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The dates on the back of a seed packet are written for the entire country, which means they’re written for nobody in particular.
I learned this the hard way. A tray of cosmos sown in early March, following the packet instructions to the letter, sitting leggy and miserable on the windowsill for eight weeks because the soil outside was still too cold to plant into. By the time it was safe to go out, the seedlings were half-exhausted and the pot was half the size it should have been. The packet hadn’t lied, exactly. It just hadn’t known where I lived.
The number that actually matters for your garden is your last frost date – the average date in spring after which you’re unlikely to get a frost cold enough to kill tender seedlings. Once you know that one date, the whole sowing year stops feeling like a guessing game.
Why seed packet dates are wrong for almost everyone
“Sow March to May” is a compromise designed to keep the packet useful in Aberdeen and Penzance at the same time. Those two places have last frost dates roughly six weeks apart. A range that wide can’t possibly tell you when to actually sow.
Three questions. Two minutes. A flower list chosen for your actual garden, not some idealised one.
Let’s find your flowers.
Packets are printed in bulk for a national market. The window has to be wide enough that it isn’t wrong anywhere, which means it isn’t really right anywhere either.
The gardener in Cornwall sows in early March and gets a head start. The gardener in the Scottish Highlands sows in early March and watches a tray of seedlings die in a late frost. Same packet, same instructions, very different outcomes.
What your last frost date actually tells you
Your last frost date is the single most useful number in your garden. It’s the average date after which you’re unlikely to get a damaging frost. Combined with your first frost date in autumn, it gives you the real bookends of your growing year.
Most half-hardy, or tender, annuals (cosmos, zinnias, strawflowers) want to be started 6 to 8 weeks before your last frost date. Hardy annuals can go in earlier, or be sown the previous autumn. Tender plants shouldn’t go outside until after the last frost has passed.
Once you have the date written down, sowing timing becomes maths instead of vibes. Count back from the date. That’s when you start.
The same seeds, three different counties
Take cosmos. The packet says “sow March to May.”
In Cornwall, where the last frost date is roughly mid-April, you’d sow indoors in late February and plant out at the end of April. Flowers from June through to October.
In Shropshire, where the last frost is closer to mid-May, sowing in February is too early. Seedlings will be leggy and root-bound by the time it’s safe to plant out. Better to sow in mid to late March, plant out in late May.
In Aberdeenshire, where the last frost can be late May or even early June, early March sowing is genuinely a mistake. Sow in early April, plant out in early June, and you’ll still get a good display, just later.
Same packet. Three different sowing weeks. The packet was technically correct for all three and practically useless for all three.
How to find your last frost date in the UK
You don’t need a spreadsheet. You need a postcode and five minutes.
There are several websites that have frost date lookup tools, I like this one lastfrostdate. Type in your location, write down the two dates (last frost in spring, first frost in autumn) and stick them somewhere you’ll actually see them. The fridge works.
A few things worth knowing:
- These are averages, not guarantees. Some years frost later, some earlier. Build in a week of buffer if you’re not sure.
- Microclimates matter. A walled garden and an exposed allotment two miles apart can have last frosts a fortnight different.
- After a year or two in the same garden, your own observations beat the average. Write down when you actually got your last frost. That becomes your real number.
What changes when you sow from your location, not the packet
The main thing that changes is that you stop feeling behind.
If you want plenty of flowers without turning gardening into a second job, The Flower Files might be just what you need.
The packet’s window starts in March, so if you haven’t sown by late April you assume the year is already ruined. It isn’t. The packet was misleading you about the urgency.
Sowing from your local frost date means fewer leggy seedlings sitting around waiting to go out. Fewer tender plants lost to an unexpected late frost. Less guilt about missing a sowing window that was never really yours in the first place.
This is what lazy gardening looks like in practice. Not doing less for the sake of it – just doing the right thing at the right time, and skipping everything else. The seed packet doesn’t know your garden. Once you find your last frost date, you do.
Frequently asked questions
How do I find my local frost dates in the UK?
Search “last frost date” plus your postcode or nearest town. There are many websites with useful tools, I like lastfrostdate. Write down your average last spring frost and your average first autumn frost, those are the two dates you actually need.
Are seed packet dates ever useful?
Yes, as a rough sanity check. If your own calculation lands wildly outside the packet’s window, double-check before sowing. But treat the packet as a backup, not the primary source.
What if I live somewhere that barely frosts?
In the mildest parts of the UK, frost dates become less critical and you can start earlier and harvest later. Pay more attention to soil temperature and day length instead.
Do frost dates change over time?
Yes, gradually. Average frost dates have shifted in parts of the UK over recent decades. If you’re working from an old chart, it’s worth refreshing it, and your own observations over a few growing seasons will be the most accurate guide you’ll find.
If you’d like your sowing schedule built around your actual frost dates rather than a packet’s best guess, that’s exactly what the Flower Grower Companion does. You tell it your location and the flowers you want to grow, and it turns your frost date into a personalised weekly schedule, so you always know what’s worth doing this week, and what can wait. Take a look at the Flower Grower Companion here.
A quick aside: I built a little app for flower growers who want gentle reminders about what to do and when - without it turning into another thing to feel guilty about. It's called the Flower Grower Companion. You tell it your location and your flowers, it tells you what actually needs doing this week (and just as importantly, when there's nothing to do at all)
Have a look
