The seedling stage is the most fragile period in a cannabis plant's life. A seedling has a tiny root system, thin stems, and no energy reserves — it relies entirely on the environment you provide. The good news: seedlings need very little. The bad news: "very little" is precise, and most seedling deaths come from too much of something, not too little.
This guide covers everything from the moment the seed cracks until the plant has 4–5 nodes and is ready for vegetative training.
The first 48 hours
What to expect
After planting a germinated seed (tap root visible) about 1/2 inch deep in moist medium:
- Hours 1–24: nothing visible above the soil. The tap root is growing downward and anchoring
- Hours 24–48: the seedling breaks the surface. A pale, arched stem pushes up, carrying the seed shell
- Hours 48–72: the stem straightens and the cotyledons (round embryonic leaves) spread open. The seed shell may fall off on its own or remain attached
If the seed shell is stuck
Don't panic. If the seed shell remains on the cotyledons after 48 hours above soil:
- Mist the shell with water to soften it
- Wait another 24 hours — most shells fall off naturally
- If still stuck after 72 hours: very gently remove it with tweezers. Hold the stem with one finger for support and peel the shell off carefully. Don't force it — if it resists, mist again and wait
If the seedling is leggy (tall and thin)
A seedling stretching toward the light with a long, thin stem is getting insufficient light. Fix immediately:
- Lower the light (or increase intensity)
- Bury the stem slightly by mounding soil around the base — cannabis stems can root from buried sections
Environment: weeks 1–3
Light
| Parameter | Target |
|---|---|
| Intensity (PPFD) | 200–350 µmol/m²/s |
| DLI | 15–25 mol/m²/day |
| Light height (LED) | 24–36 inches at 50% dimmer |
| Schedule (photoperiod) | 18/6 |
| Schedule (autoflower) | 20/4 |
Seedlings don't need much light. A 300W LED at 50% dimmer and 30 inches away is usually perfect. More light doesn't make seedlings grow faster — it stresses them. Increase intensity gradually over weeks 2–3. See our seedling lighting guide.
Temperature
| Parameter | Target |
|---|---|
| Day (lights on) | 72–78°F (22–26°C) |
| Night (lights off) | 65–70°F (18–21°C) |
| Root zone | 72–76°F (22–24°C) |
Cold root zones are the hidden killer. If your pot is sitting on a cold concrete floor, the root zone can be 10–15°F below ambient air temperature. At 60°F root temp, seedling growth stalls. Use a seedling heat mat set to 75°F under the pot.
Humidity
| Parameter | Target |
|---|---|
| Relative humidity | 65–75% |
| Method | Humidity dome, humidifier, or covered tray |
Seedlings have tiny root systems that can't supply water fast enough if the air is dry. High humidity reduces transpiration (water loss through leaves), keeping the seedling hydrated even with minimal roots.
Humidity dome: place a clear plastic dome (or a cut plastic bottle) over the seedling for the first 5–7 days. Remove it once the first true leaves are fully developed — the plant is now transpiring enough to benefit from normal humidity.
Airflow
Gentle airflow is important even for seedlings:
- A small fan on low, pointed near (not directly at) the seedling
- The slight movement strengthens the stem (mechanical stress stimulates thicker stem growth)
- Don't blast the seedling — a gentle breeze that makes the leaves barely sway is perfect
Watering seedlings
How much
Seedlings need very little water. The root system is a few inches at most — water beyond the root zone sits unused and creates anaerobic conditions.
Method: the ring technique
- Water in a small ring around the seedling, 2–3 inches from the stem
- Don't water directly on the stem (stem rot risk)
- Use 50–100 mL per watering for a seedling in a large pot
- As the plant grows, widen the ring to encourage roots to grow outward seeking water
How often
- Soil: every 3–5 days for a seedling in a large pot. The small plant uses very little water. Stick your finger in the soil — if the top inch is dry, water lightly
- Coco: every 1–2 days, but very small amounts. Coco dries faster than soil
- The biggest mistake: overwatering. Seedlings die from drowning more than from any other cause. When in doubt, wait another day. See our overwatering guide
Water quality
- Plain water for the first week (no nutrients)
- pH adjusted: 6.0–6.5 for soil, 5.5–6.0 for coco
- Room temperature: cold water shocks roots. Let water sit for an hour before using
- Chlorinated tap water: let it sit uncovered for 24 hours to off-gas chlorine, or use a dechlorinator
When to start feeding
In soil
If you're using amended soil (Fox Farms Happy Frog, Roots Organics):
- Don't feed for 2–3 weeks. The soil contains enough nutrients for a seedling
- Hot soils (Ocean Forest): may burn seedlings. Start in Happy Frog or a 50/50 Ocean Forest + perlite mix, then transplant into the hotter soil later
- First feed: at week 2–3, start with 25% of the manufacturer's recommended dose
In coco
Coco has no nutrients. Feeding must start earlier:
- Week 1: very light feed — EC 0.3–0.4 (25% strength)
- Week 2: EC 0.5–0.8 (50% strength)
- Week 3: EC 0.8–1.0 (50–75% strength)
- Cal-mag from the start — coco always needs supplemental calcium and magnesium. See our cal-mag guide
Signs of overfeeding a seedling
- Dark green leaves (too much nitrogen)
- Burnt leaf tips (nutrient burn)
- Curled or clawed leaf tips
- Slow or stunted growth despite good environment
If you see these signs, flush with plain pH'd water and skip the next feeding.
Growth milestones: weeks 1–3
| Day | What you should see |
|---|---|
| 1–3 | Cotyledons open, stem vertical |
| 4–7 | First pair of true leaves (single-blade, serrated) |
| 7–10 | Second pair of true leaves (3 blades) |
| 10–14 | Third pair (5 blades), visible stem thickening |
| 14–21 | 4th–5th node, roots reaching pot edges, accelerating growth |
By day 21, the seedling transitions to the vegetative stage. Growth accelerates noticeably — you may see a new node every 1–2 days. The plant is ready for regular feeding, training, and normal environmental conditions.
Transplanting
If you started in a small container (solo cup, jiffy peat pot): transplant when roots appear at the bottom drainage holes, usually around day 10–14.
- Water the seedling 1 hour before transplanting — moist soil holds together; dry soil crumbles and exposes roots
- Pre-moisten the new pot's medium before transplanting
- Handle by the soil, not the stem — stem damage at this stage can be fatal
- Plant at the same depth — or slightly deeper to bury a leggy stem (cannabis can root from buried stem tissue)
- See our transplanting guide
Common mistakes
- Overwatering — the #1 seedling killer. A seedling in a 5-gallon pot needs tablespoons of water, not cups. Water less than you think you should
- Too much light — a 480W LED at 100% and 12 inches will bleach a seedling in hours. Start at 50% dimmer and 30+ inches. Increase gradually
- Hot soil — Ocean Forest is too nitrogen-rich for seedlings. Start in a milder medium and transplant later
- No humidity dome — dry air (40–50% RH) dehydrates seedlings faster than their tiny roots can supply water. Use a dome for the first week
- Starting nutrients immediately — in soil, the medium feeds the seedling. In coco, start at 25% strength. Full-strength nutrients on a seedling cause nutrient burn
- Planting too deep — the seed should be 1/2 inch deep, not 2 inches. Deep planting means the seedling exhausts its energy pushing through soil before reaching light
For seed germination before this stage, see our germination guide. For lighting setup, see our seedling lighting guide. For when things go wrong, see our seedling troubleshooting guide.