Short answer. Give seedlings moderate light (≈ 150–300 µmol/m²/s for 18 h, a DLI of 10–19 mol/m²/day), warm, fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH, leaf VPD ~0.4–0.8 kPa) and small waterings in a ring around the stem. Hold off on feed until 2–3 sets of true leaves unless the medium has no starter charge (coco, rockwool, hydro). Most losses are overwatering, stretching from low light, or damping-off.
Week by week
| Days | What you see | What to do |
|---|---|---|
| 0–5 | Hook emerges, cotyledons open | Low light (100–200 µmol/m²/s), dome vented daily, keep surface just moist |
| 5–10 | First true leaves (single or 3-finger) | 150–250 µmol/m²/s; water small amounts when the plug lightens |
| 10–21 | 2nd–4th nodes | 200–300+ µmol/m²/s; start ¼–½ strength feed (EC 0.4–0.8 mS/cm); transplant when roots fill the plug (transplanting) |
Numbers that matter
| Parameter | Target | Evidence |
|---|---|---|
| PPFD | 150–300 µmol/m²/s, 18 h | Grower practice; consistent with young-plant DLI research in other crops [1] |
| DLI | 10–19 mol/m²/day | Plugs below ~8 mol/m²/day are inferior in other crops [1] |
| Air temperature | 23–27 °C (73–81 °F) | Grower practice |
| RH | 65–75 % | Grower practice |
| Medium temperature | 21–25 °C (70–77 °F) | Warm roots grow fast; cold water raises damping-off risk [2] |
| First feed EC | 0.4–0.8 mS/cm (200–400 ppm500) | Grower practice; start at true leaves [2] |
| Water pH | Soil 6.2–6.8; coco 5.8–6.2; hydro/rockwool 5.5–6.0 |
Problems and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Long, thin, pale stem, flops over | Light too weak or too far | Raise PPFD gradually; bury the stretched stem deeper at transplant (Grower practice) |
| Drooping, wet medium | Overwatering | Let it dry; smaller waterings; airier mix |
| Stem pinched and brown at the soil line, collapse | Damping-off | See damping-off |
| Cotyledons yellow at day 10–14 | Normal as true leaves take over, or no nutrients in inert media | Start light feed |
| Leaves praying hard, bleached tips | Too much light/heat | Dim or raise light |
| Seed husk stuck on cotyledons | Dry surface | Mist, wait, then gently remove |
Common mistakes
- Big pot + big watering for a tiny root.
- Dome left closed for days (mould, soft stems).
- Strong nutrients "to get it going".
- A cheap light at 1 m away.
Evidence
Young-plant light responses are Supported in horticultural research on other crops [1]; cannabis veg responds to high light with compact growth [3]. Temperatures follow hemp germination data [4] and grower practice. Fertiliser timing follows extension guidance [2].
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.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.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.The first leaves produced by the shoot tip after the cotyledons, with the plant's characteristic serrated leaflets.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.The collapse of seedlings or cuttings when soil-borne pathogens such as Pythium, Rhizoctonia solani and Fusarium rot the stem at the soil line.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.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.Transplanting moves a plant and its root ball into a new container or the ground; done gently it causes little shock, but GenetixBay advises against it for autoflowers.Input EC is the electrical conductivity of the nutrient solution going into the pot or system, in mS/cm, including the base EC of the source water.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Germination is the stage where a viable seed absorbs water, splits its shell and pushes out a root (radicle), usually within 1-7 days at 20-30 °C.
