Cotyledon
Definition: Cotyledons are the two small, rounded seed leaves that open first when a cannabis seedling breaks the surface. Formed inside the seed, they feed the young plant from stored reserves and begin photosynthesis until the first serrated true leaves take over. Their shape and color offer an early read on seedling health.
How Cotyledon works
Cannabis is a dicot, meaning its embryo carries two seed leaves. When the shoot emerges, it rises as a hook with the cotyledons often still wearing the empty seed shell. Within a day the leaves unfold, smooth-edged and oval, nothing like the jagged leaves that come next.
What cotyledons tell you
- Bright green and flat: a healthy seedling getting enough light.
- Pale or yellow early: usually too little light, or a seed with weak reserves.
- Curled down and dark: often overwatering or a soil mix that is too rich in nutrients.
- Stuck in the shell: a so-called helmet head. Mist the shell, wait an hour, then gently pull it free with tweezers if it has not dropped.
When they yellow and fall
After the plant has three or four sets of true leaves, the cotyledons have done their job. They often yellow and drop around weeks 3 to 4, which is normal. If they yellow during the first week, though, the seedling is likely starved for light or struggling at the roots, so check moisture and light distance before anything else.
Above the cotyledons, the first true leaves usually have a single leaflet, then three, then five, building toward mature fan leaves.