Floating Seeds in Water - Is This a Good Seed Viability Test? (2024)

Robert Pavlis

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?

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.

Citizen Scientists – Floating Seed Test for Viability

A number of gardeners have done tests to see how well the floating seed test works.

I had some red pepper seeds from a store bought fruit and tried floating the seeds without drying them. Half floated and half sank. I removed the floaters and used them to try the test again. Half floated and half sank. I then tested this last group of seeds for germination. The ones that floated and then sank had 8/10 germinate, and the ones that floated twice had 3/10 germinate. So it is possible that floaters have a lower germination rate, but the floaters in this test were certainly not all dead.

I tested some Camassia seeds; 38 of 48 (79%) sinkers germinated and 12 of 16 (75%) floaters germinated, after a month in the fridge using the baggy method.

Someone from our Garden Fundamental Facebook Group tested Briza maxima (quaking grass) and found better germination with floaters.

Marijuana seed that floats will germinate on top of the water in 24 hours.

Radish seed that floated had good germination.

Twelve different kinds of pepper seeds were tested in this video and both floaters and sinkers had good germination.

I’ve germinated quite a few clematis seeds and most of them have fussy tails. They all float. Many seeds have this characteristic including some grasses and pulsatilla.

The top picture in this post shows two jars. The one on the left contains black pepper seeds – they sink. You can distinguish them from papaya seeds that float, and are frequently added to spices since they look like black pepper but are much cheaper.

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. 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 for Testing Viability?

There are cases where a float test can be used to identify viable seed, but when science reports on these they are quite specific about the type of seed and the method used.

On the other hand gardeners tend to simply lump all seeds into one category and say they all work, without specifying the method that works.

As a general rule, gardeners should assume that the float test does NOT work for testing seed viability, unless there is evidence it works in a specific case.

A Better Way to Test Seed Viability

Use my baggy method if you want to test seed germination. You will actually see the root come out of the seed and know for certain that the seed is viable.