Seed germination is the process by which a seed awakens from dormancy and begins to grow into a new plant. This transformation involves absorbing water, swelling, and developing a root and shoot. Many gardeners observe that some seeds sprout more readily in the absence of light, prompting the question: why do certain seeds germinate better in the dark?
How Light Influences Germination
Light plays a significant role in seed germination, but its effects vary among plant species. Seeds are categorized by their response to light during this initial growth phase.
Some seeds are “photoblastic,” meaning their germination is influenced by light. Negatively photoblastic seeds, such as nasturtium, sweet pea, and onion, require darkness to sprout. Positively photoblastic seeds, including petunia, snapdragon, and many lettuce varieties, need light for successful germination. A third category, like cucumber, tomato, and beans, is indifferent to light, germinating equally well with or without it.
The Seed’s Internal Light Sensor
The biological mechanism behind how seeds sense light or darkness involves a specialized photoreceptor protein called phytochrome. Phytochrome exists in two interconvertible forms: Pr (phytochrome red) and Pfr (phytochrome far-red). Red light converts the inactive Pr form into the active Pfr form; far-red light, or prolonged darkness, converts Pfr back to Pr.
This conversion acts as a signaling system for the seed. For seeds that prefer darkness, Pfr’s presence, indicating light exposure, can inhibit germination. As Pfr slowly reverts to Pr in darkness, it signals the seed is likely buried underground, creating a favorable environment for sprouting. This light-sensitive pigment allows the seed to assess its environment.
Ecological Reasons for Dark Germination
Dark-requiring germination provides advantages for certain plant species. Germinating in darkness helps seeds avoid unfavorable conditions on the soil surface. Surface-exposed seeds are vulnerable to drying out from wind and sun, and are more susceptible to being consumed by birds or insects.
Darkness signals to the seed that it is likely buried at an appropriate depth within the soil. This burial provides a stable microclimate with consistent moisture and protection from temperature extremes. This ensures the seedling has a better chance of establishing roots and accessing resources before it emerges to begin photosynthesis. Waiting for burial maximizes the seed’s chances of survival.
Practical Tips for Successful Planting
Understanding a seed’s light requirements is important for successful planting. For seeds that require darkness, such as nasturtium or calendula, they should be covered with soil to exclude light. A general rule of thumb is to plant seeds to a depth roughly two to three times their diameter, which provides sufficient darkness for most dark-germinating varieties. Conversely, light-requiring seeds, like petunia or lettuce, should be sown on the surface or barely covered, allowing them access to light.
Beyond light, consistent moisture and appropriate temperature are important for germination. Keeping the planting medium evenly moist, but not waterlogged, supports the initial swelling and metabolic processes of the seed. While temperature requirements vary by species, maintaining a suitable range helps ensure optimal germination rates for all seed types.