Nutrient deficiency
Older leaves turn evenly pale green to yellow (nitrogen), get brown edges (potassium) or yellow between green veins (magnesium); with iron deficiency this happens on the youngest leaves. The plant grows slowly.
Poor or exhausted soil, compost that has not been refreshed for a long time, or a pH at which the plant can’t take up nutrients, such as chalky soil for rhododendrons.
What can you do?
- In the growing season, give houseplants a liquid feed every two to four weeks at the recommended dose.
- In the garden, work compost or well-rotted manure into the top layer in spring.
- Repot houseplants that have been in the same compost for years.
- If acid-loving plants such as rhododendrons or hydrangeas turn yellow, water with rainwater and use ericaceous compost or feed.
Prevention
Feed the soil with compost and mulch, and fertilise moderately but regularly; too much feed does more harm than good.