The seed coat (testa) is the protective layer that forms from the ovule's outer tissues and surrounds the embryo. In cannabis the testa sits tight against the pericarp (the fruit wall of the achene), so growers treat the two as one hard shell. It limits water uptake, guards the embryo from injury and microbes, and splits along its seam when the swelling embryo pushes the root out.
In practice
At germination the seed takes up water, swells, and the shell cracks along its edge; the radicle emerges first, often within 24-72 hours at about 22-26 °C (72-79 °F). A seedling that comes up still wearing its shell over the cotyledons is a "helmet head". Usually it falls away on its own within a day; misting it and waiting beats prying, which tears the seed leaves.
Scarification (lightly sanding the shell) is sometimes used on old or very hard seed. It is rarely needed for fresh, healthy stock and can let pathogens in.
Don't confuse with
- Achene: the whole fruit; the seed coat is one layer of it.
- Cotyledons: the seed leaves inside, not a covering.
- Endosperm: stored food tissue inside the seed, not the shell.
