Iron toxicity in geraniums; Photo: Syngenta Flowers
Bronze speckling, necrotic spots, and leaf edge necrosis on the bottom leaves of geraniums are caused by excessive iron and manganese in the leaves. This occurs when the pH drops to below 5.5 and iron and/or manganese are taken up in excess by the plant. Some varieties of geranium are more susceptible than others.
The team at Syngenta Flowers offers several tips for preventing and treating iron toxicity in geraniums, including:
- Leach the media well with clear water.
- Raise the pH by drenching the media with Limestone F at 16-32 fl. oz/100 gal or top-dress the media evenly using a small amount of powdered calcitic or dolomitic limestone.
- Alternatively, use potassium bicarbonate (KHCO3) drenches at rates starting from 1-1.5 lbs/100 gal.
- Return to regular fertilizers once the pH is adjusted. Continue to monitor pH with PourThrus. You can reference this guide from North Carolina State University on the PourThru method.
- Keep the pH between 6.0-6.5 — monitor the pH regularly. Low pH can cause iron and manganese toxicity, which is a common problem during the mid to late stages of production.
- Avoid continuous use of very acidic fertilizers (20–20–20, others). Adjust pH as needed with limestone additions or basic fertilizers and avoid excess micronutrient applications.
- Observe indicator varieties, like ‘Rocky Mountain Dark Red’ and ‘Americana Bright Red’, for low pH-induced iron toxicity.
- Optionally, use a custom high-lime potting mix made for your zonal geranium production to avoid low pH problems.
Learn more in these culture guides from Syngenta Flowers.
