How Long Do Cannabis Seeds Really Last? What the Science Says About Storing Seed

Watch the episode · 3:55

Seeds in a warm kitchen drawer, or in the fridge: it sounds like a small difference. But in one long-running study, cannabis seeds kept warm and slightly damp were all dead within 18 months, while cool, dry seeds were still germinating years later. Here is why, and how to store yours.

Why seeds die: slow oxidation ▶ 0:26

A dry seed is not dead. It is alive, just running incredibly slowly, and like anything alive, it ages.

Inside the seed, reactive forms of oxygen slowly build up. They damage cell membranes, proteins and even DNA, until eventually the seed is too damaged to germinate at all.

Oily seeds are especially vulnerable, because those oils oxidise. And as episode 4 on germination showed, around a third of a hemp seed is oil.

The two dials: moisture and temperature ▶ 0:58

Two things control how fast that ageing happens: the seed's moisture and the temperature. Turn both down and the clock slows right down.

The seed scientist James Harrington summed it up in famous rules of thumb. For every 1% drop in seed moisture, storage life roughly doubles. For every 5 °C or so cooler, it roughly doubles again. And a quick check: the temperature in Fahrenheit plus the relative humidity should add up to less than 100.

They are rules of thumb, not exact laws, and they work best within normal storage ranges. But they explain why cool and dry beats everything else.

The cannabis study ▶ 1:40

So what about cannabis specifically? The Canadian researchers Small and Brookes stored seeds of hemp, a medicinal strain and a wild type at four temperatures and three moisture levels, and tested them for 66 months.

At room temperature, with about 11% moisture, every variety had stopped germinating in under 18 months.

But cooling seeds to 5 °C, or drying them to about 6% moisture, made a huge difference. Going colder still gave a small extra benefit, and removing the oxygen gave no apparent benefit at all.

Storing yours ▶ 2:18

So, in practice: keep seeds dry, in their sealed pack or an airtight jar. Add a silica gel sachet to soak up moisture. Keep them cool: for most growers the fridge is ideal. And keep things steady and dark, without constant warming and cooling.

One mistake to avoid: opening a cold container straight away. Warm air condenses on cold seeds, adding exactly the moisture you were keeping out. Let it reach room temperature first.

Freezing can work for long-term storage, but only for seeds that are properly dry and sealed, and repeated thawing does more harm than good.

The world's largest seed backup, the Svalbard Global Seed Vault in the Arctic, uses the same principles: dry seeds, sealed in foil packs, at −18 °C.

Test old seeds ▶ 3:10

Not sure whether old seeds are still good? Germinate a small sample on a damp paper towel. If seven out of ten sprout, you have roughly a 70% germination rate. And remember: a swollen seed is no proof it is alive. Only a root is.

Seeds age through slow oxidation, and dry and cool slows the clock dramatically. For most growers that means the fridge, sealed, with silica gel, and always letting the container warm up before you open it. ("Old seeds are dead seeds" is one of the 7 seed myths we tested: it is only half true.)