Sunday, September 27, 2015

The Life Expectancy of Seeds

Do you store away your leftover vegetable seeds for next time? I do. Recently I went through my seed-box and discarded all my beyond their use-by date seed packets. Was I wrong?  How long do seeds really remain viable?

The oldest germinated seed

Some seeds remain viable for an incredible amount of time. In 1964 during excavations at Herod the Great’s palace in Masada, Israel a cache of date palm seeds was found preserved in an ancient jar. They had been stored and left in very dry and sheltered conditions since around 155BC . After excavation they were held in storage for another 41 years before they were carefully prepared for an attempt to germinate them. Three of the seeds were then planted at Kibbutz Ketura in the Arabah desert in southern Israel.  One of the seeds sprouted and the date palm that grew out of it is now more than two meters tall.

The date palm was nicknamed "Methuselah," after the longest lived person listed in the Bible.

Methuselah is remarkable in being the oldest known mature seed that has successfully germinated. It is now the only living representative of the Judean date palm, a tree extinct for over 1800 years, which was once a major food and export crop in ancient Judea.

A seed may not germinate if ....

  • It was never viable
Some plant species produce many empty seeds, ie. seeds that do not have an embryo or have an incomplete one.  This may be because the species' survival chances are higher when not too many young seedlings compete with each other.  
Reason can also be adverse growing conditions under which a particular plant produced seeds or its genetic make-up.  
Incomplete pollination also results in a high proportion of empty seeds. They look like seeds, but there is no embryo, or one that is only partly developed. Some plants require multiple visits to each flower by a pollinator in order to produce a good seed pod or fruit. Within one flower-head viability of seeds can vary enormously.
  • It lost viability
Seeds gradually become less viable as time goes by.
In general, germination is best when seed is really fresh, in other words, when it has just reached maturity.  Some plant-varieties produce seeds of which the viability quickly diminishes, whereas other seeds remain viable for a really long time. Good examples are parsnip (always use the freshest seed because it does not remain viable for very long) and lettuce (can still be viable after 5 years). 
Ready-for-harvest viable seeds can also lose viability while still on the plant because of wet weather.

Viability may also be reduced by less than perfect conditions after harvest. A seed may have been made unviable by irradiation or it may be un-pollinated.
  • It is ‘not yet viable’
Over generations some plant varieties have survived by not making their mature seeds immediately ready for germination, but by keeping them dormant and making their readiness for germination dependent on a trigger.
Temperature and moisture levels are examples of triggers. If the right conditions are not met, the seed remains dormant and, after a while, it might die. 
Spinach seeds will not germinate if the temperature of the soil the seeds were sown in is above 20 degrees Celsius.  On the other hand, pumpkin seeds will only germinate if soil temperature is 20 degrees Celsius or higher.  If you plant pumpkin seeds in soil that is too cold it may seem that the seed is not viable, but this is not the case. 
Plants can be very clever about this. Some varieties, for instance, spread germination of a batch of seeds over time, so a catastrophe after germination (e.g. late frosts, drought, eating by animals) does not result in the death of all offspring. 
  • It was not handled well after harvest
It may have been harvested too early, or after harvest it was not dried properly or stored properly. As a result the seed died.  More about storing seed later on.  Autumn drying can be a problem in Tasmania with cooler weather and moist air.  Good air flow during drying is vital.
  • Germination conditions were wrong
Seeds of different species have different requirements for light, soil, temperature and water.
If these conditions are not met the seed does not germinate or it dies. Perfectly viable small seeds, for example lettuce and celery, may not germinate if sown too deeply where light can't reach them.

The life expectancy of vegetable seeds 
If stored under good conditions by the farmer that produces the seed, your seed company, the retailer where you buy your seed and by you, you can expect your seeds to be viable for …

1 Year - onion, leek, parsley, parsnip, salsify

2 Years - sweet corn, okra, pepper

3 Years - asparagus, bean, broccoli, carrot, celeriac, celery, chinese cabbage, kohlrabi, spinach, pea

4 Years - beet, brussels sprouts, cabbage, cauliflower, silverbeet, chard, chicory, eggplant, fennel, kale, mustard, pumpkin, squash, tomato, turnip, watermelon

5 Years - cucumber, endive, radish, artichokes

6 Years - lettuce

The above list is only an indication of life expectancy, but it shows that there are big differences between various types of vegetable seeds.
Be aware: this is the time from seed maturity, not the time from when you got the seed.
My sources for this info can be found at the end of this blog post.

Seed companies and ‘Sow By’ dates 

Some of the seeds you buy will not come from the seed company itself, but from breeders or from contract growers who sell to a handful of major worldwide seed distributors, who in turn supply retailers.  The seed you buy could have been grown anywhere in the world, from your own neighbourhood to fields halfway around the globe.

In Australia most seed companies provide ‘sow by’ dates on their seed packets. ‘Sow By’ dates on seed packets are a best-guess by the seed company based on their own germination testing.

‘Sow By’ dates do not tell customers when the seed was harvested and are no guarantee that the seed will germinate. ‘Sow by’ dates on seed packets in hardware stores may create the impression that the seeds in it will be fine for another 2 or 3 years, but packets may have been on display in light, warm or humid conditions and therefore not live up to expectation.

