Valerian is a tall perennial grown for its clusters of small pink-white flowers and its aromatic root, long used in herbal medicine. It draws bees, hoverflies, and other beneficial insects, making it useful at the back of a border or near a vegetable garden.
It prefers moist, fertile soil and tolerates partial shade better than most flowering perennials. Plants self-seed readily and spread by short rhizomes, so deadhead spent flowers if you want to control spread. Cut stems back after bloom to keep the plant upright and tidy. Roots are harvested in fall of the second year.
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.
Aphids
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.
Valerian Leaf Spot
Brown to dark purple spots on lower leaves, sometimes with yellow halos; spots enlarge and merge in wet weather, causing leaves to wither.
Remove and destroy affected leaves. Improve air circulation by thinning and spacing plants. Water at the base to keep foliage dry. Apply a copper-based fungicide if spread continues.