Geranium Blackleg — Why Stems Turn Black and What to Do

Blackleg is a name given to a group of stem-base rots in geraniums and pelargoniums, most commonly caused by the water mould Pythium or by certain bacterial pathogens. It is most frequently seen in recently taken cuttings but also affects established plants that have been overwatered or grown in poorly drained, contaminated compost. Once the black rot reaches the vascular tissue, it usually cannot be reversed — but healthy cuttings can be saved and future plants protected.

Identifying Blackleg

The tell-tale sign is a blackening, softening, and eventual collapse of the stem at or just below soil level. The rot is wet and may smell slightly unpleasant. It spreads upward rapidly, often causing the cutting or young plant to wilt and die within a matter of days. The soil surface around the affected plant may also look darker and wetter than elsewhere. Unlike botrytis, there is no surface mould — the rot is within the stem tissue itself.

Why It Happens — Root Causes

Blackleg nearly always follows a period of excessive moisture at the stem base. Overwatering, watering onto the stem rather than the compost surface, and using dense compost that retains water around the stem all create the perfect environment for Pythium to infect. Reusing compost without sterilisation introduces pathogens. Cuttings that were taken with too little stem tissue, or that were wounded or crushed during preparation, are far more susceptible than cleanly taken, appropriately sized cuttings.

Immediate Action for Affected Plants

Remove any plant or cutting showing blackleg symptoms immediately — there is no chemical cure once the rot is established within the stem. Bin the affected material, do not compost it. Check neighbouring plants and, if any show early symptoms (a slight darkening or softening at the stem base), remove and bin those too. Wash the tray, bench, or pot with a 10% bleach solution, rinse well, and allow to dry completely before using again.

Preventing Blackleg in Cuttings

Use only fresh, proprietary cutting compost — or a 50:50 mix of perlite and fresh multi-purpose compost. Fill pots or cells firmly but do not compress. Take cuttings cleanly with a sharp, sterile blade just below a leaf node. Allow the cut surface to callus for 30–60 minutes before inserting. Water the compost once at planting, then water only from below by standing the tray in water briefly — this keeps the stem base dry. Good airflow around cuttings is equally important: a slightly open propagator lid is better than a sealed one.

Reducing Risk in Established Container Plants

Established pelargoniums rarely suffer blackleg unless they are severely overwatered or planted into contaminated compost. Ensure your potting mix drains freely — add grit or perlite if needed — and never let the pot sit in standing water. If you are repotting, use only fresh compost, not compost that has previously hosted a diseased plant. Avoid watering onto the stem, and keep the stem base clear of mulch or compost that might trap moisture against the tissue.