Growing Season Fit Calculator

How this is calculated

THIS IS THE OUTDOOR QUESTION, NOT THE INDOOR ONE. It asks whether a crop fits the frost-free season you actually have. Working out when to sow indoors ahead of that season is a different calculation with different inputs - the Seed Starting Date Calculator on this site does that half. ⚠️ YOUR FROST-FREE SEASON IS THE DAYS BETWEEN YOUR LAST SPRING FROST AND YOUR FIRST AUTUMN ONE, and this tool cannot supply it. Both dates are local, both vary by many weeks across a country, and both are probabilities rather than promises - published figures are usually the date by which frost has passed, or arrived, in a given proportion of years. Your meteorological or extension service publishes both; count the days between them and use that. A garden on a slope with good cold-air drainage can have a season noticeably longer than one in a frost pocket half a mile away. DAYS TO MATURITY IS ON THE PACKET, AND IT IS MEASURED FROM DIFFERENT STARTING POINTS DEPENDING ON THE CROP. For direct-sown crops it usually runs from sowing; for transplanted ones it usually runs from planting out, NOT from sowing indoors. Getting that wrong shifts the answer by the entire indoor period, which is often six weeks. Check which your packet means before trusting the result. ⚠️ DAYS TO MATURITY IS AN OPTIMISTIC FIGURE. It generally assumes good conditions - adequate warmth, water and nutrition - and a cool or dry season stretches it. Some crops slow disproportionately as temperatures fall at the end of the season, so the last few weeks of an autumn crop can take considerably longer than the same weeks would in midsummer. Treat the number as a floor rather than an expectation, which is part of what the buffer is for. THE HARVEST WINDOW IS OFTEN LEFT OUT AND SHOULD NOT BE. Maturity is when the first fruit is ready, not when the plant has finished. If you want to be picking for three weeks, those three weeks have to fit inside the season too - and a crop that matures exactly at the first frost gives you one harvest rather than a season of them. THE BUFFER IS YOURS AND IT IS NOT PADDING. Frost dates are probabilities, days to maturity is optimistic, and the weather does what it likes. A buffer is what turns a plan that works in an average year into one that works in most years. A NEGATIVE SLACK MEANS THE CROP DOES NOT FIT ON THESE FIGURES. The honest options are a shorter-maturing variety, starting indoors to move part of the growing time outside the season, extending the season with cover, or growing something else. The longest-variety output is there to make the first of those a concrete shopping instruction.

Common questions

How do I find my frost-free days?

Count the days between your last spring frost date and your first autumn frost date, both of which your meteorological or extension service publishes for your location. This tool cannot supply either: they are local, they vary by many weeks across a country, and both are probabilities rather than promises - published figures are usually the date by which frost has passed, or arrived, in a given proportion of years. Local variation is real too. A garden on a slope with good cold-air drainage can have a noticeably longer season than one in a frost pocket half a mile away.

Does days to maturity count from sowing or from planting out?

It depends on the crop, and getting it wrong is the biggest error you can make here. For direct-sown crops the figure usually runs from sowing. For crops that are normally transplanted, it usually runs from PLANTING OUT rather than from sowing indoors - so the six or eight weeks spent on a windowsill are not included. Using a transplant figure as though it ran from sowing makes a crop look far easier to fit than it is. Check what your packet means before trusting anything downstream of it.

Why add a harvest window?

Because maturity is when the FIRST fruit is ready, not when the plant has finished. If you want to be picking for three weeks, those weeks have to fit inside the season as well - and a crop that reaches maturity exactly at the first frost gives you a single harvest rather than a season of them, which is rarely what anyone planted it for. Leaving the window at zero is reasonable for something harvested all at once, like a storage crop lifted in one go, and misleading for anything you pick repeatedly.

Is the buffer just padding?

No - it is what makes the plan work in most years rather than only in an average one. Three things push in the same direction: frost dates are probabilities and the first frost can arrive early, days-to-maturity figures assume good conditions, and crops slow disproportionately as temperatures fall late in the season, so the last weeks of an autumn crop can take considerably longer than the same weeks would in midsummer. A buffer absorbs all three. Setting it to zero means planning for a year that behaves exactly as the averages say it will.

What does a negative slack mean?

That the crop does not fit your season on the figures you entered, and no amount of scheduling changes it. There are four honest responses: choose a shorter-maturing variety, start the crop indoors so part of its growing happens before the season begins, extend the season with cover such as a cloche, tunnel or fleece, or grow something else. The longest-variety output is there to make the first of those concrete - it is a number you can take straight to a seed catalogue and filter on.

How does the succession count work?

It divides the slack by your chosen gap and adds one for the first sowing. The logic is simple: every succession sowing must still have the full days-needed ahead of it, so the last one can go in no later than the slack allows. A dash means even a single sowing does not fit, which is the same message as a negative slack. Bear in mind that later sowings run into cooler weather and often take longer than the packet says, so the last succession is the one most likely to disappoint - the buffer helps, but only so much.

Can I make the season longer?

Within limits, yes, and it is often the difference between a crop fitting and not. Cloches, low tunnels, fleece and cold frames all buy time at both ends, and a warm sheltered wall does the same for free. Starting indoors moves several weeks of growing outside the season entirely, which is the largest single gain available and is why so many crops are raised that way. What none of these change is day length, which for some crops matters as much as temperature - a plant that needs long days will not be hurried by covering it in October.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Growing Season Fit Calculator at https://www.bfcbrilliance.com/tools/frost-date-planting-calculator.

What I entered:
- Your frost-free days (days): ___
- Days to maturity (days): ___
- How long you want to be picking (days): ___
- Buffer before the first autumn frost (days): ___
- Gap between succession sowings (days): ___

What it calculated:
- Slack left in your season: ___
- Days of your season the crop needs: ___
- Share of your season it uses: ___
- Longest-maturing variety that would fit: ___
- Succession sowings that fit: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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