Seed Saving Time: The Float Test

Tomato seed float test. Image copyright Jill Henderson Show Me Oz.wordpress.comJill HendersonShow Me Oz
Gardeners face many challenges throughout the year, but there is nothing quite as frustrating as planting seeds that don’t germinate well or at all.  You plant and wait.  And then wait some more. All the while precious weeks go by, delaying your carefully planned planting schedule and putting your future crops at risk. I have experienced this a number of times myself. That’s why I’d like to share with you what I’ve learned about the causes of poor germination and a simple test to help reduce the chances of it happening to you.

Let me start of by saying, there are a number of reasons why seeds don’t germinate. Unfortunately, one of the most common reasons is that people doesn’t seem to understand that seeds are actually living, breathing embryos. Just because you can’t see the baby plant inside, doesn’t mean it’s not alive! Seeds are basically a fully developed embryo encased in a protective shell. Once it’s outside the parent fruit, it lies dormant, waiting for the perfect time to germinate.  To ensure their survival, seeds must be nurtured and cared for.  Poorly dried or stored seeds often die or exhaust themselves trying to stay alive under adverse conditions.  To help avoid problems during storage, you might want to stop here and read this article on how to properly store seeds.

Seed Saving Time: Drying and Storing Your Home Grown Seeds

Once storage issues have been addressed, it’s time to look at a condition called inbreeding depression, which occurs when seeds are saved from too few fruits from too few plants.  This condition is basically the result of reduced genetic diversity and can expressed in various ways, including poor germination and, over repeated cycles, complete lack of germination.  Inbreeding depression is most common in outbreeding crops that have separate male and female flowers, either on each plant, like squash and melons, or on separate plants, like spinach.  Corn is notorious for inbreeding depression.

And while improper storage and inbreeding depression are the two most likely reasons for poor germination, the most common reason is more often the result of harvesting seeds from fruits that are not botanically mature. If the fruits are not ripe, then neither are the seeds within.  For example, many of the fruits we eat are fully ripe when they are at their best for being eaten. Watermelon and tomatoes are perfect examples. But others, like corn, cucumbers and green beans, which aren’t anywhere near ripe when harvested for table use.

Like human babies, seed embryos take time to fully develop to the point where they can live on their own.  But before that can happen, fleshy fruits must reach their full size and mature coloration.  Cucumbers and summer squash must go well beyond the edible stage and turn yellow, gold or tan before they are fully mature (they’re also HUUUGE!).  On the other hand, when melons, watermelons, winter squash, and pumpkins are ready to eat, most of the seeds inside will have already reached a high-level of maturation.

Removing seeds from a ripe, green-fleshed muskmelon. Image copyright Jill Henderson. showmeoz.wordpress.com

For seeds born in pods (beans, peas, okra, radish), capsules (lettuce and most Brassicas) or  inflorescences (dill, fennel), the “fruit” must be allowed to dry completely while on the plant – or at least begin to dry before being removed and allowed to dry further off the plant.  (Note: the picture above is of a fully ripe, green-fleshed muskmelon – which was incredibly delicious, by the way!)

Knowing when fruits are fully ripe and waiting until then to harvest their seeds is the most reliable way to avoid poor germination in the future.  If you’re not exactly sure what crops should look like when they are fully mature, check around online for pictures or search seed saving forums.


The Garden Seed Saving Guide by Jill Henderson   The Garden Seed Saving Guide

Save money, become more self-sufficient and avoid GMO’s by saving seeds!  This excellent resource for beginning and hobby seed savers takes you step-by-step through every aspect of saving your own seed in plain English.   Look inside!


Once you’ve got a fully ripe fruit, there is a simple way to check certain types of seeds for viability.  This is known as the Float Test.  I warn you in advance that this method is not 100% accurate and it only works with the freshly harvested seeds of certain fruits, including members of the Cucurbit Family, such as melon, watermelon, and cucumber. It also works well for all types of squash and gourds, and crops in the Nightshade Family, such as peppers, tomatillos, and tomatoes.

Also keep in mind that the float test works best on what seed savers refer to as “wet seeds” – those that are wet when extracted from the fruit.  Keep in mind that seeds of tomatoes, muskmelons, and cucumbers should always be fermented prior to the float test.  Fermentation dissolves the placental membrane covering each individual seed, which not only increases germination rates and prevents seed-born diseases, but if not removed completely, will also make seeds float whether they are ripe or not. It’s what I call a ‘false positive’.

