Seed longevity roughly doubles for each 1 percentage point of seed moisture removed and for each 5 °C of cooling, so drying a lot from 12% to 8% and cooling it from 20 °C to 10 °C multiplies its relative life by 2^4 × 2^2 = 64. The rules hold only over a limited range of moisture and temperature, they give a ratio and not a number of years, and the starting life of each species still has to be found by germination testing.
Part of Community Seed Bank Management
Seed longevity is the time a stored lot keeps its ability to germinate, controlled mainly by seed moisture and storage temperature.
Two Rules, Both Ratios
Harrington’s rules of thumb for seed storage (J. F. Harrington, 1950s-1970s, collected in Kozlowski’s Seed Biology, 1972):
- For each 1 percentage point decrease in seed moisture, the life of the seed doubles.
- For each 5 °C (about 10 °F) decrease in storage temperature, the life of the seed doubles.
The first holds roughly between 5% and 14% seed moisture; the second between about 0 °C and 50 °C. Outside those bands the rules fail. Below about 5% there is little further gain and, for some species, risk; above 14% the seed may heat, mould or sprout in storage. Because the rules are ratios, the life of any lot depends on where it started. They compare two conditions; they do not predict years.
A note on units: 10 °F is 5.6 °C, so “10 °F” and “5 °C” are the same rule rounded differently, not two different rules. Some summaries in this corpus say “every degree”; the figure is every 5 °C.
Worked Example
A lot is held at 12% moisture and 20 °C and then re-dried to 8% and kept at 10 °C.
- Moisture: 12 − 8 = 4 points, giving 2^4 = 16 times the life.
- Temperature: 20 − 10 = 10 °C, which is two steps of 5 °C, giving 2^2 = 4 times the life.
- Together: 16 × 4 = 64 times the relative life.
If the lot had a half-life of one year in the first state, the rules suggest about 64 years in the second, but the answer is extrapolated beyond what has been measured, so use it only to compare and plan a germination test every year. A measured half-life, found by testing the same lot year after year, replaces the extrapolation.
The Rule Of 100
A second rule, also from Harrington, is easier to apply with a thermometer and a hygrometer. Add the temperature in °F to the relative humidity in percent: if the sum is under 100, storage is good. Example: 60 °F + 35% = 95, good; 80 °F + 50% = 130, poor. The rule stands in for the two rules above and carries the same limits (Storage Containers).
Humidity Sets Seed Moisture
Seed gains and loses water until it matches the humidity of the air around it, so the relative humidity of the storage air sets the equilibrium seed moisture. Oily seeds hold less water than starchy seeds at the same humidity, so one seed moisture can sit at different humidities for different species. A sealed jar fixes a small volume of air with the seed: put dry seed and a desiccant in, and the jar’s humidity is set by the desiccant, not by the room (Desiccants and Moisture Control).
Colder Than A Cellar
Gene banks that follow the FAO genebank standards dry seed to roughly 3-7% moisture and freeze it at about −18 °C (FAO, Genebank Standards for Plant Genetic Resources for Food and Agriculture, 2014). Freezing is safe only for seed that is truly dry. Seed above the safe level can be damaged by ice. A lot frozen in a sealed jar must be allowed to warm to room temperature while still sealed before it is opened, or water condenses on the cold seed.
Not All Seeds Can Be Dried
The rules are for orthodox seeds, which tolerate drying and cooling; that is nearly all vegetable, grain and legume seed. Recalcitrant seeds such as citrus, avocado, mango and oak acorns die if they dry and cannot be stored by this method. They are kept moist and cool for a short time, or grown on.
Failure Modes
- Daily and seasonal temperature swings. A jar that warms by day and cools by night breathes moist air through any small leak and cycles condensation onto the seed. Prefer a steady cellar to a cool shed that swings.
- Sealing before drying. The moisture target must be met before the lid goes on.
- Wet cellar, open container. Cellar humidity of 85-95% will bring open seed to a high moisture quickly.
- Applying 5 °C rules outside 0-50 °C. The doubling stops working at the extremes.
Diagnosis
Run the germination test each year on a counted sample (Germination Rate Testing Protocol). A lot that falls faster than your calculated decline has a moisture or temperature problem that the thermometer missed: check the seal, the desiccant and any hot spots.
What’s Next
- Learn next: Desiccants and Moisture Control — reaching the target moisture
- Combine with: Seed Viability — expected life by crop