When to Plant, and What Actually Fits
The packet says 75 days. That crop needs 110 days of your season - and only half your seedlings' indoor time is actually growing.
The tools in this guide
All free, no signup. Each one has a printable sheet too.
- Seed Starting Date CalculatorWork the indoor timeline backwards from your own last frost date - sowing, emergence, hardening off and planting out, all in days you can count on a calendar.
- Growing Season Fit CalculatorDoes the crop actually fit your season? Enter your frost-free days and its days to maturity, and see the slack - or how short you are.
- Crop Rotation PlannerWork out which bed a crop family moves to in any future year, and whether your number of beds actually gives the rest interval you need.
Every date in gardening is counted backwards from a frost date, and almost every number on a seed packet describes less than it appears to. These three tools handle the counting.
"75 days to maturity" is not 75 days of your season
The Growing Season Fit Calculator is the one to run before buying anything, because it answers whether a crop fits at all.
A 150-day season and a crop listed at 75 days to maturity:
| Days to maturity, on the packet | 75 |
| Days of your season it actually needs | 110 |
| Slack left | 40 |
| Share of your season used | 73% |
The extra 35 days are the parts nobody prints on the packet: a harvest window (the crop does not all ripen on one day) and a fall buffer before the first frost. A crop that looks like it uses half your season uses nearly three-quarters of it.
Two outputs follow from that, and both change what you buy:
- The longest-maturing variety that would fit: 115 days. That is the real ceiling when you're choosing between varieties.
- Succession sowings that fit: 3. Slack is what succession planting spends, so knowing you have 40 days tells you how many staggered sowings are possible before you run out of season.
Only half of "six weeks indoors" is growing
The Seed Starting Date Calculator counts backwards from your last frost, and its most useful output is the one that shows what the indoor time is really spent on.
Six weeks before last frost, with 10 days to germinate and 10 days hardening off:
| Days before last frost | |
|---|---|
| Sow | 42 |
| Seedlings should be up by | 32 |
| Start hardening off | 10 |
| Actual growing-on window | 22 days |
52% of the indoor time is spent growing. The other half is waiting for germination at one end and acclimatising the plants at the other.
Which matters because "start six weeks before last frost" gets treated as six weeks of growth. It isn't — and if your germination is slow or you extend hardening off, the growing window shrinks from both ends while the total stays the same.
⚠️ Do not skip hardening off to buy back days. Seedlings moved straight from indoors to open ground get checked or killed by wind and UV they have never met. The ten days are not padding.
Four beds, a four-year rest, and no margin
The Crop Rotation Planner tracks which bed a family lands in, and tells you whether your rotation actually delivers the rest interval you intended.
With 4 beds and a 4-year rest interval:
| Margin against your rest interval | 0 |
Zero is not a failure — it means the rotation works exactly, with nothing spare. Four beds is precisely enough for a four-year rest, and every bed is doing necessary work.
But it also means there is no slack for the real world: skip a year, double up a family because one bed had space, or lose a bed to something perennial, and the interval breaks silently. If you want a four-year rest with room to improvise, you need five beds.
The tool also sizes the beds while it's there: 8 × 4 ft gives 32 sq ft, 128 sq ft across four beds, and at 12-inch spacing 32 plants a bed, 128 in total.
Running the three in order
- Growing Season Fit — does this crop fit your season at all, and which variety?
- Seed Starting Date — if it needs a head start, when do you sow, and when does hardening off begin?
- Crop Rotation Planner — which bed does it go in this year, and does that respect the rest interval?
The order matters because the first can rule a crop out entirely, and the third can rule out the bed you were assuming.
The habit underneath all three
Count backwards, and count the parts you can't see.
A season is shorter than its length by a harvest window and a frost buffer. Indoor time is shorter than it looks by germination and hardening off. A rotation is tighter than it looks the first time you improvise.
None of these is a large correction on its own. Together they are the difference between a plan that works and one that runs out of autumn.
⚠️ Frost dates are averages, not promises. They describe a probability across many years, not a guarantee about this one. Treat them as the centre of a range, keep fleece to hand at both ends of the season, and know your own garden's cold spots — a low corner can run a fortnight behind the rest of the plot.
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