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Freezing and Cold Storing Seeds for Long Term Storage

Since the seed supply problems caused by the pandemic, some gardeners are looking at how they can store seeds long term. Freezing helps to store food for long periods so why not freeze your seeds against future shortage.

Svalbard Global Seed Vault Photo: Cierra Martin for Crop Trust

There is precedent for freezing seeds as a hedge against disaster in the future – the Svalbard Global Seed Vault on the Norwegian island of Spitsbergen. The seeds are stored at -18ºC, the normal temperature of a domestic freezer.

To achieve this safe, long term storage and ensure seed viability when taken from the vault, they’re dried to below 14% seed moisture. This prevents ice crystals from forming when they are frozen. Ice crystals rupture cells and effectively kill the seeds.

To keep them dry and prevent oxygen reaching the seeds whilst in storage, the seeds are stored in sealed three-ply foil packages.

Can you freeze seeds at home?

Seeds in Freezer for Long Term Storage at -18ºC

Yes you can freeze seeds at home for long term storage. To do this you need to ensure the seeds are properly dry. Unless you use a specialist testing company or buy equipment to do this, that is a bit of a gamble.

Seeds bought in are most likely dried to a low moisture content level, so can be frozen safely. Once a packet has been opened the seeds will start to absorb moisture from the air. If intending to freeze and store unused seeds, they must be kept dry. An airtight container with moisture absorber like silica gel or some other dessicant in is best.

With home saved seeds they must be thoroughly dry to freeze safely. You could try a dryer or low oven but when drying seeds you should not use a temperature above 37ºC. Higher temperatures will probably kill or at least reduce the viability of seeds.

Then the seeds will need to be carefully packaged to keep them airtight and dry.

Most likely you will be OK with foil packed commercial seeds with a long shelf life but they will need extra packaging and even then there is no guarantee that they will remain viable. Seeds dried to a low water content for frozen storage will take a longer time to germinate.

Simply put, yes you can freeze seeds but there’s no guarantee they will be viable when you need them unless you know their moisture content. It’s probably not worth the gamble when you can store seeds for a quite a long time without freezing.

Increasing Seed Storage Life

There’s a standard list of the expected lifespan of seeds here: Seed Storage & Lifespan

The conditions seeds are kept in will make a huge difference to that lifespan.

Dry seeds last longer

When storing dry seeds, keep them that way. Airtight packets in an airtight tin with some dry rice to absorb moisture is adequate for home storage. Seeds will absorb moisture from the air and we need to prevent that. Above 30% seed moisture, non-dormant seeds will begin to germinate.

Taking the moisture content to between 10% and 15% will suppress activity and fungus. Below 10% few insects survive to damage the seeds. Each 1% reduction in seed moisture doubles seed life.

4-6% is the ideal seed moisture content for long term storage (more than a year).

Temperature

Seeds will keep best at low temperatures. Each 12C reduction in seed temperature doubles the seed life.

The best way to store seeds

With home saved seeds, dry them out as best you can bearing in mind that you should avoid cooking them. Whether home saved or commercial, put the seeds into airtight packets. You can buy small airtight plastic bags on Ebay.

Then put the packets into an airtight tin or plastic container. Pop some dry rice or silica gel desiccant in the container to absorb any residual moisture.

Keep the tin in somewhere cool and dark. A fridge if space allows or under a workbench in a garage. Avoid sheds as they tend to get hot in summer sunshine and we really need to keep them cool.

Once a commercial packet of seeds is opened, use what you want and then put the rest into storage immediately. This will increase the viability the following year. I put them into small sealable plastic bags and then into an airtight tin.

Seed Storage Facts

Here are some interesting facts on seed storage:

  • Each 1% reduction in seed moisture doubles seed life.
  • Each 12C reduction in seed temperature doubles seed life.
  • Above 30% seed moisture, non-dormant seeds will begin to germinate.
  • 10-15% moisture content suppresses seed activity and fungus.
  • At less than 14% seed moisture, no ice crystals will form, so you may store seeds below freezing.
  • Below 10% moisture, few insects survive (to damage your seeds).
  • 37C is the maximum temperature that should be used to dry seeds of most plants before frozen storage
  • 4-6% is ideal seed moisture content for long term storage (more than a year). Can be higher if refrigerated.
  • Seeds stored in refrigerator or freezer must be in sealed container, or they will imbibe moisture.
  • Seeds dried to a low water content for frozen storage will take a longer time to germinate.
  • Without being sure of the seed moisture content, seed freezing is a gamble.

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