Storing Seeds & Environmental Factors That Affect Viability
The length of time plant seeds remain viable for varies greatly depending on whether or not storage conditions are ideal and even on the species of seed stored.
Seeds from some species just naturally remain viable longer than other species. Some species of seed are so short lived that they should be sown and germinated in the same season that they are collected. Seeds from alpine areas generally have quite a short lifespan compared with those from lowland areas, many alpine species only survive for a about a decade even when stored under ideal conditions in a reputable seed banks such as the Millennium Seed Bank at Kew Gardens in the United Kingdom.
The seed of some species, in particular many rainforest plants, are what we term recalcitrant. Recalcitrant seeds are unable to survive being dried out in the way that seed with a traditional dormancy can be. Recalcitrant seeds must not be allowed to significantly dry between when they are planted and when they are germinated, this presents a challenge for storage as seed pathogens such as moulds thrive in the damp conditions required to keep recalcitrant seeds viable for longer than about a week. Recalcitrant seeds are typically planted as soon as possible after harvesting. The vegetable and flower seeds you buy in packets and plant in your garden at home on the other hand are examples of seeds with traditional dormancy and are quite happy to be dried out within limits.
Some seeds have a longevity that rivals some of the oldest trees alive on the planet. The record for the longest seed in the world to germinate belongs to a 2,000 year old date palm seed. Remarkably this seed wasn’t recovered from a place with ideal (stable cold and dry) storage conditions. Instead it was recovered from a place close to the Dead Sea which although very dry is also extremely hot with an average maximum temperature that ranges between 20 degrees Celsius in Winter to 39 degrees Celsius in Summer. The seed was germinated in 2006 and is still healthy and growing today. A 1,300 year old lotus seed had also been previously germinated, however it didn’t cope with the long storage period quite so well. The resulting plant was severely malformed due to genetic abnormalities.
So lets examine some of the factors that can decrease seed viability in seeds with traditional seed dormancy, that is seeds that can survive being dried and have about a 6-10% moisture content inside their walls once they are dried, and what can be done instead to maximise it.
Humidity & Seed Viability
The first big factor affecting seed viability is moisture. It’s a well known fact that seeds are alive, they are a complete blueprint to create a new plant encased in a package that both protects it and provides energy to enable germination. Because seeds are alive they respire (although very, very slowly) just like any other living thing does and require oxygen just like any other living. However air is more than just oxygen, air contains moisture and a measure of this is known as relative humidity.
Too much moisture in the air will reduce seed viability and cause seed pathogens such as moulds to grow. But as seeds are alive they need some moisture to survive, too little moisture in the air will dry the seed out too much and kill the seed. In order to maintain the ideal moisture range within the seed of 6-10% seeds should be stored in air that has a relative humidity between 30 and 45%. In order to achieve this, seed should be stored in a controlled (air-conditioned) environment set to within this ideal relative humidity range. This solution however, is rather costly and impractical except for large seed companies.
As a cheaper alternative seed can be sealed into a airtight pouch or container when the external natural humidity is within this range, this will seal the seed in air that will remain within this ideal relative humidity range even as the external relative humidity changes. Any excess air in the pouch should be squeezed out or if using containers choose a container suitable to the quantity of seed to be stored, remember seeds only need a minute amount of oxygen to maintain respiration. Sealing the seed also prevents them from being eaten by insect pests, although small insect pests will have no problem maintaining a thriving community if they are present in the seed when it is sealed. It should also be remembered that once opened the relative humidity inside the container will equalise with the external relative humidity. It goes without saying that exposing the seed directly to water will trigger germination and any seed that is germinated and dried again will die and no longer be viable.
Temperature & Seed Viability
The second big factor affecting seed viability is temperature. As a general rule the lower the temperature the seeds are stored at the longer they will remain viable, however the stability of the temperature the seed are stored at is also important. Ideally seed should be stored in a controlled environment set to a low temperature, or just in a house that is maintained via air-conditioning at a constant temperature.
I don’t have air conditioning so I tend to store any seeds under my house where it’s generally a bit cooler during Summer than inside the house. I use an old refrigerator to store the seed in, it is not turned on but the insulation in the walls of the refrigerator helps to keep the temperature more stable by reducing some of the extremes in temperature experienced in the middle of the day and night.
Some people like to keep seeds inside their grocery refrigerator but refridgerators can sometime be quite humid so seed stored this way should be kept in an airtight container. Some fridges have a low-humidity crisper section and this is the best place to store seed if you are storing your seed in this way. When removing seed from a refrigerator be sure sit the container out on the bench for awhile to allow the temperature inside the container to equalise with the outside temperature before opening, otherwise condensation can occur which will wet and ruin your seeds.
Should I Freeze Seeds?
No seed should ever be frozen unless they are first dried to a moisture level at the low end of their tolerable range as any excess moisture within the cells of the seed will form ice crystals which can rupture cell walls and cause the death of the seed. Without the sophisticated equipment some seed banks have to freeze their seed for long-term storage drying the seed to the correct level is extremely difficult and not generally achievable for home gardeners.