Seed germination is the fundamental process where a dormant seed sprouts and begins to grow into a seedling. This seemingly simple step is often a point of confusion for new gardeners, who struggle with whether to bury a seed or leave it exposed. The decision depends entirely on the specific plant species and its evolutionary adaptation. Understanding the biological signals that prompt a seed to wake up can greatly improve your success rate.
The Necessity of Darkness and Seed Size
The majority of seeds benefit significantly from being covered with a layer of growing medium. This covering serves to provide stability and ensure consistent moisture retention. A seed must absorb a substantial amount of water to activate its internal metabolic functions and break dormancy. Covering the seed prevents rapid evaporation from the surface, which is a common cause of germination failure.
Many seeds possess a mechanism known as negative photoblasticism, meaning they require darkness to trigger germination. These seeds contain photoreceptors that signal they are buried deep enough beneath the soil to begin growth. This evolutionary trait is particularly common in larger seeds, such as beans, peas, and squash.
Larger seeds have more significant stored energy reserves, which allows them to push through a greater depth of soil to reach the light. The general rule of thumb is to plant a seed to a depth two to three times its width or diameter. This depth ensures the seed is in a consistently moist, dark environment while still being shallow enough that the emerging sprout does not exhaust its energy reserves before reaching the surface.
Seeds That Require Light for Germination
Photoblastic seeds require light exposure to initiate the germination process. These seeds are typically very small, including varieties like lettuce, celery, and many ornamental flowers. For these species, light serves as the environmental cue, signaling that the seed has landed directly on the soil surface where conditions are ideal for its survival.
If a tiny seed were to germinate deep underground, it would quickly exhaust its limited stored food before reaching the sunlight needed for photosynthesis. Light acts on specialized pigments within the seed, primarily phytochrome, which then converts to an active form that promotes the growth hormones required for sprouting.
These light-dependent seeds should be surface-sown, gently pressed into the moist growing medium, and left uncovered. Even a thin layer of soil can inhibit their germination, emphasizing the importance of keeping them exposed to light.
Proper Technique for Covering Seeds
The depth rule of planting two to three times the seed’s width provides a solid starting point for most varieties. Planting too deeply is a common error that causes the seedling to use up all its energy reserves, resulting in an inability to emerge from the soil.
The material used for covering the seed should be fine and light, such as a sifted seed-starting mix or vermiculite, rather than heavy garden soil. This fine texture prevents the formation of a dense, compacted crust on the surface which a tender sprout would struggle to penetrate. A light covering medium also allows for better gas exchange, ensuring the germinating seed receives enough oxygen.
After covering the seeds, the next step is to water them very gently to settle the medium without displacing the seeds. Misting the surface or using a bottom-watering technique helps maintain the consistent moisture required for imbibition without disturbing the carefully placed seeds. Maintaining this uniform moisture and a stable temperature is the final step in ensuring a successful germination period.