Should Collected Seed Be Stored in the Fridge or Freezer?

It is fall and you have just collected some seed from your garden. Should they be stored in the fridge or freezer? Both suggestions are quite common for storing seed, but the true answer will surprise you.

In this post I will have a closer look at storing seeds from your garden.

Fridge or Freezer for Storing Seeds?

The truth of the matter is that neither option is correct.

To understand this better we need to understand a bit more about seeds and how they develop.

Orthodox and Recalcitrant Seeds

Seeds have been classified into two general groups; Orthodox and Recalcitrant (non-orthodox).

The first group, orthodox seeds, probably got their name because these seeds behave very much like the seeds that have been collected and stored for thousands of years. After collecting they can be dried and stored for a long time. This group makes up 80% of all seeds.

The second group of seeds were researched more recently and became known as recalcitrant (having an obstinately uncooperative attitude). In the early days these seeds seemed impossible to germinate, but now we know that they die when they dry out or are stored too cold.

Your Home is Not a Seed Bank

Seed banks are set up to store orthodox seeds. The seeds are dried so that the moisture content is below 10% and for some species as low as 5%. Once they are this dry they can be safely frozen for a very long time.

Gardeners have learned about these storage methods and think it is best to mimic them. They collect seed and then place it in the freezer. If it is good enough for a seed bank it should be good for gardeners, but they forget one important step – drying the seed.

Freezing seed with a moisture content higher than 10% can kill the seed as ice crystals form.

Homeowners don’t have an easy way to measure moisture content and therefore they should not be freezing seed.

When is Seed Mature?

Almost everything you read about gardening will tell you that when the seed pod gets brown and dry, it is mature and the seed is ready for harvesting. I even tell gardeners this in seminars and in my videos. The concept is easy to understand and works well for the general public.

But seed maturation is more complex than this. Many orthodox seeds continue their maturing process after the seed is black and released from the mother plant. In some cases germinability increases only after the drying process starts.

Seed from chili peppers (Capsicum annuum) had the highest rate of germination when the seeds were left in picked fruit for another 14 days after harvest. This was true for green , yellow and red fruit, with higher germination from red ripe fruit, clearly showing that for this seed, maturation was still taking place after harvest. (Note: the 14 days was an arbitrary date for testing and does not reflect the time period that produced maximum germinability.)

Similar results were found for tomatoes, eggplant, watermelon, bell peppers and cucumber.

Gabriela Costea, from BotanyCa puts it this way ” It is the natural way, if you think about it – fruits/seeds mature, fall on the ground throughout the summer and early fall. They will only experience cold gradually from late fall to winter. ”

Orthodox seed can germinate before it looks mature. Soybean and corn can germinate 20 days and 50 days, prior to full maturity, but the resulting seedlings are smaller and weaker than seedlings grown from more mature seed.

When you place freshly collected seed in the fridge, some of the chemical processes that are still taking place inside the seed, suddenly stop. The maturation process is halted, and you have just stored seed that is not fully mature.

For this reason, it is not a good idea to store your seed in the fridge. Let it sit at room temperature and continue its maturation process. By late winter or spring it will be ready to germinate.

Does Seed Get Old at Room Temperature?

Some of you may be concerned that seed sitting around at room temperature will get old and that germination rates will drop. After all, is this not the reason why seed banks store at low temperatures?

To understand this you have to consider the time horizon involved.

For most gardeners, you are storing the seed for 6 months or less. During that time period, orthodox seeds do not get old.

Seed banks are trying to store seeds for centuries. For such long term storage, cold temperatures are important.

Store in Paper

Orthodox seeds continue to lose moisture as they mature. It is important to let this moisture escape, or else the seed can get moldy. For this reason they should not be stored in closed plastic bags. Some form of paper is a much better option.

What About Recalcitrant Seeds?

Recalcitrant seeds need to be treated differently. These seeds die if the moisture levels get below 30%. Because of this high moisture content they can’t be frozen. Most of these seeds can be stored at 0 °C, but some tropical seeds are damaged even below 15 °C.

Plastic bags containing a bit of moistened vermiculite works well.

Even with this type of storage, these seeds tend to have a short life span of a few weeks to a couple of years. Even seed banks don’t have an easy way to store them long term.

When are Recalcitrant Seeds Mature?

The comments I made above about maturity are a bit simplified. In reality maturity can be defined different ways. One way to define it is to consider maturity as being the point where the seed can germinate.

Recalcitrant seeds tend to reach germinability sooner than orthodox seeds.

When the Norway maple (Acer platanoides), an orthodox seed, is compared to the sycamore (A. pseudoplatanus), a recalcitrant, researchers found that the sycamore reached germinability 10 weeks before physiological maturity (seeds look ripe). The Norway maple reached germinability 4 weeks before physiological maturity.

In another study Galanthus nivalis, the common snow drop, showed some germinability 29 days before natural seed dispersal, with a maximum germinability of 79%, 12 days before dispersal.

Many recalcitrant seeds germinate better when the seed is collected in the green stage, rather than the normal brown/black stage. By the time these seeds ‘look mature’, they are already losing viability and are getting old.

Which Storage Method Should You Use?

The take home message here is that there is no “best seed storage method” as suggested by most gardening sources. At a minimum you first need to determine which type of seed you are dealing with; orthodox or recalcitrant. To complicate the matter there is also a third category (sub-orthodox) which is half way between these two extremes.

How do you determine which type of seed you have? Unfortunately that is not easy. I have not found any good lists, but Bill Cullina, of the New England Wild Flower Society has some plants listed and uses the term ‘hydrophilic’ instead of recalcitrant. BotanyCa also does a good job of identifying seeds that need moist packaging and Genesis Nursery, Inc. also has a list.  If you can’t find your plant on a list, here are some general rules you can follow:

• 80% of all seeds are orthodox.
• Most vegetables are orthodox.
• Many North American woodland plants are recalcitrant.
• Some willows, poplars, elms, maples, oaks, hazels, walnuts, chestnuts, and hickories are recalcitrant.
• Many tropical rainforest plants are recalcitrant.

If it is an orthodox seed and it will be germinated within the next year, store it in paper and room temperature. If your goal is to store the seed for many years, make sure they are very dry and have had enough time to mature (several months), and then store them in the fridge – not in the freezer.

Recalcitrant seed should be stored moist. Use a temperature that is similar to their native environment. For temperate seeds, store at outdoor temperatures, and gradually cool them down as winter approaches. Then store in the fridge. For tropical recalcitrant seed, store between 15 to 20 °C. In both cases it is best to plant as soon as possible for best germination. Consider collecting some seed in the green stage to see if germination improves.

There is huge variation in seeds and their growth behavior. It is always a good idea to research each seed and follow the advice given for it.

More on This Topic

This topic was first discussed on our Facebook Group, called Garden Fundamentals. Feel free to post your comments there for further discussion. https://www.facebook.com/groups/GardenFundamentals/permalink/525213384615762/