Before performing the float test, rinse your seeds well to remove any pulp, sugars, and membranes.  I also like to put a little friction into the mix and once I clean and drain my seeds, they go into a small-mesh metal strainer where they are pressed and rubbed into the screen to dislodge stubborn membranes clinging to them.  But don’t get too rough or you might injure the seed’s protective shell. The idea is to get off the gel-like membranes and any minute bits of clinging flesh.

Removing membranes from melon seeds. Image copyright Jill Henderson showmeoz.wordpress.com

It is also important to note that the float test cannot be considered even slightly accurate when used on seeds that need to be fermented, but which have not been.  It also doesn’t work well for checking the viability of naturally dry seeds, like dill, coriander, corn or okra. Ditto for seeds that have started to dry out, or seeds that have previously been dried.

Basically, dry seeds usually float no matter how ripe or unripe they are and seeds that are not fermented float like balloons, thanks to that bit of placental membrane clinging to them.  Oh, and sometimes ripe seeds will float if there is even the minutest air bubble clinging to it.

Like I said, this test is not 100% perfect. But as seed savers, we often have more seeds than we know what to do with and can often afford to lose a few ripe ones.  So before we move on, I just want you to know that you can count on the fact that some of the seeds you wind up tossing out may in fact be ripe. But in my humble opinion, the float test is more effective in removing unripe seeds than any other method available – despite the few ripe floaters.

To perform the float test, start with freshly-harvested or freshly-fermented seeds.  Remove any large veins, membranes or flesh and then  simply pour the seeds into a large canning jar, add just a little bit of water, cap and shake like mad for a few seconds.  Open the jar and fill the jar almost to the top with clean water.  Shake again and let things settle for a 15-30 seconds.  Some debris will sink to the bottom and some will float.  Pour off the floaters into a waiting strainer.  Add a little more water to the jar and repeat the process until the water is nice and clear and all remaining seeds sink to the bottom.  Those are seeds that have passed the float test with flying colors! I like to think of it as a race – first one to the bottom wins!

Performing the float test of watermelon seeds. Image copyright Jill Henderson. showmeoz.wordpress.com

You may want to leave it at that…unless, of course, not all the floaters in the strainer look “immature”.  In fact, some of them – and sometimes a lot of them – are more than viable.   No one knows exactly why this happens (and uh, if you actually do know why, can you call me?).

Theories abound.  But sometimes, I’ll look in that strainer and know that I’m throwing away good seed.  And if I need more seed, I will work the strainer seeds over again, really getting in there to clean off membranes and re-test them until I’m satisfied.  A lot of times, this generates many new sinkers.  Sometimes, nothing changes.  And sometimes, when I know what I see with my own eyes, I’ll ditch the scrawny ones and keep the rest.  Only a germination test will tell me if my intuition was right.

Saving both floaters and sinkers for a future germination test. Image copyright Jill Henderson showmeoz.wordpress.com

So, if you want to try out this handy tool for sorting ripe seeds from unripe seeds and improving your overall germination rates – I highly suggest it.

Besides, it’s just fun to play with your seeds!

Happy seed saving!

© Jill Henderson  Feel free to share with a link back to the original article.


The Garden Seed Saving Guide by Jill HendersonThe Garden Seed Saving Guide
Seed Saving for Everyone!

Save money, become more self-sufficient and avoid GMO’s by saving seeds!  This excellent resource for beginning and hobby seed savers takes you step-by-step through every aspect of saving your own seed in plain English.

Available in the Show Me Oz Bookstore
Look inside!


Jill Henderson is an artist, author, and the editor of Show Me Oz . Her books, The Healing Power of Kitchen Herbs, The Garden Seed Saving Guide and A Journey of Seasons can be found in the Show Me Oz Bookstore.  Jill is a contributing author for Acres USA and Llewellyn’s Herbal Almanac and her work has appeared in The Permaculture Activist and The Essential Herbal.


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4 responses to “Seed Saving Time: The Float Test

  1. Pingback: Seed Saving Time: Drying and Storing Your Home Grown Seeds | Show Me Oz

  2. I go for a visual test. The seeds must be well coloured according to their typical appearance, and they should feel firm. If you’ve done everything else right u till then, and the seeds are properly dried I have never had a problem with germination except when age or mould was the problem. I’ve kept around 100 types of seeds viable for 15 years so far in a sealed container in the fridge. Whenever I use any of them they seem to be fine.

    • That’s awesome, Rabbit. Your visual method is spot on and every seed saver should learn to visually determine when seeds are mature. The float test is just a little helper for those seeds that are difficult to discern with the eye.

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