GenetixBayEST. 2009
Germination · Beginner · 2 min read

Seedling care: the first three weeks

Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

Supported
Draft: awaiting expert review Updated
SeedlingAny system

Short answer. Give seedlings moderate light (≈ 150–300 µmol/m²/s for 18 h, a DLI of 10–19 mol/m²/day), warm, fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH, leaf VPD ~0.4–0.8 kPa) and small waterings in a ring around the stem. Hold off on feed until 2–3 sets of true leaves unless the medium has no starter charge (coco, rockwool, hydro). Most losses are overwatering, stretching from low light, or damping-off.

Week by week

Days What you see What to do
0–5 Hook emerges, cotyledons open Low light (100–200 µmol/m²/s), dome vented daily, keep surface just moist
5–10 First true leaves (single or 3-finger) 150–250 µmol/m²/s; water small amounts when the plug lightens
10–21 2nd–4th nodes 200–300+ µmol/m²/s; start ¼–½ strength feed (EC 0.4–0.8 mS/cm); transplant when roots fill the plug (transplanting)

Numbers that matter

Parameter Target Evidence
PPFD 150–300 µmol/m²/s, 18 h Grower practice; consistent with young-plant DLI research in other crops [1]
DLI 10–19 mol/m²/day Plugs below ~8 mol/m²/day are inferior in other crops [1]
Air temperature 23–27 °C (73–81 °F) Grower practice
RH 65–75 % Grower practice
Medium temperature 21–25 °C (70–77 °F) Warm roots grow fast; cold water raises damping-off risk [2]
First feed EC 0.4–0.8 mS/cm (200–400 ppm500) Grower practice; start at true leaves [2]
Water pH Soil 6.2–6.8; coco 5.8–6.2; hydro/rockwool 5.5–6.0

Problems and fixes

Symptom Likely cause Fix
Long, thin, pale stem, flops over Light too weak or too far Raise PPFD gradually; bury the stretched stem deeper at transplant (Grower practice)
Drooping, wet medium Overwatering Let it dry; smaller waterings; airier mix
Stem pinched and brown at the soil line, collapse Damping-off See damping-off
Cotyledons yellow at day 10–14 Normal as true leaves take over, or no nutrients in inert media Start light feed
Leaves praying hard, bleached tips Too much light/heat Dim or raise light
Seed husk stuck on cotyledons Dry surface Mist, wait, then gently remove

Common mistakes

  • Big pot + big watering for a tiny root.
  • Dome left closed for days (mould, soft stems).
  • Strong nutrients "to get it going".
  • A cheap light at 1 m away.

Evidence

Young-plant light responses are Supported in horticultural research on other crops [1]; cannabis veg responds to high light with compact growth [3]. Temperatures follow hemp germination data [4] and grower practice. Fertiliser timing follows extension guidance [2].

Questions this article answers

Why is my seedling stretching and falling over?

Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

How much should I water seedlings?

Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

How far should the light be from seedlings?

Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

When should I start feeding seedlings?

Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

What humidity for cannabis seedlings?

Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

Sources

  1. Runkle E. The benefits of lighted plugs. Michigan State University Extension (plugs below DLI ~8 inferior; 10–14 mol/m²/day better roots and mass). canr.msu.edu/resources/the_benefits_of_lighted_plugs
  2. University of Minnesota Extension. How to prevent seedling damping off (fertiliser after true leaves; avoid cold water). extension.umn.edu/solve-problem/how-prevent-seedling-damping
  3. Moher M, Llewellyn D, Jones M, Zheng Y, 2021. High light intensities can be used to grow healthy and robust cannabis plants during the vegetative stage of indoor production. Preprints (not peer reviewed). doi.org/10.20944/preprints202104.0417.v1
  4. Geneve RL, et al., 2022. Temperature limits for seed germination in industrial hemp. Crops 2(4):415–427. doi.org/10.3390/crops2040029

Related

SupportedBeginner · Draft

Germination methods compared: soak, paper towel, plugs, direct sow

Cannabis seed germinates best around 20–30 °C (68–86 °F) with steady moisture and air. Every common method works if those are met: direct sowing is least handling, paper towels let you see radicles, plugs and cubes suit hydro. Lab protocols with 1 % hydrogen peroxide, short soaks or tip scarification sped up germination, mainly for old or hard seeds; they are optional for fresh seed.

EstablishedBeginner · Draft

Damping-off: why seedlings collapse at the soil line

Damping-off is seedling death caused mainly by soil-borne pathogens such as Pythium, Rhizoctonia and Fusarium: the stem pinches, darkens and collapses at the soil line, or seeds rot before emerging. It is a prevention problem: clean containers and new mix, no overwatering, warm medium, good light and airflow. Once a seedling has collapsed it cannot be saved.

Grower practiceBeginner · Draft

Transplanting: when, into what, and avoiding transplant shock

Transplant when roots fill the current container but before they circle: typically seedling plug to 1–3 L at 2–3 weeks, then to the final pot 2–4 weeks before flowering. Move the root ball intact into pre-moistened medium at the same pH and similar EC, water in lightly, and keep light and climate gentle for a few days. Autoflowers do best sown straight into their final pot.

EnvironmentSupportedIntermediate · Draft

PPFD and DLI by stage, with the evidence

In the best indoor trial, cannabis flower yield rose linearly with canopy PPFD from 120 to 1,800 µmol/m²/s at ambient CO₂ (116 to 519 g/m²) with no change in cannabinoid potency. Practical targets: 100–300 µmol/m²/s for seedlings and clones, 300–700 (up to ~900) in veg, and 800–1,200 in flower, or more with CO₂, as long as the plant shows no stress and the electricity is worth it.

EnvironmentSupportedIntermediate · Draft

VPD explained: vapour pressure deficit for cannabis growers

VPD is how hard the air pulls water out of the leaf, in kPa. Common targets are about 0.4–0.8 kPa for seedlings and clones, 0.8–1.2 kPa in veg and 1.0–1.5 kPa in flower, calculated from leaf temperature rather than air temperature. Most of these bands come from grower and greenhouse practice; cannabis-specific VPD trials are still scarce.

DiagnosisSupportedBeginner · Draft

Stretchy, leggy cannabis: a differential

Long internodes and thin stems mean the plant is reaching for light: PPFD too low, light too far away, a low red-to-far-red ratio (shade or crowding), or warm days relative to nights. Some stretch in the first 2–3 weeks after the switch to 12/12 is normal and cultivar dependent. Measure PPFD and day/night temperatures before blaming genetics.

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