Lemongrass is a tropical grass grown for its fragrant stalks, the essential herb of Thai and Vietnamese cooking, and the leaves make a lemon tea. It forms a fountain of arching blades 3 to 5 feet tall in a hot summer and is a perennial only in zones 9 to 11; elsewhere it is grown as an annual, planted after last frost when the soil is fully warm, or kept in a pot.
It needs a long season to make thick, usable stalks, so start early: buy transplants, or root a few stalks from the grocery store in a glass of water in late winter and pot them up. Set plants out 24 to 36 inches apart in full sun and rich, loamy soil, and keep them well watered and fed; drought slows it to a stop. A single plant in a 5-gallon pot does well on a sunny patio.
Harvest the outer stalks by twisting them off at the base once they reach pencil thickness, using the pale lower stem in cooking and the leaves for tea. Before frost, dig and pot a division, cut it back to a foot, and overwinter it on the sunniest window; it slows dramatically but survives. The sharp-edged leaves cut skin, so wear gloves. Deer, rabbits and mosquitoes keep their distance, the last one less reliably than the marketing suggests.
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.
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.
Spider Mites
Tetranychus urticae →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.