How do you know if your seeds are still viable? Simple, do a seed germination test. Place the seeds in some water. The ones that sink are still viable – the ones that float are dead.
This advice is all over the internet so it must work? But how reliable is it?
Testing seeds in water is a simple and quick way to check if they are still viable and likely to germinate When seeds are placed in water, viable seeds will sink while inviable seeds will float This “float test” allows you to easily separate potential seedlings from duds before planting.
While floating seeds in water is not a perfect test it can help gardeners avoid wasting time and energy planting old dead seeds. When used properly, it provides a good indication of which seeds to plant and which to throw away.
Why Seeds Float or Sink in Water
When a seed starts to germinate, it absorbs water through a tiny opening called the micropyle. This causes the seed to swell up. Viable seeds are filled with starchy food reserves that help them germinate and develop. The denser contents cause them to sink in water.
On the other hand, inviable seeds are often hollow and lightweight. They don’t have enough stored food energy left to successfully germinate. This lower density causes them to float.
Seeds may also float if the outer seed coat has become cracked or damaged. This allows air bubbles to become trapped inside making the seed buoyant.
Tips for Testing Seeds in Water
To perform the float test properly:
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Use distilled or bottled water. Tap water may affect results.
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Place seeds in a clear glass or jar with plenty of water. The container should be deep enough so seeds have room to sink.
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Allow seeds to soak for 5-10 minutes before checking results. Some seeds need time to fully absorb water.
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Agitate the water gently to dislodge trapped air bubbles. Swirl the water or tilt the jar back and forth.
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For very small seeds, pour water through a fine mesh strainer after soaking. Rinse the strainer to collect the sunk seeds.
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Check seeds individually whenever possible. Don’t just look for a pile at the bottom.
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Test seeds in small batches for easiest observation. Crowded seeds make it hard to see.
Seed Types That the Float Test Works On
The float test has proven reasonably reliable for:
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Larger seeds – Beans, peas, corn, peppers, melons, squash
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Medium seeds – Tomatoes, marigolds, zinnias, carrots, cucumbers
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Small seeds – Lettuce, cabbage, broccoli, spinach, radish
Tree seeds like acorns may also sink or float based on viability.
The test is not as effective for tiny seeds like petunia or for seeds with fluffy coverings like milkweed. It does not work well for ornamental grasses.
Always check multiple sources to verify if the float test works for a particular seed type before using it as the sole test.
Other Ways to Test Seed Viability
While floating seeds in water can be quick and convenient, it does not give definitive answers on viability. Some living seeds float while some dead seeds sink.
For more accuracy, consider these alternative seed testing methods:
Paper Towel Germination Test
- Place seeds on damp paper towel and fold over
- Put in a sealed bag or container and store in warm area
- Check daily to see if seeds sprout roots
Directly Sow Seeds
- Plant seeds densely in pots or trays
- Cover and water lightly
- Check for seedlings emerging within normal timeframe
Cut Test
- Carefully cut seed in half with a sharp blade
- Look for a white, firm interior in viable seeds
Tetrazolium Test
- Soak seeds then cut and expose to tetrazolium salt solution
- Viable seeds stain pink or red
Storing Seeds Properly
The best way to keep seeds viable year after year is by storing them correctly right from the start.
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Save seeds in air-tight, moisture-proof containers like plastic or glass. Metal allows moisture in.
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Use packet envelopes or small jars for easiest access without exposing all seeds.
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Keep seeds in a cool, dry place around 40°F. The refrigerator is ideal.
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Avoid warm, humid places like the garage or kitchen. Heat and moisture are enemies of seed longevity.
Following proper storage methods combined with occasional viability testing can help gardeners get the most potential and value out of their harvested or purchased seeds.
While not universally reliable, floating seeds in water offers a quick, simple way to gain insight into viability before planting. When used as a helpful guide rather than a definitive test, it can improve germination success when sowing older seeds. Along with proper seed storage and occasionally testing with other methods, gardeners can feel confident their seeds will sprout successfully.
Do the Floating Seed Test Properly
If you check out a number of sites that describe this test you soon realize that there are several different ways to do it. Some people add soap to the water to reduce it’s surface tension. Others put the seed in a jar and give it a good shake or they might soak the seed for 24 hours before doing the test.
There is no agreement on how to do the test properly. That means the test results reported on social media are not very reliable since they rarely include the details of the method used.
There are also silly claims like “this method is not 100% accurate and it only works with freshly harvested seeds of certain fruits such as melon, watermelon, cucumber, squash, peppers and tomatoes”. There are thousands of different types of seeds. Why would it only work on some vegetables and what does “not 100% reliable” mean? Maybe it only works 10% of the time?
