When to Start Seeds Indoors

Everything counts back from one date

Your last frost date is the reference point the whole indoor timeline hangs off — and it's a number no calculator can supply.

It varies by many weeks across a country and by days across a single valley, depending on elevation, aspect and how cold air drains off a slope. It's also a probability, not a promise: most published dates are the date by which frost has passed in a given percentage of years.

Your meteorological or extension service publishes it for your location. Someone who's gardened your ground for twenty years often knows the local version better than the map does.

So this tool works in days before or after that date. Count them off a calendar from whichever figure you trust.

A negative number means after

If you're planting out two weeks after the last frost and hardening off for ten days, hardening starts four days after the frost date — so Start hardening off reads −4.

That's not an error. It's the honest way to put one timeline against one reference point. Read a negative as "this many days after your frost date" and count forwards.

Planting out on the frost date isn't the same as planting out safely

Hardy crops can go out around it. Tender ones want warm soil as much as they want an absence of frost.

Set into cold ground they sit and sulk, and often get overtaken by plants set out a fortnight later into warmer soil. Cold wet soil also raises the risk of rot.

Days after last frost before planting out is for exactly this. Leaving it at zero should be a decision about the crop, not a default you accepted.

The number most worth looking at

Actual growing-on window is what's left once germination and hardening off come out of the indoor period. It's the time the plant actually spends becoming something worth planting.

Watch what happens at the extremes:

At three weeks, with ten days to germinate and ten to harden off, there is one day of actual growing. That's not a schedule — it's a warning that the packet's figure doesn't suit how you're doing it, especially if your propagation space is cool and germination runs slow.

At twelve weeks you have the opposite problem, which brings us to the real point.

Early is the more expensive mistake

More common than starting late, too.

A seedling held indoors waiting for the weather goes leggy, becomes root-bound, and transplants badly. It's frequently overtaken within a few weeks by one sown a fortnight later that went out at the right size.

Sowing late costs you some season. Sowing early can cost the plant's whole constitution — and a root-bound check is something plants often never fully recover from.

If you're unsure, take the recoverable error.

Use the emergence date

Seedlings should be up by is a checkpoint, not decoration. If nothing has appeared well past it, re-sow rather than wait — at that stage you still have time, and waiting another fortnight for seed that isn't coming is how a crop gets missed entirely.

Use the longer end of the packet's germination range when you plan. Cool propagation space stretches it.

Hardening off isn't optional

Indoor-raised plants have soft growth, no wind conditioning, and leaves adapted to far weaker light than they'll meet outside. Going straight out often checks them badly or kills them — scorched leaves the day after transplanting are usually this rather than disease.

The process is gradual: an hour or two out at first, building up over days, bringing them in overnight until the last stage.

Days hardening off is an input because how long depends on your weather and how sheltered the spot is. It shouldn't be zero.