When seeds float in water, gardeners often question their quality and potential for growth. A seed is a dormant plant embryo, protected by a coat and nourished by stored food reserves. Interpreting this buoyancy requires understanding basic physics and plant biology, as viability hinges on the physical reasons behind the flotation.
The Biological Mechanism of Seed Flotation
Flotation is primarily a matter of the seed’s density relative to water. A healthy, fully developed seed is typically dense, packed with tissue and food reserves, causing it to sink. A seed floats when its overall density is lower than the surrounding water, which arises from two main conditions: internal air pockets or a lack of internal mass. Air pockets may be natural or result from the drying process, trapping air within the seed coat. A lack of internal mass means the seed is a hollow shell that failed to develop a full embryo or sufficient food reserves, often caused by immaturity or pest damage.
Interpreting Flotation as a Viability Indicator
The “float test” is a traditional method used by gardeners to rapidly assess seed quality. It operates on the principle that a fully formed, heavy seed sinks, while an empty or compromised seed floats. For large seeds, such as squash, pumpkins, beans, and peas, flotation can be a useful indicator of non-viability. A floating seed often signals that the embryo or endosperm was degraded, underdeveloped, or consumed by insects, resulting in an internal void that fills with air.
Sinking seeds are generally more likely to be viable, but this correlation is not perfect. A viable seed may float temporarily due to surface tension or air bubbles clinging to its coat. If a large garden seed remains floating after ten to fifteen minutes, it is statistically more likely to be non-viable or compromised.
Seeds Where Floating is a Normal Expectation
Significant exceptions exist to the float test rule, particularly for seeds adapted for water dispersal or those with unique coatings. Some seeds possess specialized outer structures that naturally cause them to float, irrespective of their internal health. For instance, seeds like marigold or milkweed have feathery husks that capture air and maintain buoyancy. Other seeds, such as chia and flax, produce a mucilaginous, gel-like coating when exposed to moisture. This hydrogel layer causes them to float, making the float test unreliable for assessing their viability.
Practical Handling of Floating Seeds
When using the float test for larger vegetable seeds, such as corn or squash, it is practical to discard those that remain floating after a short soak. This removes the most obviously empty or damaged seeds, increasing the uniformity of the planted batch. To minimize discarding viable seeds, gently agitate the water or lightly press the floating seeds to release any trapped air bubbles before the final assessment. For expensive or rare garden seeds, or those where floating is expected, a true germination test is a more accurate measure of viability. A germination test involves placing a small sample of seeds between moist paper towels and monitoring how many sprout.