A seedling is running on stored energy and a root system the size of a hair. Your job for two weeks is simple: don't do too much.
The numbers that matter
| Factor | Target | Why |
|---|---|---|
| Light cycle | 18/6 | Vigorous growth without wasted power |
| Light distance | 45–60 cm (LED at ~30%) | Stretch means too far; bleach spots mean too close |
| Temperature | 22–26 °C day, 18–21 °C night | Cold shocks roots; heat dries seedlings out |
| Humidity | 65–70% RH | Seedlings drink through leaves while roots develop |
| Water pH | 6.0–6.5 (soil) | Nutrient availability window |
Watering: the part everyone gets wrong
Overwatering kills more seedlings than everything else combined. Roots need oxygen; saturated soil has none.
- Water in a small ring around the stem, not a flood — 30–60 ml is plenty at first
- Let the top 2 cm of medium dry out between waterings (poke a finger in; if it's damp, wait)
- A light pot is a thirsty pot — lift it and learn the weight difference
- Drooping with wet soil = overwatered. Drooping with dry soil = thirsty. Check before you pour
The ring-watering technique forces roots to reach outward toward the moisture, building a stronger root system. As the plant grows, gradually widen the ring and increase the volume. By the end of seedling stage, you should be wetting most of the pot.
For a deeper dive on overwatering vs underwatering, see the watering diagnosis guide.
Humidity: why seedlings need it high
Seedlings have almost no root system, so they absorb most of their water through their leaves via transpiration. Low humidity (below 50%) forces the tiny roots to work harder than they can, leading to wilting even in moist soil.
How to raise humidity cheaply:
- A clear dome (humidity dome, cut water bottle, or clear plastic bag over the pot) traps moisture around the leaves
- Remove the dome for 15 minutes twice a day to exchange air and prevent damping off
- A small humidifier in the tent works; aim for 65–70%
- Group seedlings together — they humidify each other
Drop humidity gradually as roots develop: 65–70% in week 1, 55–65% by week 2, then down to standard veg range (50–60%) once the plant is growing real fan leaves.
Feeding: don't
Seed leaves (cotyledons) carry the nutrients a seedling needs for the first 10–14 days, and most starting mixes carry more. Feeding now burns roots.
Start at quarter-strength vegetative nutrients only after the third set of true leaves, and only if the plant is pale. If leaves are green and happy, the medium is still feeding the plant — wait.
When you do start feeding, use a gentle seedling-strength nutrient at pH 6.0–6.5 (soil) or 5.5–6.0 (coco). See the pH guide for calibration basics.
Stretch and how to stop it
A seedling reaching for light gets tall, thin, and floppy. If yours stretches:
- Bring the light closer in small steps (watch the leaves for 24 h after each move)
- Add a gentle fan — light air movement thickens stems
- When transplanting, you can bury a stretched stem up to the first leaves
If the seedling has already fallen over, prop it up with a toothpick or small stake and fix the light distance. It will recover.
What healthy seedlings look like
- Color: bright, even green. Dark green means the medium is hot (too much nutrient); yellowing from the bottom means the medium has run out
- Growth rate: one new set of leaves every 3–5 days. Slower than that usually means overwatering or cold temps
- Stem: short, sturdy, and slightly purple is fine (some genetics). Tall and white means stretching
- Cotyledons: the round seed leaves yellow and drop around day 10–14. This is normal — the plant has used their stored energy
Common seedling problems
Damping off — the stem pinches at the soil line and the seedling topples. Caused by too-wet conditions and poor airflow. Prevention: don't overwater, use clean media, and keep air moving. There's no fix once it happens — start a new seed.
Helmet head — the seed shell stays stuck on the cotyledons. Wait 24 hours; if it hasn't dropped, mist it and gently remove it with tweezers. Don't force it — tearing the cotyledons is worse than waiting.
Curled or cupped leaves — usually heat stress or light too close. Raise the light 5 cm and check your canopy temperature.
For a full troubleshooting reference, see the seedling troubleshooting guide.
When seedling stage ends
You're in vegetative growth once the plant is producing full-sized fan leaves at a visible weekly pace — usually week 2–3. That's when feeding, training, and real light intensity begin. See vegetative stage 101 for what comes next.