Short answer. A plant that sits still is usually telling you about its roots or its light. Check, in order: watering and root health, root-zone and air temperature, light intensity and photoperiod, pH and EC, and pests on the roots and new growth. If one plant stays small with tiny leaves and short internodes while its neighbours on the same feed thrive, test it for hop latent viroid (HLVd); it is common in clone-based grows and often has no other obvious sign [3][4].
What's happening
Growth is the product of light captured, carbon fixed and roots able to supply water and nutrients. Any one of these sets the ceiling. Yield in indoor cannabis rose linearly with canopy PPFD up to 1800 µmol/m²/s in one trial [1], so under-lit plants are slow plants. Temperature also matters: photosynthesis responded to temperature, and above ~30 °C (86 °F) it declined in one cultivar [2]. Root problems (overwatering, Pythium/Fusarium root rot in hydro [6], root aphids [5]) restrict everything above ground. HLVd-infected plants may be symptomless or show stunting, smaller leaves, shorter internodes, chlorosis and brittle stems [4]; it spreads through propagation from infected mothers [3].
How to recognise it
| Pattern | Most likely | Also consider |
|---|---|---|
| Seedling in a big wet pot, droopy, pale | Overwatering | Damping-off, fungus gnats |
| Whole room slow, no other symptoms | Light too low or room too cool | Short photoperiod in veg |
| Whole room slow, pale lower leaves | N shortage or root-zone limitation | pH out of range |
| One or few plants stunted, small leaves, short internodes, maybe brittle | HLVd [4] | Root rot, root aphids |
| Stunting with wilting, yellowing, brown roots | Root rot [6] | Root aphids |
| Stunted with twisted, shiny new growth | Broad/russet mites | Ca/B deficiency |
| Clones sitting 1–2 weeks after transplant | Root establishment | Over-sized pots, cold media |
| Stalled growing tip, extra side branching | Ca or B deficiency killing the tip [7] | Mechanical damage |
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| PPFD, seedling/clone | ~150–300 µmol/m²/s | Grower practice |
| PPFD, veg | ~400–700 µmol/m²/s | Grower practice |
| DLI, veg (18 h) | ~25–45 mol/m²/day | Grower practice; 600 µmol/m²/s × 18 h ≈ 39 mol/m²/day |
| Air temperature, lights on | ~22–28 °C (72–82 °F) | Grower practice; decline above ~30 °C [2] |
| Root-zone temperature | 18–22 °C (65–72 °F) | Cold media slow everything |
| pH, hydro / soilless / soil | 5.5–6.0 / 5.5–6.5 / 6.0–7.0 | See how to diagnose |
| Typical veg growth (healthy photoperiod plant) | new node every ~2–4 days | Grower practice; cultivar dependent |
What to check first
- Watering and pot size. Lift the pot; small root systems in big wet pots are the classic seedling stall.
- Roots. Colour, smell, quantity. Insects on roots.
- Temperature of air and root zone, day and night.
- Light. Measure PPFD at canopy; calculate DLI. Check the timer actually gives 18/6 in veg.
- pH and EC of input and runoff/reservoir.
- New growth with a lens for mites.
- Compare plants. Same treatment, one plant stunted = plant-specific problem (disease, viroid, genetics).
- Test suspect mothers and stunted clones for HLVd (RT-PCR or RT-qPCR; root samples are often recommended).
How to fix it
- Root zone: correct watering rhythm, warm media, use appropriately sized containers (up-pot in steps).
- Light: raise PPFD in steps of ~100 µmol/m²/s per few days, watching for stress; extend photoperiod to 18 h in veg.
- Temperature: bring air into ~22–28 °C and roots to ~20 °C.
- pH/EC: correct as per system.
- Root aphids or mites: see root aphids, broad and russet mites.
- HLVd positive: there is no cure for an infected plant. Remove positives from propagation, sanitise tools with an agent shown to inactivate viroids, and restart from clean, tested stock. See HLVd.
How to confirm it worked
- Node spacing and leaf size increase on new growth within 7–10 days.
- Root mass visibly increases (white tips at the pot edge or in the net pot).
- HLVd: clean stock tests negative on repeat tests and grows at the expected rate.
Common mistakes
- Feeding harder to "kick-start" a plant with a root problem.
- Blaming genetics before checking light and temperature.
- Keeping a slow, "just a weak pheno" mother that is actually viroid-infected and seeding the whole facility.
Evidence
- Light-limited yield is Established for cannabis [1].
- Temperature effects are Supported [2].
- HLVd stunting and spread via propagation: Established, though symptom severity varies widely and incidence estimates are Contested [3][4].
- Root aphid on indoor cannabis: Established [5]. Root rot in hydro: Established [6].
