It happens every year—a customer receives a packet of seeds and, rightfully skeptical of ordering from a new company, decides to test them to see if they’re good.
And more often than not, that test involves a glass of water.
A quick search online will tell you that “good seeds sink” and “bad seeds float.” It sounds simple. Reassuring, even. But if you’re standing there with a packet—or worse, a water glass full of seeds—you deserve a clearer answer than that.
Let’s talk about where this idea comes from—and where it falls apart.
Tomato seeds undergoing fermentation. At this stage, a good seed is just as likely to float as it is to sink. In a few days, however, the sink/float test will come in handy.
Where the Sink-or-Float Idea Comes From
This method didn’t appear out of nowhere. It has roots in traditional seed saving—especially for wet-processed crops like tomatoes and peppers.
When seeds are being cleaned, they’re already fully wet, often fermented, and then rinsed or sluiced to separate heavier seeds from lighter debris. You can learn more about that process here.
In that context, floaters can sometimes include:
- Empty seed coats
- Poorly developed seeds
- Pulp and debris
And yes—sometimes removing floaters can improve overall seed lot quality. In fact, we use this technique ourselves to harvest and clean tomato and pepper seeds.
But here’s the key difference:
This is done during processing, not as a dry-seed viability test.
While the sink/float test works for some pepper varieties, others can be quite stubborn. These Aji Amarillo peppers, for example, produce a good number of floaters that, as it turns out, germinate just as well as the sinkers.
Where It Starts to Break Down
Once seeds are dried and stored, everything changes.
Float vs. sink becomes much more about:
- Air trapped in the seed coat
- Surface tension
- Seed structure and density
- Even subtle varietal differences
And that’s where the rule stops working.
We’ve seen this firsthand. For years, when cleaning Aji Amarillo peppers, we let large quantities of floating seeds run off the end of the sluice, assuming they were inferior.
Then we tested them.
The result? The floaters actually germinated slightly better than the sinkers.
We’ve seen similar behavior in certain squash varieties as well.
And with squash, there’s another wrinkle:
Once seeds are fully cured and dried, they often won’t sink at all—no matter how viable they are. The air trapped inside the seed combined with its broad, flat shape make it no match for the forces of surface tension.
Pumpkins, oh pumpkins—nothing causes more frustration to the seed saver. Their wide, flat seeds have a tendency to float, even when wet and fully viable. Constant stirring is required for wet processing, just to break the tension. Float testing dry seeds is an exercise in futility.
When the Test Sometimes Works
To be fair, the sink/float method isn’t completely useless—it’s just limited.
Here’s a more accurate way to think about it:
| Seed Type | Does Sink/Float Help? | Why |
|---|---|---|
| Tomatoes (fresh/wet) | Yes | Helps remove obvious debris and duds during processing (after fermentation) |
| Peppers (fresh/wet) | Usually | Similar to tomatoes, but inconsistent by variety. Unreliable in dry seeds as shape and trapped air confound results. |
| Beans & Peas | Often | Reliable for large-seeded varieties, less so for others |
| Cucumbers (wet/dry) | Often | Wet seeds can be easily sorted, cured seeds less reliable |
| Squash | Rarely | Wet viable seeds usually sink, but not always. Cured seeds usually float, regardless of viability. |
| Small/light seeds | No | Surface tension dominates behavior |
The key pattern:
It’s most useful during cleaning—not after the fact.
Nasturtium have large, bumpy seeds that frequently trap air. You're better off to germ test a few (in soil) if you're dubious about their quality.
The Bigger Problem: It Can Mislead You
If you’re using this test on a packet of dry seeds, you can easily:
- Throw away perfectly good seeds
- Keep seeds that won’t germinate
- Or worse—damage them through unnecessary soaking
And that last point matters.
Unplanned soaking can:
- Trigger early germination
- Increase risk of rot
- Reduce overall viability if seeds are dried improperly afterward
And some seeds respond especially poorly to soaking. You can learn which ones here.
It’s also worth noting that even when the sink/float test “works,” it’s only identifying seeds that were never viable to begin with—typically underdeveloped or poorly filled seeds. There isn’t some hidden density change that happens when a seed dies. A well-formed seed can lose viability over time while still looking and behaving exactly the same. Even older dry beans, for example, will usually sink when soaked, despite having very low germination. The test can’t tell the difference, which is what makes it so misleading.
Beans are commonly float tested to identify duds, but it's important to note that this only identifies seeds that were defective from the start. Loss of germination potential over time cannot be monitored through soaking—old beans still sink, but probably won't germ.
You Don’t Need to Do This
We understand the instinct. Seeds offer no visual reassurance of their quality—and you don’t know us from Adam. It’s only natural to want some kind of proof that they’re viable.
But that’s our job.
Reputable seed companies test germination and stand behind their seed. We do both. In fact, we’ve warrantied seeds for all sorts of reasons—when they didn’t germinate (no questions asked), when seedlings struggled after emergence, even when the peppers turned out hotter than expected. One time, we replaced dozens of packets after a gardener’s toddler dumped them all together in a tub of water. We get it—we're parents too.
The point is simple: a good company stands by their seeds.
Ours are guaranteed for one year past the sell-by date, so there’s no need to second-guess them with a glass of water.
Whether they're your own saved seeds or purchased ones, the most reliable test is a germination test. If you're curious, place a few seeds on a wet paper towels or better yet, in soil. Within a few days, you'll know what's up (pun intended).
If You Want a Real Answer
If you truly want to test seeds, skip the float test and do a germination test:
- Place seeds in a moist paper towel
- Keep them warm
- Check for sprouting over the expected timeframe
It takes a few extra days—but it tells you something real.
Healthy plants start with healthy seeds. Choose a reputable company, or save your own, and monitor their quality through simple germ testing. No better test exists for monitoring a seed's potential.
A Final Thought
The sink-or-float test isn’t entirely wrong—it’s just been taken out of context.
In the seed world, context matters.
A method that works during processing doesn’t always translate to your kitchen counter. And as we’ve seen, sometimes the seeds you’d toss aside are the very ones that grow best.
So if you’ve got seeds in a glass right now, don’t panic.
Plant them.
You might be surprised by what comes up.
Enjoying this content? Leave a comment below. We'd love to hear what other questions you have about gardening and seed saving. Or visit our Blog Page to view all of our gardening articles.
