Short internodes, compact growth and small leaves

Likely causes, most likely first

Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.

1. PossibleHop latent viroid (dudding)

   DiseaseAct within a day or twoRead: Hop latent viroid (HLVd, 'dudding') and other cannabis viruses

   Fix: No cure in a plant. Test mothers (roots and leaves) by RT-PCR or RT-qPCR, discard positives, disinfect tools with 10% bleach or heat (alcohol and peroxide don't inactivate it), and use meristem tissue culture to rescue valuable genetics. Don't take cuttings from stunted plants.

   Recovery: Negative tests on new mothers; no more duds in the next cycle.

   How to confirm it

   • Lab test for hop latent viroid (RT-PCR or RT-qPCR). Send root and leaf samples (roots detect HLVd earlier in young plants) to a lab offering HLVd RT-PCR or RT-qPCR. Test mother plants and any stunted plant among healthy siblings. A positive plant cannot be cured; tissue culture of meristem tips can rescue genetics.
   • Compare with siblings or the same cut. Put the affected plant beside siblings or clones of the same cut in the same conditions. A trait shared by all of a cultivar (purple stems, foxtailing, pale colour) is genetic; a problem on one plant points to its roots, a virus or its genetics; a problem on every cultivar points to the environment or feed.

2. Less likelyLight burn: too much light for the plant's climate and CO2

   LightDeal with it this weekRead: Heat and light stress: taco leaves, bleaching and foxtails

   Fix: Dim or raise the light 10-20%; ramp intensity over 7-10 days after a light change. Seedlings and clones need only 100-300 µmol/m²/s. High PPFD works only with leaf temperature, VPD, roots and feed all in range.

   Recovery: Top leaves flatten within 1-3 days.

   How to confirm it

   • Measure PPFD at the canopy top. Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
   • IR leaf temperature at the canopy top. Point an IR thermometer at the top leaves under the light (5-10 cm away, emissivity 0.95) and at shaded leaves. Leaf temperature above about 30 °C (86 °F) at the top only points to heat or radiant load; leaf VPD can be calculated from leaf temperature and RH.

3. Less likelyCold stress

   EnvironmentDeal with it this weekRead: How cold is too cold for cannabis?

   Fix: Keep days at 22-28 °C (72-82 °F) and nights no more than 3-6 °C (5-10 °F) cooler and above ~15 °C (59 °F); warm the root zone. Cold-for-colour is real in purple cultivars, but claims of more terpenes or resin are unsupported.

   Recovery: Growth resumes within a week; new leaves come in green.

   How to confirm it

   • Check temperature and RH at canopy height with a calibrated sensor. Put a calibrated sensor (salt test: a sealed bag with damp table salt reads 75% RH after 8-12 hours) at canopy height, out of direct light, and log for 24 hours including lights-off. Look for lights-off RH spikes and dew point.

4. Less likelyZinc deficiency

   NutrientLow urgencyRead: Zinc (Zn) in cannabis: small leaves, margin burn and the phosphorus link

   Fix: Lower high pH, stop excess phosphorus 'boosters' (high P ties up Zn), then supply about 0.1-0.5 mg/L Zn as zinc sulfate or chelate.

   Recovery: New leaves reach normal size with clean margins in 7-14 days.

   How to confirm it

   • Measure input pH and root-zone (runoff or reservoir) pH. Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
   • Magnifier check of leaf undersides and new growth. Use a 10-20x loupe for spider mites, thrips, aphids and whiteflies, and a 30-60x lens or clip-on phone macro for broad and russet mites. Check the undersides of 10 affected leaves across several plants and the newest shoot tips, in white light. Look for moving dots, eggs, shed skins, webbing, slender insects with black specks, and glassy specks on new growth.
   • Leaf tissue analysis. Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.

5. Less likelyCO2 set far too high

   EnvironmentDeal with it this weekRead: CO₂ enrichment: when it pays, how much, and how to stay safe

   Fix: Hold 800-1,200 ppm; above that there is little further gain in cannabis. Leaf rolling and chlorosis at very high CO2 are documented in tomato and other greenhouse crops. People: keep occupied rooms under the 5,000 ppm 8-hour limit and never enter a room above 30,000 ppm.

   Recovery: Symptoms stop on new growth after CO2 is lowered.

   How to confirm it

   • Measure CO2 during the light period. Read a CO2 monitor at canopy height through the light period. A sealed room without enrichment can fall below 300-350 ppm and stall growth; enriched rooms should hold 800-1,200 ppm. Keep occupied spaces below the 5,000 ppm workplace limit.

Look for

Stack of tight nodes at the top with small leaves; compare with the leaf size lower down..