Short answer
The cannabis seedling stage lasts about 2–3 weeks from emergence, ending at 3–4 nodes of true leaves with multi-fingered leaflets. Good light (150–300 µmol/m²/s, 18 h) and warmth (23–27 °C / 73–81 °F) keep it short. Grower practice.
Why
A seedling is limited by its tiny root and few leaves. Growth accelerates once true leaves are photosynthesising and roots have filled a small container.
Light sets the pace. Young plants given a DLI of about 10–14 mol/m²/day root and grow better than dimly lit ones in other crops [1], and cannabis responds to higher light in veg with compact, robust growth [2].
Stretch is the sign of too little light. The stem elongates looking for it, giving a tall, weak seedling that can flop.
The end of the seedling stage is practical, not botanical: when roots fill the plug or cup it's time to pot up [3] and treat the plant as veg.
The numbers
| Days from emergence |
What you see |
Light (µmol/m²/s) |
| 0–5 |
Cotyledons open |
100–200 |
| 5–10 |
First true leaves (1–3 fingers) |
150–250 |
| 10–21 |
Nodes 2–4, leaves with more fingers |
200–300+ |
| ~14–21 |
Roots fill plug or cup; enter veg |
300+ as veg begins |
What to do
- Count nodes, not days.
- Measure PPFD; raise it if the stem stretches.
- Keep the medium just moist, never soggy.
- Start a light feed at 2–3 sets of true leaves.
- Pot up photoperiods when roots reach the edges; leave autos in their final pot.
Common mistakes
- A weak light 1 m away, then wondering why it stretched.
- Calling it "veg" by the calendar while it still has two nodes.
- Holding a seedling in a tiny cup until it is root-bound.
A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.The first leaves produced by the shoot tip after the cotyledons, with the plant's characteristic serrated leaflets.One of the separate serrated blades that make up a palmate cannabis leaf; a mature fan leaf typically has five to eleven leaflets.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Daily light integral: the total number of PAR photons a square metre of canopy receives in one day, in mol/m²/day; PPFD multiplied by hours of light.The stretch is the rapid increase in height in the first two to three weeks after switching to flower, when many cultivars grow 1.5 to 3 times taller.Up-potting moves a plant into progressively larger containers as its roots fill each one, instead of starting in the final pot.One of the two rounded seed leaves packed in the embryo that open first after germination and feed the seedling until the true leaves take over.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.18/6 is the standard vegetative light schedule: 18 hours of light and 6 hours of darkness each day, keeping photoperiod cannabis from flowering.24/0 means lights on 24 hours a day with no dark period; GenetixBay endorses it for veg, as cannabis does not need a sleep period before flowering.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.A root-bound plant has filled its container so densely that roots circle the walls, water runs straight through, and growth slows or the plant wilts fast.
