How do I fix phosphorus deficiency on cannabis?

Older leaves develop dark green to purplish discoloration, especially on the undersides. Growth is stunted, root development is poor, and flowering may be delayed. Warm the nutrient solution to 18–21 °C (official chart). Apply a P-K booster during transition and early flower. Verify pH is in optimal range.

Visual Guide

Phosphorus Deficiency.

Warm the nutrient solution to 18–21 °C (official chart)

← All problems

Dark green or purplish leaves, especially undersides and stems

Photo

Deficiencies

Older leaves develop dark green to purplish discoloration, especially on the undersides. Growth is stunted, root development is poor, and flowering may be delayed.

Looks like this

Dark green or purplish leaves, especially undersides and stems
Stunted growth and weak root development
Delayed or reduced flowering

Solve it with Vitaponix

VitaBulk →

Replaces VitaBase from week 7 at 5 ml/L; steers the plant towards heavier flowering.

How to fix it

Warm the nutrient solution to 18–21 °C (official chart)
Apply a P-K booster during transition and early flower
Verify pH is in optimal range

Why it happens

Cold root zone temperatures (<15°C) limiting uptake
Low pH or alkaline substrate
Insufficient P in the feed during transition and flower

Prevent it

Keep root temperatures stable and warm
Use a flowering booster from the start of transition
Maintain pH at 5.8–6.3 (official chart)

Official feeding chart

Target pH (official chart): 5.8 – 6.3
Water temperature: 18 – 21 °C

Doses and targets come from the official Vitaponix feeding chart. Vitaponix nutrients are plant nutrition, not pesticides or fungicides.

Prevent it next time

Get the full 10-week feeding schedule.

Open the feeding chart Shop Vitaponix →