Whether you've saved seeds from your garden, or you have extra seeds from last spring, you want to make sure they're properly stored so they will germinate when needed. Once your seeds are well-dried, package them into airtight containers. Snap top mason jars are excellent for seeds, though glass jars with thicker rubber seals and metal clasps make a better seal for long term storage. Remember that many seed envelopes can fit inside one jar! At very least, package the seeds on a dry day. Ideally, run a dehumidifier in the room and bring the relative humidity reading down to 35% or lower. This decreases the amount of moisture in the jar. Once sealed, don’t re-open your jar as this will immediately equalize the moisture in the jar with that in the surrounding air!
Seeds are living things, each with a little baby plant sleeping inside and some stored food to feed that plant until it sprouts. That's the key to a seed's shelf life - it lives until its food runs out. Old seeds don't grow because they ran out of food.
In other words, to make your seeds last longer, you have to make them consume their stored food more slowly. The deeper they sleep, the slower they eat, and the longer they live. With seeds, the word "dormant" is typically used. What makes a seed dormant? Precisely the opposite of what makes them sprout. When you want to germinate a seed, you give it warmth, moisture, and light, so to make a seed dormant you would make it cold, dry, and dark. That forces it to slow down its metabolism, eat its food slower, and therefore last longer in the jar or envelope.
Seed storage is not difficult, but it makes the difference between seeds that last for years, and seeds that refuse to sprout after only a few months. The trick is to keep them dry and cold, and dry is more important than cold. Darkness might make a small difference, but it isn't nearly as important as dryness.
In Canada, our winter air is usually quite dry, so by keeping seeds in breathable packages or bags until November or December, in typical indoor conditions (e.g. not in a humid greenhouse, or beside the shower), they should dry well enough to keep for several years.
Here's a simple trick for choosing a good place to store your seeds: measure the temperature in degrees Fahrenheit, and the percentage of relative humidity. If the sum of those numbers is less than 100, you've found a pretty good place to keep seeds in the open air.
°F + %RH < 100
The lower the temperature the better, and especially the lower the humidity the better, so try these methods too:
- Put your well-dried seeds in air-tight jars, and store the jars in a cold cellar. You should never keep seeds open to the air here, because it’s much too humid, but inside the jars the seeds will stay dry. Just remember to open and close the jars in dry air when you fetch the seeds, so you don't trap humidity inside.
- Silica gel, sold at most craft stores for drying flowers, is a material that absorbs moisture. Put an envelope of silica gel and an envelope of seeds in a sealed jar for two days. The silica will remove excess moisture from the air and also from the seeds. Remove the silica to prevent over-drying, and keep the seeds sealed in the dry jar for years.
Exactly how long your seeds will last depends on many variables. Generally, bean seeds usually last for 3-5 years if they're dry and room temperature, tomato seeds can live for up to 10 years in the same conditions, and other vegetable and flower seeds normally keep somewhere between 2-6 years.
Here are some tips to get you thinking about seed storage:
- Every 6°C (about 10°F) of lower temperature will double a seed's lifespan (at typical temperature ranges).
- Every 1% of moisture removed from a seed will double its lifespan (to a certain point; you can fatally overdry them if you use heat).
- Seeds dried in silica gel, or very dry open air, will not be damaged by freezing. Then you can really make them dormant!
- Remember to keep seeds in air-tight jars in a fridge or freezer, because the humidity is higher than you think. (There wouldn't be frost in your freezer if there weren't moisture in the air).
Where to Store Your Seed Containers
Here are six good and bad places to store your seeds during the winter.
1. In the basement. (NO)
Usually, basements are fairly humid. Even if they are finished, most basements don't have the same ventilation as the upper floors of a house so moisture from the walls can enter the air. If you have any problems with water or humidity in your basement, that is definitely not a good place to keep your seeds.
2. In air-tight containers in the basement (YES)
Once your seeds are fully dry you can store them in a humid basement if they're in air-tight containers. Even if the basement is a bit humid, the dry air inside the containers plus the cool downstairs temperature should help your seeds to last a long time.
Glass jars are good for this, but plastic bags leak more air than you might expect so they aren't really good enough.
The important thing is to make sure the seeds are fully dried, for at least a month, before putting them in containers because any remnant of moisture can cause them to mould.
3. In the fridge (NO)
You might think a fridge would be a good place to keep seeds cold, but that can be a real mistake. Fridges are cold, but they're also very wet and humid. Even chilly seeds will expire quickly if they absorb moisture from such a humid place.
4. In a main-floor closet (YES)
The main floor of your house might not be as cool as the basement, but it is almost certainly drier. Since DRY is more important than COLD for seed longevity, a dry closet is a pretty good place.
5. In a heated garage (NO)
Maybe you have a garage built onto your house, where some warmth comes in during the winter. It's probably a convenient place to keep seeds, because it's handy to the garden and where you keep your tools. Unfortunately during winter those kind of garages tend to warm up when the door is closed, then suddenly cool down when the door is opened. That leads to condensation on all surfaces in the garage, including on your seeds. When they absorb the moisture, the seeds can lose some dormancy, but worse, they can be damaged if that moisture freezes.
6. In an unheated garage or shed (MAYBE)
Sometimes, an unheated garage or shed is a good place. As long as your seeds have been able to dry completely, they can freeze without a lot of damage. Since changes of temperature are gradual outdoors, there is less danger of condensation, and winter air tends to keep the seeds dry anyway. The important thing is make sure that the place stays dry. Or you can put your seeds in glass jars and let them freeze - as long as they're well dried when you seal the jars.
Also, be sure to protect your seeds from mice if you keep them in a shed!
There are lots of places where you can store seeds successfully, as long as you avoid humidity and moisture, and only put seeds in air-tight containers after they have had a least a month to fully dry out. If you can keep them dry and cold, that's perfect. If you can only find a place that's dry and warm, or moist and cold, then make sure you keep them dry at least.
Written by Bob Wildfong, Seeds of Diversity's executive director