Another site says, “the test only works for melons or cucumbers if the seeds are fresh and have not dried out.” So it doesn’t work on purchased seed. This same site went on to state that you need to ferment tomato seeds to get them to germinate, and I have already shown that this is a myth.
This gardening technique is so poorly defined that it is not possible to know how to do it correctly.
Science on Seed Viability Using the Water Float Test
Acorns have very low germination because many seeds don’t develop completely inside the nut and because various pests lay their egg in viable seeds which are subsequently eaten by the larvae. Floating them is a common way to eliminate many of the non-viable seeds. Even with this test, too much agitation of the water will cause viable seed to float.
Juniperus polycarpos, the Persian juniper, also produces a low number of viable seeds. Floating in water is not a reliable means of separating the good from the bad, but floating in a sugar solution does work. Sugar water has a higher density than water and this difference can be used separate seeds of various densities. The heavier viable seed sinks.
The float test “works well with hard-seeded peas in the family Fabaceae (e.g. Daviesia, Chorizema, Gastrolobium and Gompholobium) and Mimosaceae (e.g. Acacia), and has also been used on species in Hemigenia with good success. Do not attempt this test on seed of Allocasuarina. Allocasuarina seed is mucilaginous. This means it has a mucous membrane around the seed that gets very sticky on wetting.”
Arabidopsis seed forms a sticky mucilage on the outside of the seed as it absorbs water. Mutations of arabidopsis have been found that don’t produce this coating, allowing them to be separated from normal types with a float test. This is an example where within a single species, some seed floats and some does not, depending on genetics that has nothing to do with seed viability.
“Wheat was used in one set of experiments, and the average of all tests showed a germination of 68.3 per cent for the sunken seeds and 72 per cent for those that floated (reference no longer available). In another set of experiments lentil was used, and it was found that 75.4 per cent of the sunken seeds and 86.7 per cent of those that floated germinated.”
The floating characteristic of seeds depends very much on their weight, surface coating, shape and specific gravity. Some seeds do develop a large seed coat which can be empty and these likely float. The specific gravity of a seed is controlled by the environment (moisture) and internal enzymes and hormones. Some dead seeds sink, while some spongy seeds like spinach float even if viable.
Does the Seed Float Test Work?
FAQ
How to check if seeds are good in water?
Larger seeds, like peas and beans, can be tested using the water method. When good seed is placed in a bowl of water, they’ll sink. If they float, trash them. I like to give them 5-10 minutes.
Should seeds float or sink in water?
Floating seeds: partly right. No need to soak overnight, an hour in slightly warm water will do. Make sure no bubbles are attached to seeds. The ones that sink will generally be the good seeds. But usually you can see the difference between poor, thin seeds and the fatter healthier ones without soaking. 1.
What is the easiest method of seed testing?
A seed germination test is probably the most simple: seeds are given the needed resources (air, water, warmth, and light) to germinate and grow into a seedling. Simply place seeds in the soil or in a pot of soil and see how many grow.
What is the float test for seeds?
The idea is you put the seeds in water and if they float, then they are no good. They did not, however, take the floating seeds out of the water, and plant them just to see if they grew.
What is the water test for seed viability?
One method to check for seed viability is the water test. Take the seeds and place them in a container of water. If the seeds sink, they are likely viable and will germinate. If they float, they may be too old or damaged to germinate.
How to check seed viability at home?
One of the easiest methods for checking seed viability at home is the water float test. The water float test is based on the principle that viable seeds will sink in water while non-viable seeds will float. Here’s why Healthy seeds are denser than water due to the embryo and endosperm inside. This density causes them to sink.
How do you know if a seed is viable?
The quickest – though least reliable – method to test seed viability is simply to drop them into water and see if they sink or float. ‘This is an easy way to test larger seeds, such as beans and peas,’ says Matt. ‘If more than half float to the top then it’s best to use a fresh packet.’
How do you test a seed?
If you check out a number of sites that describe this test you soon realize that there are several different ways to do it. Some people add soap to the water to reduce it’s surface tension. Others put the seed in a jar and give it a good shake or they might soak the seed for 24 hours before doing the test.
How do I know if my seeds are still good?
When properly stored in a cool, dry place, seed’s shelf life can be extended. Yet, even then, there is no guarantee that they will still be productive for next season’s planting. There are two easy tests you can take to check to see if there is life left in your old seeds. Water test: Take your seeds and put them in a container of water.
Why do seeds float in water?
The water float test is based on the principle that viable seeds will sink in water while non-viable seeds will float. Here’s why Healthy seeds are denser than water due to the embryo and endosperm inside. This density causes them to sink. Non-viable seeds are often hollow inside.