Cannabis Seedling Stage Week by Week: 1, 2, 3 and 4 Week Old Plants

Cannabis growth stages at a glance

From germination to harvest a cannabis plant goes through five stages. The growth phase in the broad sense covers the first three of them.

With photoperiod strains you set the length of the vegetative stage yourself: the longer the plants grow, the bigger they get — and the more room they need later, because during the stretch in the first weeks of flowering their height often doubles again.

1 week old weed plant: the cannabis seedling

In the first week after breaking through the soil, a cannabis seedling still looks modest: two round, smooth-edged cotyledons (seed leaves) and, between them, the first pair of true, serrated leaves, usually with one to three fingers. The stem is short and the plant is 2–5 cm (1–2 in) tall. For a labelled diagram of every part, from the cotyledons to the trichomes, see our guide to cannabis plant parts.

• Light: gentle. Hang the LED a little higher or dim it; on a windowsill a bright spot without harsh midday sun is enough.
• Watering: only around the stem, a few sips at a time. The soil should be moist but never wet.
• Feed: none. The seedling lives on the reserves in the seed and on the soil.
• Climate: 22–26 °C (72–79 °F) and 60–70% relative humidity.

If the stem gets long and thin and the seedling falls over, it is getting too little light — it has stretched. Bring the lamp closer or turn it up, and plant the stem a little deeper when you transplant.

2 week old weed plant

At two weeks old, a weed seedling has 2–3 pairs of true leaves that already show the typical shape with three to five fingers. The cotyledons are still there but slowly fading. The plant is about 5–8 cm (2–3 in) tall, and its roots will soon fill a small starter pot.

• Light: increase the intensity step by step, 18 hours of light and 6 hours of darkness.
• Watering: only water again when the pot feels noticeably lighter. Too much water is the most common mistake at this age.
• Feed: none yet in pre-fertilised soil. In very light soil you can start now at most, with about a quarter of the manufacturer's dose.

Yellowing cotyledons are normal in week 2 — the plant is using up its reserves. If the true leaves turn pale or yellow instead, it is usually short of nitrogen or the roots are too wet.

3 week old weed plant

In week three the seedling moves into the vegetative stage. You will see about 4 leaf pairs, the leaves now have five to seven fingers, and the first side shoots appear in the leaf axils. The plant is roughly 12–15 cm (5–6 in) tall, and the cotyledons yellow or drop off.

When to transplant cannabis seedlings

If your seedling is still in a small pot, now is the right time to transplant it: as soon as roots show at the edge of the pot, move it into a 3–11 litre (1–3 gallon) pot. Water the day before so the root ball holds together. Autoflowers, on the other hand, are best sown straight into their final pot, because every transplant costs them valuable growing days.

First feed

Once the soil's own nutrients run out, start with a nitrogen-rich grow fertiliser at half strength. How to recognise the need and what a feeding schedule looks like is explained in our cannabis feeding schedule guide.

4 week old weed plant

At four weeks the seedling has become a small bush: 5–6 leaf pairs, large fan leaves and strong side shoots. Depending on the strain and the light, the plant is 20–30 cm (8–12 in) tall. From now on it grows fast, often several centimetres a day.

• Light: full intensity, 18/6.
• Watering: water thoroughly until a little runs out of the bottom, then let it dry out.
• Feed: grow fertiliser with every second or third watering.
• Training: start topping or low-stress training (LST) now, while the shoots are still flexible.

Autoflowers often show the first pre-flower hairs at this age. With photoperiod strains, pre-flowers usually appear in the leaf axils between weeks 4 and 6 — that is when you can tell the sex of the plant. How to do it is explained in male vs female cannabis plants. With feminized seeds you get practically only female plants.

Weeks 5–8: the vegetative stage

From week 5 the plant grows mainly in width and height. A photoperiod strain stays in the vegetative stage for as long as it gets 18 hours of light. When to switch to 12/12 depends on your space: as a rule of thumb, flip when the plant has reached about half of the available height, because during the stretch it grows another 50–100%.

• Small grow tent (up to 120 cm / 4 ft tall): flip after 4–5 weeks of growth.
• Larger tent or heavy training: 6–8 weeks of growth for an even, wide canopy.
• Outdoors: the plant keeps growing until the days get shorter in late summer — in the northern hemisphere flowering usually starts in August.

Cannabis seedling week by week: the table

The figures are a guide for photoperiod plants in soil under LED. Strain, light and pot size shift them considerably.

Light, temperature and humidity for cannabis seedlings

Light

Indoors, cannabis in the growth phase gets 18 hours of light and 6 hours of darkness. Some growers use 20/4, but more brings hardly any benefit. What matters is a spectrum with plenty of blue — modern full-spectrum LEDs cover that. Keep the distance the lamp manufacturer recommends for the vegetative stage: too close burns the leaf tips, too far makes the plants tall and thin.

Temperature and humidity

Ideal is 22–28 °C (72–82 °F) during the day and no lower than 18 °C (64 °F) at night. Seedlings like 60–70% relative humidity; in the vegetative stage 50–60% is right. A small fan keeps the air moving, strengthens the stems and helps prevent mould.

Watering

The rule for the whole growth phase: water less often but thoroughly, rather than often and a little. Lift the pot — if it feels light, it is time to water. In soil, the pH of your water should be 6.0–7.0.

Common cannabis seedling problems

• Stretched seedlings: long, thin stems from too little light. Bring the lamp closer and plant the stem deeper when transplanting.
• Yellow leaves: cotyledons yellow normally. If true leaves turn yellow, the cause is usually too much water or too little nitrogen.
• Drooping leaves: after watering, a sign of too much water; in dry soil, a sign of too little.
• Damping off: the stem turns thin and brown just above the soil and the seedling falls over. It is caused by a fungus in wet conditions with poor air movement — let the soil dry out, add a fan and use clean pots.
• Burnt leaf tips: too much feed or the light too close.

Autoflower or photoperiod: differences in the growth phase

Autoflowering seeds have a short, fixed growth phase of about 3–4 weeks from germination, then they flower regardless of the light. They hardly recover from mistakes in the first weeks — transplanting, too much water, feeding too early. So: sow them straight into the final pot, handle them gently and use only light training such as LST.

Photoperiod strains give you more control: you can let them grow for as long as your space and time allow, and make up for mistakes during the growth phase. Both routes work for a first grow — for an overview, see our guide how to grow cannabis.

What comes next: the flowering stage

As soon as you switch to 12/12 — or the autoflower starts on its own — flowering begins. What happens week by week, from the stretch to ripe trichomes, is covered in our guide cannabis flowering stage week by week.

Robust strains for a healthy growth phase

Healthy seedlings start with good genetics. Our seeds come straight from the breeder — Pyramid Seeds has bred and produced its own genetics since 2002 — with a germination guarantee and discreet shipping. Forgiving choices for a first grow include Northern Lights, its autoflowering version Auto Northern Lights and Super OG Kush.