Cannabis cultivation begins with assessing the quality of the seed itself. Before committing resources, growers must determine the seed’s viability—its capacity to germinate and develop into a healthy plant. To quickly gauge this potential, cultivators often use simple, non-destructive methods to distinguish robust seeds from those that are empty or damaged.
The Float Test: Sinking Versus Floating
The most common preliminary assessment is the float test, a simple water-based method that provides a quick indication of a seed’s potential. To perform this test, seeds are gently placed into a glass of room-temperature water. A seed that is considered viable is expected to absorb water and sink to the bottom within one to two hours. Conversely, seeds that remain floating on the surface are generally viewed as non-viable or weak, although this is not a definitive rule.
For the test, the water used should be non-chlorinated and maintained at room temperature, typically 68–72°F (20–22°C). It is important not to leave the seeds submerged for an extended period. Prolonged soaking can deprive the internal embryo of oxygen, potentially causing damage. A maximum soak time of 24 hours is considered the limit before the risk of drowning the seed increases.
The Science Behind Seed Density
The principle behind the float test is rooted in the physical property of density, which is mass per unit volume. A viable cannabis seed has a higher density than water, causing it to sink. This increased mass is a result of a fully developed and well-hydrated internal structure, specifically a healthy embryo surrounded by nutrient-rich endosperm. The hard outer shell, or testa, is intact and fully contains these denser biological components.
Seeds that float are less dense than water, usually because they contain excessive internal air space. This buoyancy often indicates the seed is hollow, has been poorly stored, or is immature, meaning the embryo and endosperm did not fully develop. Older seeds that have dried out significantly over time may also float, as the loss of internal moisture is replaced by air pockets. However, a viable, healthy seed may initially float due to air trapped on its shell. It should sink once the shell absorbs water and releases that trapped air.
Beyond the Water Test: Next Steps for Germination
Once the float test is complete, the sunken seeds are ready to proceed to the next stage of germination. These seeds should be carefully moved to a humid and warm environment to encourage the emergence of the taproot. Common, successful methods include the paper towel technique, where seeds are placed between moist, not soaked, paper towels inside a sealed container. Alternatively, they can be directly planted into a light, well-draining soil mix or a sterile medium like rock wool.
Seeds that initially floated should not be immediately discarded, as some may have a resistant shell that took longer to absorb moisture. These can still be placed into a germination medium. Growers should maintain a stable temperature between 68°F and 77°F (20–25°C) and ensure complete darkness for the best results. The appearance of a small white taproot means the process is successful, and the seedling is ready to be planted root-down into a starter container.