I grow a wide variety of crops (often several varieties of each one) every year, which means that I end up with a lot of half empty seed packets at the end of the growing season. Fortunately most seed remains viable for at least 3 years (even if just sitting in a drawer in your kitchen) so you can use up the rest of the packet the following year. A few kinds of seed (corn, onion, leek, chives and parsnip) are only considered dependable for 2 years, so you should try and use these up as soon as possible. Others (cabbage, chicory, endive, cucumber, squash and tomato) may last as long as 6-8 years.
The exact length of time a seed will remain viable is determined by how it is stored. Seeds are living plants in a much reduced form and have to respire to maintain life processes. This slowly depletes their food reserves, so vigor declines until the seed runs out of food and is no longer able to germinate (this is why 5 year old seed isn’t usually as vigorous as one year old seed).
If seed is to remain in good condition for future use, it must be stored properly. If you are planning to use the rest of the seed in the following year, all you need to do is keep them in a dry, cool place. If you want them to survive as long as possible you must keep them very dry and very cool. This means drying them thoroughly (put them in a glass jar with a moisture absorbent such as silica gel) and then keeping them in a place that remains cool even in summer. For longest possible storage they may be kept in a freezer, though any seed you store in this way must be very dry, otherwise moisture in the seed may freeze and damage it.
Garden centers often sell off year old seed packets cheaply when the new seed arrives in February or March and this can be a good way to save a little money and try some new varieties. It should still be good for at least another year and often a lot longer (just be aware of which seeds are long lived and which aren’t). The best deal is seed that comes in packets lined with aluminum foil, as this keeps the seed very dry and extends its life considerably (I’ve had good germination from 12 year old lettuce seed packed in foil).
Factors in seed storage
The factors responsible for seed losing its vigor are the same ones necessary for seed germination: moisture and warmth. They speed up seed metabolism (in preparation for germination) causing it to respire at a faster rate and use up its reserves of food more quickly . This is why it is not good to leave seed packets in warm humid places such as the greenhouse (and never let them get wet.)
Moisture is the most critical factor in shortening a seeds life span. Seeds absorb moisture from the air very readily and their moisture content fluctuates with humidity. In very dry air it may be as low as 5%, while in very humid air it could be as high as 14%. For every 1% rise in the moisture content of a seed (above 5%) its storage life is halved. This means that for longest life you should keep your seeds as dry as possible. If they get wet they lose their viability very rapidly.
High temperature shortens seed life in the same way as moisture, by increasing the rate of respiration. For every 18° rise in temperature above 32°F the storage life of seed is halved. This isn’t quite as critical as moisture (seeds keep very well when simply stored in unopened foil packets), but the two have a synergistic effect.
Relative humidity should be low: 40% is good. There is a rule of thumb that temperature and humidity added together should be less than 100. In low humidity areas the seed may dry sufficiently in the open air, in which case you just need to ensure that it stays dry.
How long seed will remain viable at room temperature and average humidity depends on the species. The numbers in the table below are only approximations, mostly useful for comparison. Seed longevity isn’t very predictable however and they often last longer than you might expect (or longer than they should).
Generally it is better to grow out a batch of seeds every few years (the exact time depending upon its estimated longevity), rather than try and save seed for a long period.
Seeds must be very dry if they are to remain in good condition for any length of time. The safest way to do this is to put the seed packets in a sealed jar with a desiccant. You can use dry powdered milk (from a freshly opened packet), but silica gel is much better. This is usually treated with cobalt chloride, to make blue crystals that turn white when they absorb moisture.
Drying the seeds is simple enough. Measure out an amount of silica gel equal in volume to the amount of seed. This is roughly an ounce of seed to one tablespoon of silica gel (or 3 tablespoons of fresh powdered milk). Seal it into the airtight jar with the packets of seed. When the silica gel turns white from absorbing moisture, it should be replaced with fresh gel (do this quickly so as not to admit too much moisture). The white gel can then be re-dried for later use. Repeat this operation as necessary until the gel in the jar no longer turns white. You should then change the gel routinely every six months or so, to ensure the seeds are kept dry. When the seeds are thoroughly dry they will shatter when crushed, rather than simply being flattened.
It is possible to dry seeds slowly in a cool (100°F max) oven for up to 6 hours. You have to take extreme care not to overheat them though, as this will definitely kill them.
Seed storage ideas
If you want to store seeds for any length of time you must keep them dry and cool. There are a number of ways to do this, depending on how long you want to keep them. Obviously if you are only going to keep the seed until the next spring planting season, then good storage conditions aren’t as important as if you are starting a seed bank and hope to store them for 50 years.
Commercial paper packaging doesn’t do anything to protect the seed. This doesn’t matter if the seed is fresh and is to be used promptly, but means that the seed will lose viability at the normal rate when stored at room temperature and humidity (see the table below).
The best commercial seed packaging is the aluminum foil pack that keeps out all moisture until opened. I recently germinated some 20 year-old foil packed kale seed that grew very well, even though stored at room temperature and humidity. Once the foil packet is opened the seeds start to deteriorate at the normal rate, unless you quickly re-seal the packet with tape.
Seeds last longer if stored in the refrigerator at 35 – 40°F, however they must be kept in an airtight container, as refrigerators tend to be very humid.
The best way to store seeds long term is to dry them with a desiccant (see below) and then freeze them to about 15°F. Remember the colder you store the seeds the longer they will live. The seed must be thoroughly dry before it is frozen though, as undried seed may be damaged or killed by ice crystals forming inside it.
Once seed is in storage it is best to leave it undisturbed. Opening a container and removing seed lets in moisture (and if it is cold enough may result in condensation on the seed). Several rounds of freezing and thawing are not good. In this situation you may want to have several packets of a variety, so you can take some out without affecting the rest.
For very long term storage you can vacuum seal the seed in a plastic bag and store that in the freezer in a glass jar full of desiccant (plastic itself may gradually allow moisture to pass through it).
Whenever you save seeds it is very important that each lot be labelled clearly and that it can’t wear off, or otherwise disappear over time.
Any container can be used for seed storage, so long as it is moisture proof , airtight (some plastics are actually permeable) and pest proof. The most popular container is a screw top glass jar with a rubber gasket (cut from an old inner tube) in the lid. Seal-able plastic freezer bags can also be used, if you suck out the surplus air before sealing. Ideally you will put an open container of silica gel in the container so you can see its color (if this changes to indicate it has absorbed moisture, you can replace it) Always remove the seeds and re-seal the container as quickly as possible).
Seed longevity table
Any list of seed longevity is only an estimate, because of all the variables involved.
|Seed Longevity in years|
|Bean, Bush / Pole||2||4||5|
|Bean, Scarlet Runner||3||4||6|
|Celery / Celeriac||2||4||5|
|New Zealand Spinach||3||5||6|
|Potato (True seed)||4||5||8|