Your best bet to get consistently fresh seed is really your local seed business, because you are often dealing with people who take pride in what they sell and do extensive germination testing to make sure their seeds live up to expectation. 

If you get good germination results the first time you sow a particular bunch of seeds, chances are you can save leftover seed for the following year and see at least average results, assuming you're not dealing with short-lived seed such as onion, leek, parley or parsnip.

If your initial results are not great (less than 6 out of 10 seeds germinate), I would not count on the seed lasting another season, even under ideal storage conditions. This assumes that you stored your seeds well, kept them moist once sown and so on.

Tasmanian seed companies Southern Harvest and Inspirations (for details see the Useful Links page on this blog) are good examples of small seed businesses that, in my opinion, are preferable over multi-national seed companies.

Inspirations test their seeds four times per year and are confident that all the seed they stock has a germination rate of around 95% at the time of harvest.  You can read more about their germination testing in their February 2015 newsletter here:

I recently discarded many of my old seed packets. In fact, I emptied the content into a jar and am going to sow them all in a spot in the garden that I don’t use for anything else, just to see which seeds past their ‘sow by’ date will come up. Here is what I am going to sow:

Other people might give such a wonderful out-of-date mix of seeds to their chooks.

The seeds I choose to sow 

If you sow today you will know 3 or 4 weeks from now what percentage of your seeds were viable. For me, that is too much of my growing season wasted if the seeds were not viable. In most cases seed is not expensive. I prefer to buy new seed with an expiry date far into the future (always check or ask), rather than use seed that is near its sow-by date and lose valuable weeks. 
For the same reason I prefer having to thin out too-thickly sown seeds to having to ‘spot-sow’ where seeds did not come up.

Checking and testing your seed

You can test your seed in the following three quick ways:

  • A visual inspection: a basic assessment to differentiate healthy, plump (viable) seeds from seeds that are shriveled, affected by insects, or immature (green or pale). 
  • A float test: determines if a seed is full or empty. Empty seeds will float.  After this test make sure to thoroughly dry the seeds (not in the sun) or plant them immediately.  This method may not work for tiny very light seeds.
No doubt: the seeds on the left sank, the ones on the right floated
  • A cut test: determine the internal state of a few seeds you are willing to sacrifice. Are they full, shriveled or empty? Again, this method may not work for tiny seeds. 
You can also do a seed-viability test. It takes 7 – 10 days. You need a sealable plastic sandwich bag, paper towels and a sunny window and to do the following:
  • Moisten the paper towel enough so seeds will receive moisture at all times. 
  • Fold 10 seeds into the towel (10 seeds makes it easy to determine germination percentage. If 8 seeds sprout, you have 80 percent viability). Some people argue that 20 seeds makes a more representative sample. 
  • Seal the paper seed-filled towel in the bag and then mark the bag to identify the seeds. 
  • Place the bag in a location where the temperature is around 20 degrees Celsius. 
  • Wait 7-14 days (even for slowly germinating varieties there should be some change over that time). 
  • Be sure the paper towel or filter does not dry out during this time. 
  • Count the number of seeds that germinate and calculate the percentage. If less than 70%, you could still use these seeds, but you would sow a lot thicker. 
With some seeds (peas for instance), if you very carefully sow the seeds at the end of the test, they will continue to germinate and you have not lost any time doing the test.

The fact that a seed germinates, does not mean it will grow into a healthy plant. If only 20% of seeds germinate, the ones that do, may be weak and not reach maturity.

Storing seed:

  • Make sure seed is completely dry before storing it 
  • Store it in a spot that is continually under 20 degrees Celsius. One seed company reported keeping its seeds at a constant 7 degrees Celsius.  Your average backyard shed might not be a good place for storing seeds as temperatures will go up and down all the time.
  • Store seeds in a place that is dark. 
  • Ideally humidity is low and does not vary.
    Rosalie's well-organised seed stash
  • Storing seeds in their original seed packets can be fine, but brown paper bags are better, because they better absorb any moisture that might be in the environment.  Some people recommend paper bags in zip-lock plastic bags.
  • Many of us, and some of the professionals as well, use glass jars with lids that close tightly. Some people put their seeds in paper bags in glass jars or add silica crystals to every jar to absorb any moisture that might be there.
  • Always make sure that you know what seeds you are storing and when they were stored.  Both name and date are vital to avoid disappointment later. If you bought your seed, you could write date of purchase on the original seed packet and keep that with the seed.
  • While out in the garden sowing, expose seeds, seed packets and jars as little as possible to sun and moisture.

Some seeds can be kept in a freezer or fridge, but only if they are in air-tight and moisture-proof containers. Paper bags and containers with plastic lids or containers made entirely out of plastic are not okay in this environment. Some people report seed remaining viable for much longer when kept in fridges or freezers. Other people report that their seeds died in their freezer or fridge. Both opinions may be correct, because they may apply to different types of seed.
It would be great to hear from people who successfully store seeds in freezers and fridges. It would be interesting to know what seed varieties keep well in this environment.

Thank you Sam and David!

Some of the info in this blog post is the result of feedback provided by David of Inspirations Garden Centre and Sam of Southern Harvest.  If you like to contact these Tasmanian seed companies, have a look at the Useful Links page of this blog.

Happy sowing!

My information re life expectancies for vegetable seeds came from:

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