Will This Crop Fit My Growing Season?

The outdoor question

This is about whether a crop fits the frost-free season you actually have. Working out when to start it indoors ahead of that season is a different calculation — the Seed Starting Date Calculator on this site does that half.

Your frost-free days

Count the days between your last spring frost and your first autumn frost. Both come from your meteorological or extension service, and neither can be guessed by a calculator.

They're local — 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. And both are probabilities: published figures are usually the date by which frost has passed, or arrived, in a given proportion of years.

The mistake that changes everything

Days to maturity is measured from different starting points depending on the crop.

• Direct-sown crops: usually from sowing
• Transplanted crops: usually from planting out — not from sowing indoors

So the six or eight weeks on a windowsill often aren't 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.

Maturity is the first harvest

Not the last. If you want to be picking for three weeks, those weeks have to fit inside the season too.

A crop reaching 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.

That's why How long you want to be picking is a separate field. Leaving it at zero is reasonable for a storage crop lifted in one go, and misleading for anything you pick repeatedly.

The buffer isn't padding

Three things push the same way:

1. Frost dates are probabilities — the first frost can arrive early
2. Days-to-maturity figures are optimistic, assuming good warmth, water and nutrition
3. Crops slow disproportionately as it cools, so the last weeks of an autumn crop take 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.

When it doesn't fit

Slack left in your season goes negative, Share of your season it uses goes over 100%, and Days short tells you by how much.

At that point there are four honest responses:

• A shorter-maturing variety
• Start indoors, moving part of the growing outside the season
• Extend the season — cloche, tunnel, fleece, a warm sheltered wall
• Grow something else

Longest-maturing variety that would fit makes the first one concrete. It's a number you take straight to a catalogue and filter on. At the defaults that's 115 days — and setting maturity to exactly 115 lands on zero slack, which is the tool agreeing with itself.

Successions

Succession sowings that fit divides the slack by your gap and adds one for the first sowing. Every later sowing still needs the full days-needed ahead of it.

A dash means even a single sowing doesn't fit — the same message as negative slack.

Worth knowing: later sowings run into cooler weather and often take longer than the packet says. The last succession is the one most likely to disappoint, and the buffer only helps so much.

What season extension can't fix

Cloches, tunnels, fleece and cold frames all buy time at both ends, and starting indoors is the largest single gain available.

None of them change day length. For crops that care about it as much as temperature, a plant needing long days won't be hurried by covering it in October.