Seed viability is the ability of a seed to germinate and grow into a healthy plant. Assessing this potential is important for gardeners, especially when using older seed stock. A non-viable seed will not sprout, meaning time and effort spent planting it will be wasted. Gardeners often seek a quick way to estimate if a batch of seeds is worth planting before committing them to the soil.
Why Viable Seeds Sink
The water test is a rapid, non-destructive method used to triage seeds based on their potential to sprout. The general rule is that seeds which sink are considered viable, while those that float are likely non-viable and should be discarded. To perform the test, seeds are gently dropped into room temperature water. Allow the seeds to soak for about 15 minutes before making a determination.
This test is particularly effective for larger seeds, such as beans, peas, corn, and squash varieties. Seeds that settle on the bottom are ready for planting or further germination testing. Floating seeds are usually skimmed off, as their inability to absorb water or low internal mass suggests a compromised ability to sprout. Sinking confirms that the internal components of the seed are intact and dense.
Internal Structure and Density
The physical principle behind the water test is density. Water has a density of approximately one gram per cubic centimeter. A viable seed is dense enough to have a specific gravity greater than water, causing it to sink. This density comes from a healthy internal structure, including a firm, fully developed embryo and a solid reserve of food storage tissue.
A viable seed also contains a certain level of internal moisture. Non-viable seeds, particularly those that are old or improperly stored, tend to dry out over time. As internal tissues degrade, the seed loses mass and develops large internal air pockets. This loss of dense material significantly lowers the seed’s overall density, causing it to float on the surface.
The protective seed coat of a healthy seed allows water to penetrate slowly, increasing the seed’s weight and helping it overcome surface tension. If the internal structure is decayed or the seed is empty, air pockets trap the seed at the surface. Sinking indicates a heavier, well-preserved internal mass and the necessary structural integrity for germination.
Exceptions to the Rule
The float test is not a universal indicator of viability for all species. Some seeds, even when healthy and fresh, naturally float due to their size, shape, or specific adaptations. Very small and lightweight seeds, such as those from carrots or lettuce, may float because the water’s surface tension supports their minimal weight.
Certain plant families, particularly those that produce seeds within a fleshy fruit like tomatoes, cucumbers, and melons, require specific preparation. These “wet seeds” are often encased in a highly buoyant, gel-like membrane. If this membrane is not removed (typically through fermentation and thorough rinsing), the seeds will float, leading to a false positive result. In these cases, a reliable test of viability involves a full germination trial, where seeds are placed in a moist environment and monitored for sprouting.