How Long Do Seeds Stay Viable — and How Do I Test Old Seeds?

Seed viability is one of those gardening questions where the honest answer is "it depends". Some seeds last 10 years in good storage; others are essentially dead after one season. Knowing the viability range for your seeds, and how to test them before committing precious time and compost to sowing, prevents wasted effort and explains the patchy germination that plagues old seed collections.

Seed Viability by Crop

Here are approximate viability periods under good storage conditions (cool, dry, dark):

• Onions and leeks: 1–2 years — among the shortest-lived
• Parsnips: 1–2 years
• Sweet corn: 2–3 years
• Tomatoes: 4–6 years
• Peppers and chillies: 3–5 years
• Cucumbers and courgettes: 5–7 years
• Brassicas (cabbage, broccoli, kale): 4–5 years
• Beans and peas: 3–5 years
• Lettuce: 3–4 years
• Celery and celeriac: 3–4 years

These ranges assume good storage. Seeds kept in a warm, damp kitchen drawer will deteriorate much faster — sometimes within a single season.

The Paper Towel Viability Test

Before committing to sowing old seed, test it. Place 10 seeds from the packet on a damp sheet of kitchen paper. Fold the paper over the seeds, slide into a labelled plastic bag, and keep in a warm place (around 20 °C) for the normal germination period for that crop. Count how many germinate:

• 8–10 of 10: excellent viability — sow normally
• 5–7 of 10: acceptable — sow more thickly than usual to compensate
• Fewer than 5: poor viability — consider buying fresh seed

Storage Conditions That Extend Viability

The two enemies of seed viability are heat and moisture. Every increase of 5 °C in storage temperature roughly halves seed lifespan. A dry, sealed container in a cool cupboard or fridge (in a sealed bag with silica gel sachets to absorb moisture) can extend viability significantly beyond the typical ranges listed above. Never store seeds near a boiler, in a greenhouse, or in a kitchen with humidity fluctuations.

When to Buy Fresh Seed

If the viability test shows under 50% germination and you need a reliable crop from the variety — particularly for slow-growing crops like celery or leeks where a failed sowing costs you the season — buy fresh seed. Saving money on a £2 packet of seed is not worth risking a whole growing season.