Seed stays alive longest when it is dry, cool, dark and sealed against air and pests: every 1% reduction in seed moisture and every 5 °C reduction in temperature roughly doubles how long it lives. Dry the seed first, seal it in an airtight container with a desiccant, and keep it in the coolest steady place you have, such as a cellar.
Part of Community Seed Bank Management
Seed storage conditions are the moisture, temperature, light and air around a seed lot, which decide how long it stays able to germinate.
Dry First, Then Cool
Seeds are alive but dormant. They breathe very slowly and use up their food reserves, and the rate depends on moisture and temperature. Both rules of thumb come from the plant physiologist James Harrington and cover the ordinary working range: each 1 percentage point less water in the seed roughly doubles its life, and each 5 °C (about 10 °F) cooler roughly doubles it. The two effects multiply (Storage Conditions, Temperature and Humidity for Seed Longevity).
The seed bank carrier’s targets are seed below 8% moisture, a temperature of 40-50 °F (about 4-10 °C), darkness, and relative humidity below 40%.
What To Set Up, In Order
- Dry the seed. Spread it thin, indoors, in warm airy conditions for 1-2 weeks, turning daily. Small seeds should shatter under a hammer, medium seeds should not dent under a fingernail, and large seeds should snap when bent. These are the carrier’s tests.
- Choose airtight containers. Glass jars with tight lids are best; metal tins that seal are good (Container Selection).
- Add a desiccant to absorb what moisture is left (Desiccants and Moisture Control).
- Label inside and out with the variety, year and number of parent plants.
- Store cool and dark. A root cellar with steady 4-10 °C is better than a kitchen shelf; the attic is worst because of the summer heat.
- Test germination every year for important lots (Germination Rate Testing Protocol).
Species Differ In How Long They Last
The rules halve or double life, but they do not say how long life starts out. Onion, leek and parsnip seed are short-lived; tomato, brassica and lettuce seed last far longer in the same conditions. The Phase 2 viability guide has a table by crop (Seed Viability). Treat the table as a guide and the germination test as the answer.
Common Mistakes
- Sealing seed that is still damp. The jar then holds the water in with the seed, and mould follows.
- Storing in a warm place because the container is good. Heat shortens life even in a sealed jar.
- Opening a cold jar in a warm, humid room. Moisture condenses on the seed. Let the closed jar come to room temperature first.
- Never testing. A lot that quietly dropped to a low percentage is found at planting time, when it is too late.
What’s Next
- Learn next: Temperature and Humidity for Seed Longevity — the arithmetic
- Combine with: Record Keeping & Germination Testing — knowing what you have