Purple leaves, bracts or sugar 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. PossibleGenetic 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. PossibleNormal late-flower fade (senescence)

   Normal ageingLow urgencyRead: Yellowing lower leaves on cannabis: a differential

   Fix: Nothing to fix in the last 2-3 weeks of flower. Keep feeding at stage strength with N tapered; don't 'flush' (pre-harvest flushing starves the plant while it packs on final weight and resin, and controlled studies show no quality gain). Fade before week 5-6 of flower suggests N was cut too early.

   Recovery: Buds keep swelling and ripening normally while fan leaves fade.

   How to confirm it

   • 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.

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.

Look for

Even purple colour on leaves and bracts, tissue healthy, no spots..