Seeds & growth
Seed viability
Also known as: seed viability
Whether seed is still alive and able to germinate; viability declines with age and varies by crop, which is why fresh seed is more reliable.
Seed viability is simply whether a seed is still alive and capable of growing. A viable seed, given the right moisture and warmth, will sprout; a dead one will sit in the compost and rot, however perfect your conditions. It is the single biggest reason a sowing fails for no obvious reason, and it is easy to overlook because old seed looks identical to fresh.
Viability fades with age
No seed lasts forever. Inside every seed is a tiny, dormant embryo with a finite store of energy, and over time that store runs down and the embryo slowly dies. The packet may still show 95% germination on the day it was filled, but a few years later that might be 60%, then 20%, then nothing. Warmth, damp and light all speed the decline, which is why a packet left on a sunny shed windowsill ages far faster than one kept cool and dark.
Rough lifespans by crop
How long seed stays viable depends heavily on the crop. Some are notoriously short-lived: parsnips are the classic example and are best bought fresh each year, with leeks, onions and carrots not far behind. Others are remarkably tough. Tomatoes, cucumbers, courgettes and other cucurbits, plus beans and peas, will often keep for four, five or more years if stored well. As a loose rule, the small, oily or thin-coated seeds fade quickest, while plump, dry, hard-coated ones last longest.
Storing seed to keep it alive
You cannot reverse ageing, but you can slow it right down. The enemies are heat and moisture, so keep opened packets somewhere cool, dry and dark — a sealed tub or biscuit tin in an unheated room or the bottom of the fridge works well, ideally with a sachet of silica gel or a spoon of dry rice to absorb damp. Always reseal packets, label them with the year, and never leave them in a humid greenhouse or a warm kitchen drawer. This matters just as much for seed you have saved yourself, so let home-saved seed dry thoroughly before it goes into store.
A simple germination test
If you are unsure about an old packet, test it before you waste a sowing window. Lay ten seeds on a piece of damp kitchen roll, fold it over, slip it into a labelled freezer bag and keep it somewhere warm, around 18–20°C. After the crop's usual germination time — a week or two for most veg — count how many have sprouted. Seven or more, and the seed is fine to sow as normal. Three or four, sow it thickly to compensate. None at all, and it is time to buy fresh.
In a UK garden
UK seed packets carry a 'sow by' year, but kept cool, dark and dry, many crops stay usable well past it — while parsnips rarely do.
Example
Counting how many of ten chitted seeds sprout on damp kitchen roll tells you, roughly, what to expect from the rest of the packet.