a green dot between neighbouring squares that benefit each other and a red dot between squares that are better kept apart, so you see the conflict while you are still planning.
Companion planting in NiwaHow much soil do I need to fill a raised bed?
Multiply length × width × depth in feet to get cubic feet, then divide by 27 for cubic yards. A 4×8 bed at 12 inches deep needs 32 cubic feet, roughly 1.2 cubic yards or sixteen 2-cubic-foot bags.
Raised bed soil calculatorDoes Niwa work offline, and do I need an account?
The whole plant library and all your layouts are stored on the device, so the app works with no signal — useful on an allotment. There is no account to create and your garden data is never uploaded to a server.
See what's includedWhich growing zones and countries does Niwa support?
Niwa works worldwide. It recognises USDA hardiness zones in the United States and derives frost dates from your location elsewhere, including the UK, Canada, Australia and Europe.
Find your frost dates and zoneHow is Niwa different from other garden planner apps?
Three things: it is a native iPhone app rather than a desktop planner viewed in a mobile browser, it shows companion-planting conflicts on the grid as you place plants instead of in a separate chart, and it works fully offline with no account.
Compare Niwa with other plannersIs Niwa available on Android?
Not yet. Niwa is a native iPhone app and needs iOS 18.0 or later. An Android version is not currently available.
Read the gardening guides in the meantime