French tarragon — not to be confused with the seedy, inferior Russian tarragon — is propagated only from cuttings or division, never from seed. The anise-forward flavour is the defining herb of French cuisine, essential in béarnaise and fines herbes. Plant in full sun in very well-draining soil; it rots quickly in wet, heavy ground. Cut back by half in midsummer to encourage a flush of tender new growth.
Divide clumps every 2–3 years to maintain vigour; tarragon gradually loses potency if left undivided. In Zone 6b it is reliably perennial with good drainage and light mulch.
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.
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.
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.