Gentian

If you've never seen a gentian in flower, you haven't experienced blue. I don't say that lightly — I've grown delphiniums, meconopsis, agapanthus, every blue flower the British garden offers. Gentiana sino-ornata makes them all look like they're trying. This is blue at its absolute, undiluted, electric maximum. A trumpet-shaped flower, maybe two inches long, in a colour so intense it seems to generate its own light. Cobalt, ultramarine, something beyond both — a blue that doesn't exist in any paint chart.

The flowers appear in September and October, lying close to the ground on trailing stems, opening their trumpets to the low autumn sun. Each flower has fine stripes of darker blue and greenish-white running along the outside of the trumpet, which adds complexity to what could otherwise be overwhelming. From a distance, a patch of flowering gentian looks like someone has scattered fragments of autumn sky across the ground.

It's not the easiest plant. It wants acid soil — lime will kill it — and it wants moisture but not waterlogging. In practice, this means a peaty, humus-rich soil in a position that gets sun but doesn't bake dry. A rock garden, a raised bed with ericaceous compost, or the edge of a woodland garden where the soil is leafy and damp. It's worth the fuss.

I grow mine in a shallow stone trough with ericaceous compost and a top-dressing of fine grit. In October, when the rest of the garden is winding down into browns and golds, those impossible blue trumpets open and every visitor stops and stares. It's the most asked-about plant in my garden. Everyone wants to know what it is, and nobody believes me when I tell them it's easy.