Purple or red stems and petioles

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. LikelyGenetic purple colour (anthocyanin)

   GeneticsLow urgencyRead: Purple or red stems and leaves on cannabis: a differential

   Fix: Nothing to fix: purple stems, petioles and bracts are mostly genetic and harmless, brought out by cool nights 5-10 °C (9-18 °F) below day. Purple is bag appeal, not potency.

   Recovery: Leaves stay healthy and green apart from the colour; no spotting.

   How to confirm it

   • 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 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.

3. Less likelyCold root zone (below about 16 °C / 61 °F)

   Root zoneDeal with it this weekRead: Temperature by stage: day, night and root-zone targets

   Fix: Raise root-zone temperature to 18-22 °C (64-72 °F): lift pots off cold floors onto insulation, warm the room at night, use an aquarium heater in reservoirs (with a thermostat), water with room-temperature solution.

   Recovery: Growth resumes and new leaves green within a week.

   How to confirm it

   • Measure reservoir and root-zone temperature at the end of the light period. Measure the solution in the reservoir and in the root chamber or bucket, at the warmest time of day. Target 18-22 °C (64-72 °F). Above about 23-24 °C (73-75 °F), dissolved oxygen falls and Pythium risk rises; below 16 °C (61 °F) uptake slows.
   • Lift the pot and check moisture at depth. Lift the pot right after a full watering to learn its wet weight, then lift before each watering. Water when it is clearly lighter (about half the added water gone). Push a finger or a moisture probe 5 cm (2 in) down. Heavy for 3-4+ days with droop: overwatering. Very light with limp papery leaves: thirst.

4. Less likelyPhosphorus deficiency

   NutrientDeal with it this weekRead: Phosphorus (P) in cannabis: what it does, deficiency, excess and how much is enough

   Fix: Warm the root zone to 18-22 °C (64-72 °F) and nights above ~15 °C (59 °F) first; cold blocks P uptake. Correct pH (soilless 5.5-6.5, soil 6.0-7.0). If genuinely short, supply about 30-60 mg/L P; controlled cannabis trials found no yield or cannabinoid gain above roughly 25-60 mg/L, so skip 'bloom boosters'. Soil: a balanced liquid feed; living soil: a slow P amendment acts in weeks.

   Recovery: No new olive spots on leaves that open after the fix, within 7-10 days; growth rate picks up. Spotted leaves stay damaged.

   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.
   • Measure runoff EC against input EC. Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
   • 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.
   • 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

Red-purple petioles and stems on otherwise healthy green leaves..