Annual geranium (Pelargonium) is the zonal geranium of window boxes and porches: a tender perennial grown as an annual in cold-winter zones, with rounded clusters of red, pink, white or salmon flowers above scalloped leaves, often banded with a darker zone, from late spring until frost. It reaches 12 to 24 inches and tolerates heat better than most bedding plants.
Set plants out after the last spring frost, 12 to 18 inches apart, in full sun and soil that drains, or in a container with a good potting mix. Let the soil dry slightly between waterings; constant wet soil causes stem and root rot, and it is the usual way a geranium dies. Remove the spent flower heads at the base of the stalk and it blooms continuously. Feed lightly every few weeks in a pot.
Plants can be carried through winter indoors on a sunny windowsill, as cuttings taken in late summer, or as bare roots hung in a cool, dry cellar and potted up in late winter. The scented foliage keeps deer and rabbits off, and the only common trouble is botrytis on the flowers in a wet spell. It is native to South Africa; the hardy cranesbills, the true Geranium genus, are a different plant.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Damping Off
Seedlings collapse at the soil line with thin, water-soaked stems; seeds may fail to emerge or rot before sprouting.
Sow in well-draining mix, avoid overwatering, and ensure good airflow. Remove affected seedlings and let the surface dry between waterings.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.
Bacterial Blight of Geranium
Small water-soaked or brown spots with yellow halos on leaves, V-shaped yellow wedges along leaf margins, wilting, and blackened stems.
Remove and destroy infected plants; avoid overhead watering and handling wet foliage. Use clean tools and disease-free cuttings, and space plants for airflow. No chemical cure once infected.