Comfrey is one of the most useful plants in the garden even though it barely belongs in the cutting garden — its value is as an accumulator, companion, and compost activator rather than as a cut flower. The deep taproot mines minerals from subsoil and brings them to the surface in leaves that compost or decompose rapidly. Chop and drop the foliage as mulch, or steep in water for a high-potassium liquid feed that dahlias and tomatoes respond to dramatically.
The flowers — small, nodding, lavender-to-pink bells — are attractive to bumblebees at a time when little else is available. The plant spreads by root and is very difficult to remove once established; plant it with intention, not casually. The cultivar 'Bocking 14' is sterile and does not self-seed, which is strongly recommended.
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.
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.
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.