Seed is alive, and it ages fastest when it is warm and damp. Keep it cool, dry and dark, in a sealed jar at 40 percent relative humidity or less, and most vegetable seed stays good for three to five years; onions and parsnips fade in one or two. A simple test before planting tells you whether an old packet is still worth sowing.
Why cool and dry.
A dry seed is a plant on pause, still breathing slowly and using up its stored food. Heat speeds that up. So does moisture, which a seed pulls from the air around it, so a seed stored in humid air ends up damp inside even if it went into the envelope dry.
Nearly every vegetable, grain and flower seed is what seed scientists call orthodox: it keeps longer, predictably, as it gets drier and colder. A few seeds are recalcitrant and die if they dry out, among them acorns, avocado and many tropical trees. If your seed library shares tree or native-plant seed, those need their own handling.
Harrington's rules.
James Harrington, a vegetable crops professor at UC Davis, gave growers two rules of thumb that are still in use:
- Moisture. Each one-point drop in the seed's moisture content roughly doubles its storage life.
- Temperature. Each drop of about 10°F (often written as 5°C) roughly doubles it again.
They hold within limits, roughly 5 to 14 percent seed moisture and temperatures from freezing to about 120°F. Below about 5 percent moisture there is no further gain, and some seed is harmed. A related rule of thumb, widely credited to Harrington, is the Hundred Rule: storage is good when the temperature in degrees Fahrenheit plus the relative humidity in percent adds up to 100 or less. At room temperature, around 70°F, that means humidity at 30 percent or lower, which is hard to hold without a sealed jar and a desiccant. In a refrigerator at 40°F, up to 60 percent passes. Stricter versions of the rule also keep the temperature itself at 50°F or below, which is why a refrigerator is the best spot most homes have.
These are approximations. Seed scientists use the Ellis and Roberts viability equation, which fits constants for each species, to predict how fast a seed lot loses germination. Even that model assumes steady conditions; a jar that swings between cool nights and hot afternoons ages much faster than its average temperature suggests.
Storing seed at home.
- Dry it first. Seed has to be fully dry before it goes into a sealed container, or it molds. See the snap test in saving seed.
- Seal it. Glass jars with good lids or metal tins keep out humid air and pests. Paper envelopes inside a jar keep lots separate and labeled.
- Add a desiccant for the valuable lots. A silica gel packet in the jar pulls out the last of the moisture. Indicating silica gel changes color when it is spent.
- Pick the right spot. A cool closet on an inside wall beats a garage or shed, where temperature and humidity swing hard. A refrigerator works well for sealed jars.
- Freeze only dry seed. Wet seed is damaged by freezing. Freezing is not needed for seed you will use within about two years. When you take a jar out, let it warm to room temperature before opening it, so moisture does not condense on the cold seed.
How long seed lasts.
| Group | Years | Crops |
|---|---|---|
| Short-lived | 1 to 2 | Onion, leek, parsnip, parsley, sweet corn |
| Middle | About 3 | Beans, peas, carrot, spinach, pepper (2 to 4), okra (about 2) |
| Long-lived | 4 to 5 or more | Beet and chard, the cabbage family, squash, pumpkin, cucumber, melon, watermelon, eggplant, tomato, lettuce, radish |
From University of Nebraska Extension G2090 and Colorado State Extension 7.221, cross-checked against Seed Savers Exchange. Sources disagree at the edges (tomato is given as anywhere from 3 to 6 years), so test anything more than a year old before you count on it.
Testing old seed.
The rag-doll test takes a week or two and a paper towel.
- Count out seeds: ten for a quick check, 25 to 50 for a figure you can trust.
- Lay them on two sheets of damp, firm paper towel, roll it up, and put the roll in a plastic bag to hold the moisture.
- Keep it warm, around 70 to 75°F, and check every few days, keeping the towel damp.
- After the crop's usual sprouting time (a week for most, up to three for slow ones like peppers and parsley), count what sprouted. Sprouted divided by total is your germination rate.
At 80 percent or better, sow normally. Between about 50 and 70, sow more thickly. Below 50, replace the seed.
For a seed library's cabinet.
Most seed libraries keep their collection in a card catalog or cabinet out in the public part of the building, living with whatever the heating and cooling gives them. The Hundred Rule says what to watch for. Keep the cabinet away from sunny windows, heat vents and outside walls, keep the most valuable and the hard-to-replace lots in sealed jars with desiccant, and drop the short-lived crops, onions and parsnips, after a season or two. Test donated lots when they come in and again after a year, and write the date and the result on the packet.
Measure it
Guessing the humidity inside a cabinet is how seed goes bad quietly. An SHT-30 temperature and humidity sensor on an ESP32 costs a few dollars, reports every few minutes, and tells you whether the drawers stay under the Hundred Rule through a humid summer.
Frequently asked questions.
How do you store seeds so they last?
Dry them fully, seal them in a glass jar or tin, add a silica gel packet for valuable lots, and keep the jar somewhere cool, dark and steady, at 40 percent relative humidity or lower. A refrigerator works for sealed jars.
How long do vegetable seeds last?
Under good storage, onions, leeks, parsnips, parsley and sweet corn last one to two years; beans, peas and carrots about three; and beets, the cabbage family, squash, cucumbers, melons, tomatoes and lettuce four to five or more.
Can you freeze seeds?
Yes, if they are fully dry; seed with moisture in it is damaged by freezing. Freezing is not needed for seed you will use within about two years. Let the jar warm to room temperature before opening it.
What is the Hundred Rule for seed storage?
A rule of thumb widely credited to seed scientist James Harrington: seed stores well when the temperature in degrees Fahrenheit plus the relative humidity in percent adds up to 100 or less.
How do I know if old seeds are still good?
Do a rag-doll test: roll 10 to 50 seeds in a damp paper towel, keep it warm in a plastic bag, and after the usual sprouting time count how many sprouted. At 80 percent or better sow normally; below 50 percent replace the seed.
Is it OK to store seeds in the garage?
Not for long. Garages and sheds swing hard in temperature and humidity, and seed ages fastest in warm, damp spells. An inside closet or a refrigerator is better.