Wednesday, February 20, 2013

Long Term Storage of Seeds

Gardeners are often required to grow out their vegetable varieties to keep each kind of seed they have viable. Though there are many charts available online to help gardeners determine how long they can expect their seeds to last after harvest, many of these charts don’t help gardeners who may want to only grow their harvest out to seed every other year, instead of every year. What if I don’t want to grow onions or parsnip every year – does this mean that I have to buy new seed each time or anticipate that only half my seed will germinate after being stored for 2 years?

Some supplies for Long-term storage of vegetable seeds

Gardeners frustrated with the idea of growing out vegetable varieties just to preserve their hard-earned seed may take comfort in knowing that there are other alternatives out there. A few of the options to extend seed life include desiccating (or drying out) seed, storing seed in airtight containers and storing the seed at very low temperatures.

Silica Gel Beads from Southern Exposure Seed Exchange

Before putting seeds in a container, they need to be dried out properly. The reason they need to be dried is so that the seed's cell walls do not expand and burst when being frozen. Most garden seed uses moisture as one of the triggers to know when to begin the plant’s growth. However, when moisture is present without the presence of other growing conditions, moisture can cause vegetable seed age more quickly and, over time, the seed will fail to germinate. While several methods of drying seeds out exist, it is often difficult to know exactly how dry seeds are without the use of some form of material to desiccate the seeds. In order to prepare seed many seed-savers are now utilizing silica gel (or beads) to ensure that their seeds are dried out properly before moving their seeds into long-term storage. This is similar to the silica gel you may find in a container with new shoes, however this gel is used for garden applications. The silica gel is able to bring the seeds to a 2-3% moisture rate. I obtained my silica gel from Southern Exposure Seed Exchange. They sell a food-grade silica bead that is much safer to use around food than other varieties. When drying out seeds I put an equal amount of seeds, per weight, as silica beads in a glass jar to dry out. Note: if your seeds are already dry then be careful of drying them out any more! Over-drying seed can desiccate them so much that it can kill the seed.

Putting orange-to-green silica beads and seeds in jar

Storage container ratings from McCormack, 2004.

Once dried properly, the seeds should be kept in an airtight container to keep out the moisture. Although polyurethane (plastic) bags are often used to store seeds, these bags often let in small amounts of water. As a result, though storing seeds in plastic bags might be better than storing them in the open air, storage in glass jars with airtight lids will make the seeds last much longer. If mason jars are used, a new or unused seal is recommended for greater protection from leaking. There are other storage methods that are used, including metal lined polymer bags.  In general, the less the container that stores seed leaks - the longer the seeds will last. One research-based online article tells of several good ways to store seeds. 

A clean break when snapping a seed (right) is evidence of proper drying.

There are a number of studies that show that frozen seed can last longer than seed that is stored at room temperature including one from North Carolina State University and another one from a journal entitled Cryobiology. Additionally, some seed may be supported, rather than hindered, by being frozen. However, any seed that cannot be dried properly should not be frozen. When seeds with excess water content are frozen, the expanding water can damage the cell walls of the seed, causing the seed to be damaged and unusable. Remember that excessive desiccation of seed can lead to seed mortality as well.

Freezing Vegetable Seeds can help them last longer

All melon seeds from those that were not dried sprouted

All melon seeds from those that dried also sprouted

All seed sprouted - Strange that the frozen ones grew faster

Taken together – proper drying, storage and freezing of seeds may be the best way for seed savers to preserve their seed for future growing seasons and future generations. I experimented by sprouting seeds that had not been dried, had been dried, and had been frozen. All three were watered and sprouted within 3 days. My experience has shown that when proper drying, storing and freezing are used together, the germination of frozen seed is equal to original germination percentages. So next time you want to store some of that seed you have worked hard to harvest you might consider drying, bottling, and freezing your seed.

Can you sprout and grow seeds that have been frozen? Yes!


  1. I had never considered this. I am excited to start saving seed... perhaps this year I will have time finally. I have that same book in your first photo.

