The “float test” is a simple, time-honored method used by gardeners to quickly assess the viability of old or stored seeds. This technique involves placing seeds in water to determine which ones are most likely to sprout successfully. By separating the seeds that sink from those that remain on the surface, a gardener can make an initial judgment about the quality of their planting material before planting.
The Physical Science Behind Seed Buoyancy
A seed’s buoyancy, or its ability to float, is fundamentally governed by its overall density compared to the density of water. A healthy, viable seed typically possesses a high density because its internal structures, primarily the embryo and its nutrient-rich endosperm, are solid and compact. When submerged, a dense seed rapidly absorbs water, a process called imbibition, which increases its weight and causes it to sink to the bottom.
Floating is often a sign that the seed’s density is lower than that of water. This usually occurs due to large internal air pockets within the seed coat or where the endosperm should be. Air pockets can result from the seed being immature or from the deterioration of internal tissues over time, leaving an empty shell. A less dense, floating seed may also have a damaged or compromised seed coat, preventing successful water uptake.
The float test relies on the density change caused by water absorption. Sinking indicates a successful, rapid uptake of water, suggesting the internal components are intact and ready to begin germination. A floating seed has not absorbed enough water to overcome its buoyancy, signaling potential problems with its internal structure or ability to hydrate properly.
Performing and Interpreting the Float Test
The float test is a straightforward procedure designed to provide a quick evaluation of a seed batch. To perform the test, simply drop a small sample of seeds into a container of room-temperature water. It is helpful to gently tap any initial floaters to dislodge microscopic air bubbles that may be clinging to the seed’s surface and artificially keeping it afloat.
After the initial soaking, the seeds should be left undisturbed for a specific period, typically between 15 to 30 minutes for most common vegetable seeds. This brief immersion time allows the water to penetrate the seed coat and begin the imbibition phase. The seeds are then ready for interpretation based on their position in the water.
The sinking result is the favorable outcome, as these seeds have absorbed enough water to become denser than the surrounding liquid. These sinkers are the most promising candidates for planting and generally show a high probability of successful germination. Floating seeds should be viewed with skepticism, as their inability to absorb water quickly suggests a lack of dense internal material or a compromised seed coat. While some floaters may still germinate, the practical recommendation is to discard them or plant a larger number to account for the lower viability rate.
Important Exceptions to the Floating Rule
The float test is not universally applicable, and relying solely on it can lead a gardener to discard perfectly viable seeds. Certain seeds, particularly those that have evolved to disperse via water, are naturally buoyant and will float even when completely healthy. Examples include some aquatic plant seeds or those with specialized, lightweight structures, such as the feathery pappus found on some seeds.
Another significant exception involves seeds with exceptionally hard or thick seed coats, which are designed to resist rapid water penetration. These seeds may require a much longer soak time, often 12 to 24 hours, to allow the water to breach the protective layer and reach the embryo. If removed after the standard 30-minute test, these viable, hard-coated seeds will still be floating and could be mistakenly discarded.
For these types of seeds, an initial float indicates seed coat impermeability, not non-viability. To improve their chances of sinking and encourage germination, these seeds often require a pretreatment like scarification. Scarification involves lightly scratching or nicking the seed coat to create an entry point for water.
Alternative Methods for Assessing Seed Viability
When the float test is inconclusive or unreliable for a specific seed type, alternative methods offer a more definitive assessment of viability. A simple visual inspection can reveal immediate problems, such as discoloration, mold, or obvious physical damage to the seed coat. Seeds that appear shriveled, dusty, or have visible holes from insect damage should be immediately discarded as their internal structures are likely compromised.
The most accurate and accessible non-water-based method is the actual germination test, often called the paper towel method. This involves placing a small, representative sample of seeds between layers of moist paper towel, rolling it up, and placing it in a plastic bag in a warm location. After a species-specific period, typically 5 to 10 days, the gardener can count the number of seeds that have sprouted.
This direct germination percentage provides a precise measure of viability, which is used to calculate the necessary planting density for the entire batch. For instance, if 8 out of 10 seeds sprout, the batch has 80% viability. This method eliminates the guesswork associated with buoyancy and provides a reliable, actionable number for planting.