Seed Starting Date Calculator

How this is calculated

⚠️ YOUR LAST FROST DATE IS NOT A NUMBER THIS TOOL CAN SUPPLY. It is local, it varies by many weeks across a single country and by days across a single valley, and it is a probability rather than a promise - most published dates are the date by which frost has passed in a given percentage of years, not a guarantee. Your national meteorological service or extension service publishes it for your location, and a neighbour who has gardened the same ground for twenty years often knows the local version better than the map does. Everything here is expressed in DAYS BEFORE OR AFTER that date so you can count backwards on a calendar from whichever figure you trust. THE WEEKS-BEFORE FIGURE IS ON YOUR SEED PACKET. It differs enormously by crop - some want six to eight weeks of indoor growing, some resent transplanting and want direct sowing, and a few want only two or three weeks. The packet knows your variety. This tool does the arithmetic that follows. ⚠️ PLANTING OUT ON THE LAST FROST DATE IS NOT THE SAME AS PLANTING OUT SAFELY. Hardy crops can go out around it or before; tender ones want the soil warm as well as frost-free, which is usually a couple of weeks later or more, and cold soil stalls them even when nothing freezes. That is what the days-after-frost field is for, and setting it to zero is a decision rather than a default to accept. A NEGATIVE NUMBER MEANS AFTER THE FROST DATE, NOT AN ERROR. If you are planting out two weeks after the last frost and hardening off for ten days, hardening starts four days AFTER the frost date - so the figure comes out negative. It is the honest way to express one timeline against one reference point. THE GROW-ON WINDOW IS THE FIGURE MOST WORTH LOOKING AT. It is what remains after germination and hardening off, and it is the time the plant actually spends growing into something worth planting. If it is very short, the packet's weeks-before figure may not suit the way you are doing it - and if it is very long you are heading for root-bound seedlings in small cells, which check growth in a way they often do not recover from. STARTING TOO EARLY IS A REAL MISTAKE, AND MORE COMMON THAN STARTING TOO LATE. An over-large seedling held indoors waiting for weather goes leggy, becomes root-bound and transplants badly - and it is frequently overtaken by one sown two weeks later. Late is recoverable; leggy and root-bound is not. HARDENING OFF IS NOT OPTIONAL. Indoor-raised plants have soft growth and no wind conditioning, and going straight out often checks or kills them. The days you allow are yours to set here.

Common questions

Why won't it give me an actual date?

Because it would need your last frost date, and that is local in a way no calculator can guess - it varies by many weeks across a single country and by days across a single valley, depending on elevation, aspect and how cold air drains off a slope. It is also a probability rather than a promise: most published frost dates are the date by which frost has passed in a given percentage of years, not a guarantee. Your meteorological or extension service publishes it for your location, and everything here is in days before or after so you can count off a calendar from whichever figure you decide to trust.

Where does the weeks-before figure come from?

Your seed packet, and it differs enormously by crop. Some want six to eight weeks of indoor growing, some are best sown direct because they resent root disturbance, and a few want only two or three weeks before they are ready to go out. Variety matters too - two cultivars of the same crop can differ by a fortnight. The packet knows what you actually bought, which is why it is an input here rather than a number this page asserts.

Why is one of the figures negative?

Because it falls after the frost date rather than before it. If you are planting out two weeks after the last frost and hardening off for ten days, hardening starts four days AFTER that date - so expressed as days-before it is minus four. That is not an error; it is the honest way to put one timeline against one reference point. Read a negative number as 'this many days after your last frost date' and count forwards instead of back.

Should I plant out on the last frost date?

Only for hardy crops, and even then it depends on your soil. Tender plants want warm soil as much as they want an absence of frost - set out into cold ground they sit and sulk, and often get overtaken by plants set out two weeks later into warmer soil. Cold soil also increases the risk of rot in wet conditions. The days-after-frost field is for exactly this, and leaving it at zero should be a decision about the crop rather than a default you accepted.

What is the grow-on window for?

It is the figure most worth looking at, because it is the time the plant actually spends becoming something worth planting - what remains once germination and hardening off are taken out of the indoor period. If it comes out very short, the packet's weeks-before figure may not suit how you are doing things, particularly if your germination is slow because the propagation space is cool. If it comes out very long, you are heading for root-bound seedlings in small cells, which is a check to growth that plants often do not fully recover from.

Is starting early safer than starting late?

No, and it is the more common mistake of the two. An over-large seedling held indoors waiting for the weather goes leggy, becomes root-bound, and transplants badly - and it is frequently overtaken within a few weeks by one sown a fortnight later that went out at the right size. Late sowing costs you some season; early sowing can cost you the plant's whole constitution. If you are unsure, the recoverable error is the late one.

Can I skip hardening off?

Not sensibly. Indoor-raised plants have soft growth, no wind conditioning and leaves adapted to far weaker light than they will meet outside, and going straight out often checks them badly or kills them outright - scorched leaves on a bright day after transplanting are usually this rather than disease. The process is gradual exposure over days: an hour or two out at first, building up, and bringing them in overnight until the last stage. The days you allow are an input here because how long depends on your weather and how sheltered the spot is.

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 Seed Starting Date Calculator at https://www.bfcbrilliance.com/tools/seed-starting-date-calculator.

What I entered:
- Weeks indoors before planting out (weeks): ___
- Days to germinate (days): ___
- Days hardening off (days): ___
- Days after last frost before planting out (days): ___

What it calculated:
- Sow this many days before your last frost date: ___
- Seedlings should be up by: ___
- Start hardening off: ___
- Total time indoors: ___
- Actual growing-on window: ___

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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