New leaves almost white

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. PossibleLight bleaching at the canopy top

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

   Fix: Raise or dim the light by 10-20% so top-canopy PPFD sits around 800-1,100 µmol/m²/s at ambient CO2 (up to ~1,500 with CO2 and good climate). Keep the canopy even so one cola doesn't sit in a hotspot. Bleached tissue won't re-green.

   Recovery: No new bleaching within 3-5 days; new bracts and leaves at the top stay green.

   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.

2. PossibleHigh root-zone pH locking out iron, manganese, zinc, boron and copper

   pHDeal with it this weekRead: pH and nutrient availability: what pH really controls, by medium

   Fix: Bring root-zone pH into range: soil 6.2-6.8, coco and rockwool 5.8-6.2, hydro 5.5-6.2. Lower input pH by 0.2-0.3 units at a time rather than crashing it; neutralise alkaline water with a mineral acid (phosphoric, nitric or sulfuric; wear gloves and eye protection, add acid to water). Use an iron chelate stable at your pH (DTPA or EDDHA above 6.5).

   Recovery: Runoff or reservoir pH back in range at two consecutive checks; new leaves open green within 5-10 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.
   • Check the meters against fresh standards. Put the pH probe in fresh pH 7.0 and 4.0 buffer and the EC probe in a 1.413 mS/cm standard. If pH reads more than 0.1-0.2 off, recalibrate; if it can't be calibrated or the slope is below ~90-95%, replace the probe. Many 'lockouts' are a drifting meter.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • Pour-through extraction for container pH and EC. Water normally, wait 30 minutes to 2 hours, pour enough pH-adjusted water on top to collect about 50 mL of leachate (roughly 75 mL for a 15 cm pot, 150 mL for a 4 L pot, 350 mL for 11 L), and measure pH and EC of that 50 mL. Less disruptive than full runoff and more repeatable.

3. Less likelyIron deficiency (usually high pH)

   NutrientLow urgencyRead: Iron (Fe) in cannabis: yellow new growth, chelates and pH

   Fix: Bring root-zone pH into range first: soilless 5.5-6.2, soil 6.0-6.8. Neutralise alkaline water with acid. Supply 1.5-3 mg/L Fe as a chelate matched to pH: EDTA to about pH 6.5, DTPA to about 7.5, EDDHA to about 9. A foliar iron chelate spray (veg only) greens leaves in days.

   Recovery: The next new leaves open green within 5-10 days; the whitened leaves re-green partly or not at all.

   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.
   • Get a water report (or test your water). Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
   • 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.
   • Watch new growth for 5-10 days after one change. Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.

Look for

Cream to white newest leaflets; note whether the whitening is only nearest the lamp (light) or also on shaded tips (nutrition)..