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A planting calendar built from your frost dates, not your zone
Generic planting charts tell you to sow tomatoes “in spring”. Niwa finds your average last and first frost from your location, then gives every crop in your beds its own sow, transplant and harvest window on a month-by-month timeline.
Almost every planting mistake is a timing mistake, and almost every timing mistake comes from using a chart written for someone else's garden. Two dates fix it: your average last spring frost and your average first autumn frost. Niwa reads them from your location and counts backwards for each crop — tomatoes 6–8 weeks indoors before last frost, peas direct-sown 4–6 weeks before it, garlic in the ground weeks before the first autumn frost.
Why zone-based charts get the dates wrong
A USDA hardiness zone is a measure of average annual minimum winter temperature. It answers “will this shrub survive the winter here?” It does not answer “when can I put tomatoes out?” — and those are different questions with different answers.
Seattle and parts of northern Alabama share zone 8. Their last frost dates differ by nearly a month, and their summer heat is not remotely comparable. A chart that says “zone 8: sow tomatoes in March” is right for one of them and wrong for the other.
The rule worth internalising: hardiness zone tells you what survives the winter. Frost dates tell you when to plant. You need both, and only one of them is on the seed packet. Full explanation →
The two dates everything hangs off
| Date | What it is | What it governs |
|---|---|---|
| Last spring frost | The average date after which frost is unlikely | When tender crops can go outside; when to start seeds indoors, counted backwards |
| First autumn frost | The average date frost returns | The deadline for a crop to mature; when to sow autumn and overwintering crops |
| Growing season length | Days between the two | Whether a 100-day crop like winter squash can finish at all |
Both are averages, not guarantees — roughly a 50% chance of frost after the “last” frost date. Cautious gardeners add ten days before putting out anything tender. Find yours →
How Niwa builds your calendar
It reads your location once
On first run Niwa establishes your frost dates from your location and shows them in the calendar header — Personal calendar active · Last frost: 1 Mar 2026 · 30 days season. Everything downstream is derived from that, and it works for the UK, Canada, Australia and Europe as well as USDA zones in the US.
Each crop gets its own timeline
Rather than one master chart, every plant you have placed in a bed appears as its own row across Jan–Dec, with coloured bands for the phases: start indoors, transplant or direct sow, and harvest. Crops you have not planted do not clutter the view.
The offsets are per crop, and they are what matters
These are the standard counts Niwa applies, all relative to your last frost date:
| Crop | Start indoors | Outside | Notes |
|---|---|---|---|
| Tomato | −6 to −8 wk | +1 to +2 wk | Wait for soil at 60 °F / 15 °C |
| Pepper & chilli | −8 to −10 wk | +2 wk | Slowest to germinate; needs warmth |
| Eggplant | −8 to −10 wk | +2 wk | Very heat-dependent |
| Basil | −4 to −6 wk | +1 to +2 wk | Collapses at the first cold night |
| Cucumber | −3 wk | +1 wk | Dislikes root disturbance; sow in modules |
| Courgette / zucchini | −3 wk | +1 wk | One plant is usually enough |
| Broccoli & cabbage | −6 wk | −2 to −4 wk | Cool-season; tolerates light frost |
| Lettuce | −4 wk | −3 wk | Bolts in heat; re-sow every 2–3 wk |
| Spinach | direct | −4 to −6 wk | Very cold-tolerant |
| Peas | direct | −4 to −6 wk | Sow as soon as soil is workable |
| Radish | direct | −4 wk | 25–30 days to harvest |
| Carrot | direct | −2 to −3 wk | Never transplant |
| Bush beans | direct | +1 to +2 wk | Rots in cold soil |
| Garlic | direct | 4–6 wk before first autumn frost | Overwinters; harvest next summer |
Reading your calendar in practice
Plan the beds first, then open the calendar
The calendar is generated from what is in your beds. Lay out the square foot grid first and the schedule builds itself.
Find the earliest indoor-sowing band
Usually peppers, 8–10 weeks before last frost. That date is when your gardening year actually starts — not the date the garden centre puts bedding plants out.
Buy seed from the calendar, not the rack
You now have a dated list of exactly what you need. This is the step that prevents the annual drawer of unopened packets.
Check the harvest bands for collisions
If six crops all finish in the last week of August, spread the sowings. Succession planting is easier to arrange on a timeline than in your head.
Work backwards from first frost for autumn sowings
Take the crop's days-to-maturity, add two weeks for slower autumn growth, and count back from your first frost date. That is your last safe sowing day.
Timing the second half of the season
Most gardeners plan spring carefully and let autumn happen by accident, which wastes the warmest soil of the year. The arithmetic is simple: first frost date − (days to maturity + 14 days) = last safe sowing date.
With a 15 October first frost, a 60-day crop like beetroot must be sown by roughly 1 August. Kale, chard, spinach, turnips, radish and lettuce all fit comfortably into that window, and several actually taste better after a light frost.
Micro-climate beats the map. A south-facing brick wall, a frost pocket at the bottom of a slope, or being three streets from the coast can shift your real frost date by a fortnight in either direction. Use the calculated date for the first year, then adjust it from what you observe — your own records beat any dataset.
Related reading
- When to start seeds indoors — month-by-month schedule for 20+ crops
- USDA hardiness zones explained — what your zone really means
- Frost date and zone lookup — get your two dates
Get the dates for your garden, not your zone.
Every crop in every bed, counted back from your own last frost. Free on iPhone.
Questions gardeners ask
How do I know when to plant in my area?
Work from two dates: your average last spring frost and your average first autumn frost. Everything else is counted from them. Warm-season crops such as tomatoes, peppers and basil go out after the last frost; cool-season crops such as peas, spinach and radish go in weeks before it. The gap between the two frost dates is your growing season length, which tells you whether a 100-day crop will finish in time.
Look up your frost datesIs a hardiness zone the same as a planting calendar?
No, and confusing the two is the most common planting mistake. A USDA hardiness zone describes average annual minimum winter temperature, which predicts whether a perennial survives the winter. It says nothing directly about when to sow an annual vegetable. Two places in the same zone can have last frost dates four weeks apart.
What hardiness zones do and don't tell youIs 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