Lemongrass is a tropical grass grown for its fragrant stalks — the essential herb of Thai and Vietnamese cooking. In Zone 6b it is grown as an annual, planted after last frost when the soil is fully warm. It grows large and fast in a hot summer but needs a long season to produce thick, usable stalks; start early indoors or buy transplants.
Harvest outer stalks by twisting them off at the base when they reach pencil thickness. The inner stalks and heart are used in cooking; the tougher outer leaves are used for tea and aromatics. Bring a pot inside to overwinter on a very sunny windowsill — it will slow dramatically but survive.
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.
Spider Mites
Fine stippling or bronzing on leaves, faint webbing on undersides, foliage drying out during hot, dry weather.
Spray foliage with a strong jet of water to dislodge mites. Apply insecticidal soap or horticultural oil to leaf undersides; repeat every 5–7 days as needed.
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.