The cannabis vegetative stage runs from the seedling with its first true leaves until the switch to flowering. Indoors it usually lasts 3–6 weeks for photoperiod plants: you set the length yourself, based on the height available and the number of plants. During this period plants need 18 hours of light and 6 of darkness (18/6), an intensity of 300–600 µmol/m²/s, 22–28 °C, 55–65% humidity and nitrogen-rich nutrients.
In brief
- Duration: 3–6 weeks indoors for photoperiod plants; autoflowers move into flowering on their own after 3–5 weeks, often 3–4.
- Light: an 18/6 cycle at 300–600 µmol/m²/s, raising the intensity gradually.
- Climate: 22–28 °C with lights on, no lower than 18 °C at night, humidity at 55–65%.
- Nutrients: more nitrogen (N) than phosphorus and potassium, at rising doses; pre-fertilised soil is enough for the first 2–4 weeks, depending on the type.
- Switch to 12/12 when the plant has reached about half the final height you can allow: it keeps stretching in flowering.
How long the vegetative stage lasts
A photoperiod plant stays vegetative as long as it gets long days: you decide when it flowers by switching to 12 hours of light and 12 of darkness. Autoflowering varieties, by contrast, follow a genetic timetable and start flowering on their own 3–5 weeks after germination (often 3–4), whatever the light cycle. The right length depends on the space to fill and how many plants you grow:
| Situation | Typical duration | Depends on |
|---|---|---|
| Autoflowers | 3–5 weeks (often 3–4) | Genetics; light does not change it |
| Rooted cuttings, several small plants | 1–2 weeks | Cuttings are already mature enough to flower |
| Plants from seed, standard grow (4–9 per m²) | 3–6 weeks | Available space and growth rate |
| 1–2 plants with LST or scrog | 5–8 weeks | Time needed to fill the whole area |
Plants grown from seed show their first pre-flowers at the nodes after about 4–6 weeks: a sign they are mature enough to flower well. Switching them to 12/12 earlier still works, but flowering gets off to a slower start. With cuttings that are already rooted, 1–2 weeks are enough, because they are as old as the mother plant. The full timeline, from seed to harvest, is in the guide on how long cannabis takes to grow.
Light: 18/6 cycle and intensity
The standard is 18 hours of light and 6 of darkness, controlled by a timer: the dark hours also let the tent cool down. A 20/4 cycle is possible but uses more electricity for only a modest gain in growth. More in the guide to the photoperiod.
| When | PPFD (µmol/m²/s) | DLI at 18/6 (mol/m²/day) |
|---|---|---|
| First week after transplanting | 200–300 | 13–19 |
| Full vegetative stage | 300–500 | 19–32 |
| End of the vegetative stage | 500–600 | 32–39 |
PPFD is measured at canopy height with a quantum sensor; otherwise follow the distances and dimmer settings given by the lamp manufacturer. Dimmable LED lamps let you increase intensity step by step without moving the lamp every few days. If the top leaves bleach or curl upwards, the light is too strong or too close.
Climate: temperature, humidity and air
| Parameter | Vegetative value | Note |
|---|---|---|
| Temperature, lights on | 22–28 °C | Measured at canopy height |
| Temperature, lights off | 18–22 °C | Avoid dropping below 18 °C |
| Relative humidity | 55–65% | Up to 70% in the first weeks |
| VPD | 0.8–1.2 kPa | Calculated from leaf temperature |
| pH in soil | 6.0–7.0 | Ideal 6.2–6.8 |
| pH in coco coir and hydroponics | 5.5–6.5 | Ideal 5.8–6.2 |
In winter in Switzerland, central heating often pushes indoor humidity below 40%, and the extractor fan pulls that dry air into the tent. When humidity is too low, plants transpire too much, drink more and grow more slowly: a small humidifier from the humidification collection, run by a humidistat, brings the values back into range. An oscillating fan that gently moves the leaves also strengthens the stems.
Water and nutrients
In the vegetative stage the plant builds leaves and stems and mainly needs nitrogen (N). Grow-phase fertilisers therefore contain more nitrogen than phosphorus and potassium. A few practical rules:
- Pre-fertilised soil: usually enough on its own for the first 2–4 weeks, less with "light" mixes; start feeding when the lower leaves turn slightly paler.
- Doses: start at a quarter to half of the feeding chart dose and increase according to how the plant responds.
- Coco coir and hydroponics: here you feed from the start, at an EC of 0.8–1.6 mS/cm depending on the plant's age, with a product that contains calcium and magnesium.
- Signals: lower leaves yellowing evenly point to a lack of nitrogen; very dark green leaves with claw-like tips point to too much.
- Watering: water thoroughly and wait until the pot feels light before the next watering, rather than giving a little every day.
Repotting and training
Seedlings usually start in 0.25–1 litre pots and are transplanted once white roots fill the root ball, generally after 2–3 weeks. Indoors, a final pot of 11–20 litres suits most photoperiod plants; autoflowers are sown straight into their final pot. Do the last repotting at least a week before switching to 12/12, so the roots have time to colonise the new medium. You will find various sizes among the pots.
The vegetative stage is also the time to shape the plant. Topping is done at the 4th or 5th node and needs about a week of recovery; LST starts at 3–5 nodes and continues until the stretch ends, around the 2nd–3rd week of flowering. A few days before 12/12 it pays to remove the low shoots that would otherwise sit in shade.
When to switch to flowering
During the first 2–3 weeks of flowering the plant keeps stretching, usually by 50–100%, and in some varieties even more. That is why you switch to 12/12 when the plant has reached about half the final height the grow tent allows. To work it out:
- Start from the internal height of the grow tent.
- Subtract the pot height and the space taken by the lamp and hangers.
- Subtract the minimum lamp-to-canopy distance given by the manufacturer.
- The result is the maximum plant height: switch to 12/12 once the plant has reached about half of it.
Example: in a 180 cm tall tent with a 30 cm pot, 20 cm for lamp and hangers and 35 cm between lamp and tips, about 95 cm is left for the plant. It is therefore worth switching to flowering when the plant measures 45–50 cm above the pot. With a scrog the rule is different: you switch to 12/12 when the net is 70–80% full.
Frequently asked questions
How long should the vegetative stage last?
Indoors, for photoperiod plants, usually 3–6 weeks: shorter with many small plants, longer with one or two trained plants meant to fill the tent. Autoflowers move into flowering on their own after 3–5 weeks, often 3–4.
How many hours of light in the vegetative stage?
The standard is 18 hours of light and 6 of darkness (18/6), controlled by a timer. A 20/4 cycle is possible but uses more electricity without much benefit.
When should you switch to flowering?
When the plant has reached about half the final height the tent allows, because it stretches by a further 50–100% in flowering. With a scrog, switch when the net is 70–80% full.
Which fertiliser should you use in the vegetative stage?
A grow-phase fertiliser with more nitrogen than phosphorus and potassium, at rising doses starting from a quarter to half the stated dose. In pre-fertilised soil you often need nothing for the first 2–4 weeks.
Grow-phase fertilisers from several brands are in the Gro.w.LaB fertilisers collection.
Updated on 13 October 2026.