Globe thistle is one of the most striking plants available to the cutting garden: perfectly spherical steel-blue flower heads the size of golf balls on tall, branching stems, 3 to 4 feet, above deeply cut, silver-backed leaves. The blue is intense and unlike anything else, and bees and butterflies work the spheres constantly through midsummer. It is also one of the easiest perennials in difficult conditions: drought, poor soil and neglect do not bother it. 'Veitch's Blue' is the deepest-coloured cultivar.
Plant it 24 to 30 inches apart in full sun and poor to average soil that drains; rich ground makes it flop and wet ground rots the taproot. Water until it is established and then not at all. Cut the spent stems to the basal leaves for a smaller second flush, or leave the heads for the goldfinches and for winter. It self-sows moderately and is long-lived; divide only when the clump opens, and expect it to resent it.
Cut for the vase when the spheres show full blue but before the tiny florets open; at that stage they last 7 to 10 days. For drying, cut a little earlier and hang upside down; the spheres hold their shape and colour perfectly. The spiny leaves keep deer and rabbits off. Native to southern Europe; hardy in zones 3 to 9.
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.
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.
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.