Lemon balm is a perennial herb in the mint family grown for its lemon-scented leaves, used fresh in tea, in fruit salads and desserts, and rubbed on the skin as a folk insect repellent. It forms a bushy clump 18 to 24 inches tall and attracts bees when it flowers; the Latin name Melissa is the Greek for honeybee, and beekeepers once rubbed hives with it. Where winters freeze it dies back and returns reliably from the roots.
Like other mints it spreads, though by seed far more than by runner. Cut the plant back hard before it flowers, in early summer, to stop the seeding and to bring a flush of fresh leaves, and cut it again in late summer. It takes part shade and poor soil, but the best flavour comes from regular harvesting. Plant 18 to 24 inches apart; a single plant is usually enough.
The leaves lose most of their scent when dried, so use them fresh or freeze them in ice cubes. Deer and rabbits leave it alone. If you let it flower, the small white blooms are busy with bees and hoverflies through midsummer. It is native to southern Europe and hardy in zones 4 to 9.
Powdery mildew
White or grey powdery coating on leaves - usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Rust
Orange to brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infections cause leaves to yellow and drop.
Remove and destroy infected leaves. Avoid overhead watering and improve air circulation. Apply a sulfur or copper-based fungicide if it spreads. Clear plant debris in fall.
Septoria Leaf Spot
Small tan to brown spots with dark borders on leaves, often with tiny black specks at the center. Spots merge and lower leaves yellow and die.
Remove infected leaves and avoid overhead watering. Space plants for airflow and rotate planting location each year. Apply a copper-based fungicide if spread continues.