Cotyledons are the embryonic leaves formed inside the seed. Cannabis is a dicot, so each seedling opens two smooth, oval, unserrated cotyledons on top of the hypocotyl. They hold stored reserves and start photosynthesising as soon as they turn green, carrying the seedling until the first true leaves expand.
In practice
Healthy cotyledons are firm, even and green. Pale or stretched seedlings with cotyledons held high on a long, thin stem usually need more light, not more feed. Yellowing cotyledons after two or three weeks are normal as their reserves run out. Early yellowing combined with pale new growth points to the root zone or too little light.
Seedlings need little or no fertiliser while the cotyledons and the first one or two pairs of true leaves are still working. Damaged or missing cotyledons slow a seedling but rarely kill it.
Occasionally a seedling shows three cotyledons (tricot). It is a developmental quirk and says nothing reliable about vigour or sex.
Don't confuse with
- True leaves: the first serrated, compound-type leaves that follow the cotyledons.
- Hypocotyl: the stem section below the cotyledons.
- Fan leaves: the large mature leaves of an established plant.