    1. Dear Donna,
      I always enjoy your comments.
      The first chapter of "From Seed to Seed" does a good job explaining proper seed saving techniques. It is definitely worth a read.

  2. Love your experiments... and it is very curious that the frozen seeds did so well. Made me wish my Science Fair days weren't over. That would have been a great one. (Raising eight kids we were always on the look out for new ideas.)

  3. I really enjoyed this info. Saving seeds for success has always been a challenge.

    1. Thanks for the comment Lynne. Hopefully, as we all learn to store our seeds well we will have them for a long time to come!

  4. Enjoyed the article. Thanks. The link " One research-based online article tells of several good ways to store seeds. " does not seem to work. Do you have an updated link?

    1. Dear Shew,

      Thank you for your response and question. I have found a cached version of the article in text-only form. I changed the link, though I am not sure how long the cached version will remain online.

      Since creating this post I have found that a slow cooling and slow warming up results in better germination than in a quick freeze and quick warming up. I would suggest putting properly dried and prepared seeds in the refrigerator for 48 hours before freezing and putting them back in the refrigerator for 48 hours (or longer) after freezing to avoid altering the tissues in the seed too quickly.

  5. Is there a short list of vegetable or fruit seed that could work with freeze drying processes ? That is, the freeze dryer from harvest right freezes first then dries with warmth on the drying trays, then vacuums the moisture. Harvest right says someone tried it then found the seeds did germinate, but there was only one person and no list of working seeds.
    I welcome sheer speculation, and thanks, just came across your site and felt welcome.

    1. Dear Lynne,

      I don't believe I have ever used a freeze drying process, but I don't see why it wouldn't work, to a point. There gets to be a point at which the seeds can dry out so much that they are no longer viable. I would experiment by starting with a large batch of very good seed (90%+ germination rate), then freeze a very small amount of that batch for 80% of the time that normal freeze drying of that much substance would require, then take them out. Then I would put them in the refrigerator for at least 12 hours, then in room temperature for at least 12 hours, then try to sprout them using wet paper towel at the optimal germination temperature. If the germination drops significantly from your initial germination rate, then decrease the amount of time you put the seeds in your freeze dryer until you figure out the right amount of time per ounce or gram you are trying to preserve.

      To elaborate on the process: When experimenting with freezing seeds, I have found that the seeds remain much more viable when I allow the frozen seed at least 12 hours in the refrigerator between when they in the freezer and when I expose them to room temperature air.

    2. Oh, and thanks for the complements, Lynne!

  6. nice information sir. it is helpfull. but i want to raise one question. that im looking for store my large quantity of hybrid watermelon seeds in frozen for 2 years. temperature may be -8 C. then will it be last long for 2 years. real time test for germination shows 95% germ. rate. will it decrease or any problem occured?

    im worried that watermelon seeds can frozen or Not? please guide

    1. Dear Radha,
      You ask a good question. Optimal temperature and humidity levels for long-term storage of seed can vary, depending upon what kind of seed you are trying to store. I have not found a large volume of information on this topic to help me in determining how to best store my seed. However, from my experience in the long-term storage of seed, here is what I advise: Begin with a test batch. Desiccate (dry) a portion of your seed until it splits clean when snapped with your fingers. Then, place 25 of these dried seeds in a double air tight container in the freezer at the -8 C for two days. Next, take this small batch and put it back in the refrigerator for two days. Finally, place these seeds out of the refrigerator for an hour and let them come to room temperature. Separate 10-20 of these seeds to germinate (based on how many you can spare). My preferred method of sprouting seed is in a lightly moist paper towel kept in an airtight bag and on a mildly warm surface. If, after 3-7 days they do not germinate then the drying percentage was to low/high, or the temperature at which they were stored was not conducive for survival of the seed.

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  8. Have you tried this with onion seeds? How long?

  9. Dear Lee Tullberg,

    I have not tried this yet. I'll have to try that another time.

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