Centaurea montana Blue
Perennial cornflower opens from a scaly bud that looks like a small artichoke, then unfurls into a ragged star of deep blue outer florets around a red-purple centre. Centaurea montana holds these on soft grey-green stems above broad, downy leaves, and it starts early and keeps going, flowering from October right through to February if the spent stems are cut away.
Easy colour over a long season makes this a cottage garden regular. It fills gaps in a mixed border, works in meadow-style planting and holds a bank together, and the blue sits well with soft yellows and silver foliage. Bees and bumblebees work the flowers hard in spring. The stems are usable in a vase, though they are short.
Plant in full sun or part shade in almost any free-draining soil, including chalky and stony ground. Two habits are worth knowing before you plant. It creeps outward on rhizomes to form a mat, and it seeds itself freely wherever seed is allowed to drop. Deadhead and lift stray growth each year, or plant it somewhere it can be left to colonise.
Reaches around 0.45 to 0.5 m tall, spreading 0.5 to 1 m and continuing outward.
Trimming: Shear the whole plant back hard after the first heavy flush, taking it almost to the ground. It regrows a clean mound of foliage within a few weeks and flowers again. Cut back fully in late autumn.
Deadheading: Deadhead regularly. This is the single most useful job on this plant, because it both extends flowering and stops the free self-seeding that otherwise puts cornflower through the whole border.
Fertilising: Little is needed. A light general fertiliser in early spring is enough, and feeding richer than that produces floppy stems and more leaf than flower.
Watering: Water through the first summer, then only in extended dry weather. Established plants are tough and cope on rainfall in most NZ districts.
Pest Control: Aphids gather on the buds and stems in spring and hose off. Slugs and snails may graze the soft new leaves after cutting back.
Disease Management: Powdery mildew commonly greys the foliage in midsummer, especially on plants left uncut. Shearing back hard after the first flush removes the affected leaves and solves it in one step.
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