Newest leaves limp while older leaves stay firm

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. PossibleHeat stress

   EnvironmentAct within a day or twoRead: Heat and light stress: taco leaves, bleaching and foxtails

   Fix: Bring canopy leaf temperature under about 30 °C (86 °F): more exhaust (one tent air exchange every 1-3 minutes), raise or dim the light, add canopy airflow (0.3-1.0 m/s), keep RH so VPD stays 1.0-1.5 kPa, water well. Above ~30 °C photosynthesis falls unless CO2 and light are high.

   Recovery: Leaves flatten within a day; no new foxtails or crisping.

   How to confirm it

   • 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.
   • 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.

2. Less likelyCopper deficiency

   NutrientLow urgencyRead: Copper (Cu) in cannabis: limp, distorted new growth and root-burning excess

   Fix: Very rare with a complete micro mix. Supply about 0.03-0.1 mg/L Cu; do not overshoot (excess Cu damages roots).

   Recovery: New leaves regain size and turgor in 10-14 days.

   How to confirm it

   • 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.

3. Less likelyBoron deficiency

   NutrientDeal with it this weekRead: Boron (B) in cannabis: distorted tips, dying roots and a narrow safe window

   Fix: Rule out broad mites first. Supply about 0.2-0.5 mg/L B (boric acid ~1.1-2.9 mg/L, or a complete micro mix). The gap between enough and too much is narrow: never add B 'just in case' on top of a complete feed.

   Recovery: New shoots grow normally from the tip or from side buds within 10-14 days.

   How to confirm it

   • 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.
   • Look at the roots. Hydro: lift the net pot and look at the root mass in good white light. Containers: tip the plant out with a hand over the medium and look at the edges and bottom of the root ball. Healthy roots are white to cream with fine fuzzy tips. Brown roots whose outer layer slides off between the fingers leave a bare thread: rot. Firm, evenly tan roots with white tips in coloured solution are staining.
   • 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.

Look for

Close-up of the top two nodes with soft, drooping leaves above firm older leaves..