Plant problems · Nutrients · Flowering · Act this week
Phosphorus deficiency
What you see
- Dark, dull or bluish green leaves with brown to purple patches, often on older leaves
- Leaf stems turn red or purple (red stems alone are often genetic)
- Spots look bronze and feel dry
- Buds develop slowly in flower
Why it happens
Phosphorus is mobile and the plant needs a lot of it in flower. Cold roots (below about 15 C), a pH above 7 in soil or a starved medium make it hard to take up.
What to do
- Check pH first and keep the root zone warm
- Feed a bloom nutrient with phosphorus at the label dose
- Do not add extra phosphorus on top of a full bloom feed: too much locks out zinc and iron
Next time
- Keep roots above 18 C
- Switch to bloom nutrients as flowering starts
Easy to confuse with
- Potassium deficiency: Leaf edges and tips turn yellow, then brown and crispy, starting on older leaves
- Cold stress: Leaves and stems turn purple (beyond what the strain normally shows)
- Septoria leaf spot: Round yellow spots on lower leaves that turn brown with a yellow halo
Not sure?
Take a photo of an older fan leaf with spots, and the leaf stem where it meets the branch and ask the Grow Doctor, or send it to the grow companion on Telegram.