A cannabis seedling needs gentle light (100–300 µmol/m²/s) on an 18/6 cycle for the first three weeks after germination, air at 22–25 °C with 65–75% humidity, and small amounts of water given close to the stem. In pre-fertilised soil no feed is needed for 2–4 weeks; in coco, which holds almost no nutrients, you feed at a reduced dose from the first waterings. The most common mistake is watering too much, not too little.
In brief
- Light: 100–200 µmol/m²/s in the first week, up to 300 µmol/m²/s in the third; dimmed LED or hung higher.
- Climate: 22–25 °C by day, 20–22 °C at night and never below 18 °C, humidity 65–75%.
- Water: 20–50 ml at a time around the stem, then let the surface dry out.
- Transplant into a bigger pot when roots reach the edges or after 2–3 weeks.
- Feeding: not needed for 2–4 weeks in pre-fertilised soil; in coco from the first waterings at a quarter to half the dose.
The first three weeks: what happens
| Period | What you see | Light (18/6) | Water |
|---|---|---|---|
| Days 1–7 | round cotyledons, then the first serrated single-blade leaves | 100–200 µmol/m²/s | 20–50 ml around the stem |
| Days 8–14 | 3- and 5-blade leaves, first node visible | 150–250 µmol/m²/s | 50–100 ml in a wider circle |
| Days 15–21 | 3–4 nodes, 5–7-blade leaves, faster growth | 200–300 µmol/m²/s | enough to wet the whole small pot |
Timings vary with variety, temperature and growing medium. A seedling that has started well is short and compact, with closely spaced nodes and mid-green leaves. How to get this far is explained in the guide to germinating seeds.
Light: intensity, distance and cycle
Seedlings have few leaves and small roots: too much light bleaches or curls the leaves, too little makes the stem stretch. Use an 18/6 cycle and adjust the intensity.
- Dimmable LED: start at 25–40% power and follow the manufacturer's seedling distances, which are usually higher than for flowering.
- No dimmer: hang the light higher and raise it further if the leaves cup upwards or turn pale.
- T5 tubes or compact lamps: suitable for seedlings, at 15–30 cm depending on wattage.
- Measure with a PAR meter or a smartphone app: apps only give an estimate, but it is enough to avoid extremes.
A suitable DLI rises from about 6 to 19 mol/m²/day between the first and third week: with 18 hours of light that equals 100–300 µmol/m²/s (PPFD × 18 × 0.0036). Distances and values for each stage are in the guide on LED grow light distance. In a large tent, a few small seedlings waste electricity: a compact 30–60 W LED in a small space is enough for the first weeks. At 0.30 CHF/kWh, a 50 W LED on 18/6 costs 0.05 kW × 18 h × 21 days = 18.9 kWh, about 5.70 CHF over three weeks.
Temperature and humidity
With roots still small, a seedling takes up little water, and in dry air it loses more through its leaves than it can replace. Keep the air at 22–25 °C by day and 20–22 °C at night, never below 18 °C, with 65–75% humidity. In winter, humidity in heated Swiss homes often falls below 40%: a small humidifier or a clear dome for the first few days solves it. Avoid both still air and a direct draught: a gentle fan that barely moves the leaves and strengthens the stem is enough.
Watering: little and close to the stem
The medium should stay moist, never soaking: in constantly wet soil the roots lack oxygen and the seedling stalls or rots.
- Use water at room temperature with a pH of 6.0–6.8 in soil or 5.8–6.2 in coco.
- In the first week pour 20–50 ml in a 5 cm circle around the stem, using a cup or a spray bottle.
- Water again when the top centimetre of the medium is dry or the pot feels clearly lighter.
- Widen the circle each week so the roots grow out towards the edges.
If you sowed straight into a large pot (11 litres or more), water only near the stem for the first 2 weeks: wetting the whole pot means soggy soil for days.
Typical problems
- Long, thin stem: light too weak or too far away, or too much heat. Move the light closer or raise the intensity in small steps; at the first transplant you can bury the stretched part up to the cotyledons.
- Damping off: the stem thins and darkens at the base and the seedling topples. It is caused by fungi encouraged by a wet medium and still air; an affected seedling can rarely be saved. Prevent it with fresh medium, clean pots, less water and air exchange.
- Yellow leaves: cotyledons turning yellow after 2–3 weeks is normal. If the first true leaves yellow, the usual causes are too much water, the wrong pH or, in poor media, a lack of nutrients.
- Curled leaves or burnt spots: light too intense, heat under the lamp or feed that is too strong.
First transplant: when and how
If the seedling is in a 0.25–0.5 litre pot or a cube, transplant after 2–3 weeks, when white roots reach the edges or come out of the drainage holes.
- Water the small pot lightly the day before so the root ball holds together.
- Prepare the new pot (3–4 litres as an intermediate step, or the final pot straight away) with a light growing medium and a hole the size of the small pot.
- Turn the small pot upside down, hold the stem between your fingers and slide the root ball out without pulling the plant.
- Set the seedling in the hole at the same height or slightly deeper and press the soil gently around it.
- Water lightly and keep the light a little dimmer for 1–2 days.
When to start feeding
Many growing soils are pre-fertilised ("light mix" a little, "all mix" more) and feed the plant for 2–4 weeks. In these media no feed is needed for the first weeks. In coco, rockwool or poor media, feed from the first waterings with a quarter to half of the feeding chart dose (EC around 0.4–0.8 mS/cm), then increase according to how the plant responds. In soil, the signal to start is even yellowing of the lowest leaves after the 3rd–4th node. After these three weeks the plant moves into the vegetative stage, with more light, water and nutrients.
Frequently asked questions
How much light do seedlings need?
Around 100–300 µmol/m²/s on an 18/6 cycle, for example a dimmable LED at 25–40% hung 35–45 cm above the seedlings. Increase it gradually from the first to the third week.
Why is my seedling stretching so much?
Usually because the light is too weak or too far away, sometimes because of too much heat. Move the light closer or raise the intensity and, when transplanting, bury the stem up to the cotyledons.
When should a seedling be transplanted?
After 2–3 weeks, when the roots reach the edges of the small pot or come out of the holes. Water the day before so the root ball does not crumble.
Do seedlings need feeding?
Not in pre-fertilised soil for the first 2–4 weeks. In coco or rockwool, yes, from the first waterings, at a quarter to half the dose.
Small pots, propagators and domes for the first weeks are available in the cuttings and germination section at Gro.w.LaB.
Updated on 13 October 2026.