Seed germination is a complex biological event requiring specific environmental cues, including light. This response to light is known as photoblastism. Seeds inhibited by light are termed negatively photoblastic. They require darkness as a signal that conditions are optimal for survival, ensuring the seedling does not sprout on the vulnerable soil surface.
Seeds That Require Darkness
Many common garden plants germinate best in total darkness, a strategy ensuring they are safely buried beneath the soil. These seeds are often larger and possess sufficient energy reserves to push through a layer of soil. Examples include popular flower varieties like Delphinium and Larkspur. Certain vegetables and herbs also show a strong preference for darkness, such as cilantro (coriander) and some Allium species like onions. This requirement acts as a natural depth-sensing mechanism, protecting the seed from harsh surface conditions.
The Biological Reason for Darkness
The scientific explanation for the darkness requirement lies in phytochrome, a pigment system acting as a photoreceptor within the seed. Phytochrome exists in two interconvertible forms: the inactive form, Pr (absorbing red light), and the active form, Pfr (absorbing far-red light). Exposure to red light, abundant in direct sunlight, converts Pr into the active Pfr form, which often acts as a biological inhibitor of germination in negatively photoblastic seeds. When the seed is buried, it is shielded from red light, allowing Pfr to slowly revert back to the inactive Pr form through dark reversion. This removal of the Pfr inhibitor, which requires prolonged darkness, triggers the necessary events for the seed to break dormancy and sprout.
Planting Techniques for Optimal Germination
Successfully germinating dark-requiring seeds depends on simulating a consistently moist, dark environment. The most practical application is ensuring the correct planting depth to act as a light barrier. A general guideline is to cover the seed with fine soil or vermiculite roughly two to three times the seed’s diameter. For very small seeds, a thin covering of about three millimeters is sufficient to block light without impeding emergence; if using trays, cover them with an opaque material, like foil or dark plastic, to guarantee total darkness until the first sprouts appear. Once seedlings emerge, immediately remove the covering to provide light for photosynthesis, preventing the young plants from becoming weak and “leggy.”