Every gardener has the same drawer or tin: a jumble of half-used packets, some dated three years ago, some with no date at all, and no idea which are still worth sowing. Late August is exactly the right moment to deal with it. The autumn sowing window is open, you will be ordering for next spring within weeks, and ten minutes of testing now saves an empty seed tray and six wasted weeks in March. The good news is that most seed lasts far longer than people assume — and for the ones that do not, there is a simple test that gives you a straight answer rather than a guess.
- Short-lived (1–2 years) parsnip, onion, leek, chives, delphinium — buy fresh each year
- Medium (3–4 years) carrot, pea, bean, sweet corn, pepper, spinach, aster
- Long-lived (5–10 years) tomato, cucumber, courgette, squash, brassicas, lettuce, radish, sunflower, poppy
- The test 10 seeds, damp kitchen paper, warm spot — count what sprouts, multiply by 10 for a percentage
What "viability" actually means
A seed is a living embryo in suspended animation, running on a finite store of energy. Viability is simply whether that embryo is still alive and capable of germinating. It does not switch off on a date; it declines gradually, and the packet date is a guide rather than a verdict. Seed does not go from 90% germination to zero overnight — it goes to 70%, then 40%, then 10%, and the seedlings you do get from old seed tend to be slower and weaker as well as fewer.
Two things drive that decline: moisture and warmth. Cool and dry buys you years; a warm, humid shed costs you them. That is why storage is the single biggest variable in this whole subject, and why the same species can last two years in one house and eight in another. Our guide to storing and preserving seeds covers how to keep them properly; this article is about working out what you have got and whether to sow it.
How long different seeds last
Seed longevity is a species trait, and the differences are large. As a rule of thumb, seeds with hard, oily or dense coats last longest, while thin-coated seeds from the carrot and onion families are the first to go.
One to two years. The onion family is notoriously short-lived: onions, leeks and chives are worth replacing annually. Parsnip is the classic offender — fresh seed germinates well, year-old seed disappoints, two-year-old seed usually fails. Parsley and delphinium belong in this group too.
Three to four years. Carrot, pepper, spinach, sweet corn, broad beans and peas, basil, aster and zinnia sit comfortably in the middle. Perfectly usable in year three; test before you rely on them in year four.
Five years and more. The cucurbits are the champions: cucumber, squash, pumpkin and melon routinely germinate after eight or ten years. Tomato, lettuce, radish, kale and the rest of the brassicas, plus sunflower, poppy, nigella, cornflower and calendula, all keep well for five years or more in decent conditions.
The germination test, step by step
- Count out ten seeds Ten makes the arithmetic trivial: each seed that sprouts is 10%. If the packet is precious and nearly empty, use five and multiply by 20 — but accept that a smaller sample is a rougher answer.
- Damp paper, not wet Moisten a sheet of kitchen paper, wring it out, and space the seeds across one half. Fold the other half over. Sodden paper drowns the seeds and invites the mould described in our guide to damping off.
- Bag it and label it Slide the folded paper into a food bag or lidded box to hold humidity, and write the variety and date on it. Put it somewhere warm — 18–22 °C suits nearly everything, as explained in our germination guide.
- Check against the packet's germination time Radish may show in three days, parsley in three weeks. Look up the expected period — our guide to understanding seed packets explains what the figures mean — then wait that long plus a few days before counting.
- Count roots, not shoots, and do the sums A seed has germinated once the root emerges. Seven out of ten is 70% germination. Above 70%, sow as normal. Between 40 and 70%, sow thicker. Below 40%, only bother if the variety is irreplaceable.
- Do not waste the sprouted seeds Carefully pot them up or plant them out — they are alive and well ahead of schedule. Handle by the seed leaf, never the root.
What to do with the result
70% or better
Effectively fresh seed. Use it with confidence.
- Sow at the spacing on the packet
- Fine for direct sowing outdoors
- Fine for module sowing, one or two per cell
- Re-seal and store cool and dry for next year
Under 70%
Still usable — adjust how you sow it.
- Sow half as thick again at 50–70%
- Sow double at 30–50%, and thin later
- Start in modules rather than direct, so gaps are visible
- Under 30%: replace the packet
One caveat worth stating plainly: low germination is not always the seed's fault. Cold soil, dry compost, sowing too deep and slugs all produce the same empty tray, and old seed gets the blame. That is precisely why the paper test is useful — it isolates the seed from every other variable. If the test comes back at 90% and your outdoor sowing still fails, the problem is elsewhere, and our guide to why seeds are not germinating will point you at it.
Common mistakes
- Treating the packet date as an expiry date — it is a "sow by" recommendation, not a switch. Well-stored squash seed will outlive it by years.
- The float test — dropping seeds in water and discarding the floaters is unreliable for most species and does not tell you whether the embryo is alive. Sprout them instead.
- Testing too few seeds — three seeds tells you almost nothing. Ten is the minimum for a meaningful percentage.
- Counting too early — slow species look like failures at day five. Wait for the full germination period before you write a packet off.
- Storing packets in the greenhouse or shed — heat and damp swings are what actually kill seed. A cool cupboard indoors is far better.
- Not writing the year on home-saved seed — label it as you collect it, using the methods in our seed saving guide.
Frequently asked questions
Can I sow seeds that are out of date?
In most cases, yes. The date on a packet is a recommendation based on average storage, not a hard expiry. Long-lived species such as tomato, squash and lettuce frequently germinate well several years past it. Run a ten-seed germination test first and adjust your sowing rate to the result.
Does freezing seeds make them last longer?
It can, but only if they are properly dry first — moisture inside a seed expands as it freezes and destroys the embryo. For home gardeners, an airtight jar in the fridge with a sachet of silica gel is safer and nearly as effective. Always let a jar reach room temperature before opening it, so condensation forms on the outside rather than on the seed.
Why did my germination test work but my outdoor sowing fail?
Because the test removes every variable except the seed. Soil that is too cold, too dry or too wet, seed sown too deep, birds and slugs all cause failures that have nothing to do with viability. Compare the two and you have isolated the cause.
How should I store seed once the packet is open?
Fold the packet closed, put it in an airtight container with a desiccant sachet, and keep it somewhere cool, dark and stable — a cupboard indoors, not a shed. Cool and dry roughly doubles the useful life of most seed.
Spend an evening on the seed tin now and you will start next season knowing exactly what you have. Sort by species, test anything over three years old, throw out the onion and parsnip packets without sentiment, and write a short list of what to replace. Then get on with the season: see what to sow in August, read up on autumn sowing hardy annuals or green manures, keep the harvest rolling with succession sowing, and check timings in our sowing calendar, vegetable calendar and flower calendar. Browse fresh vegetable seeds, flower seeds, herb seeds and organic vegetables — or, if you have a cat and a spare sunny corner, our new guide to catnip vs catmint. All our seeds and growing guides are a click away.
