Valerian is a tall perennial, 3 to 5 feet, grown for its domed clusters of small pink-white flowers in early summer and the sweet, slightly musky scent they carry on the air. The root has a long history in herbalism, which is what most people know the name from, but in the garden it is a back-of-border plant that draws bees, hoverflies and other beneficial insects.
It prefers moist, fertile soil and tolerates part shade better than most flowering perennials. Sow in spring or set out divisions 24 inches apart, and cut the stems back after bloom to keep the plant upright and tidy. It self-seeds readily and spreads by short rhizomes, so deadhead the spent heads if you want to hold it in place; it can naturalise in damp meadows and ditches.
Cats are drawn to the roots as they are to catnip and will dig at a young plant, so protect it the first season. The flowers are good for cutting and last a week. Deer leave it alone. It is native to Europe and western Asia 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.
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.
Fungal leaf spot
Brown or purplish spots on the leaves, often ringed with a yellow halo, that spread and merge in warm, humid, or wet weather; heavily spotted leaves yellow and drop early.
Pick off spotted leaves and clear fallen debris to remove overwintering spores. Improve air circulation by thinning, and water at the base so foliage dries quickly. If it persists, apply a copper- or sulfur-based fungicide at the first sign.
Aphids
Aphididae →Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.