# Purple or red veins, mostly on leaf undersides: likely causes and fixes

> Purple or red veins, mostly on leaf undersides on cannabis is most often Genetic purple colour (anthocyanin), Cold stress or Phosphorus deficiency. How to tell them apart, the fix for each and what recovery looks like.

Also called: purple veins, red veins under leaves, veins turning purple, purple leaf veins.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=old&s=purple-veins

## Likely causes, most likely first

### Genetic purple colour (anthocyanin) (Likely)

- Kind: Genetics; urgency: Low urgency
- 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.
- Confirm: 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.
- Article: [Purple or red stems and leaves on cannabis: a differential](https://genetixbay.com/knowledge/diagnosis/purple-stems-leaves.md)

### Cold stress (Less likely)

- Kind: Environment; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Article: [How cold is too cold for cannabis?](https://genetixbay.com/knowledge/answers/how-cold-is-too-cold.md)

### Phosphorus deficiency (Less likely)

- Kind: Nutrient; urgency: Deal with it this week
- 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.
- Confirm: 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.
- Confirm: 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.
- Confirm: 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.
- Confirm: 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.
- Article: [Phosphorus (P) in cannabis: what it does, deficiency, excess and how much is enough](https://genetixbay.com/knowledge/nutrients/phosphorus.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.