Monkshood

Monkshood is the flower I never entirely trust, which is part of why I love it. Every part of this plant is seriously poisonous — roots, leaves, flowers, the lot. Aconitum has been used as an arrow poison, a wolf poison, and, according to several grim historical accounts, a murder weapon. It's the most toxic plant commonly grown in British gardens. And it's absolutely gorgeous.

'Spark's Variety' flowers late — September and October, when most blues in the garden have packed up — and the colour is a deep, saturated indigo that borders on navy. The hooded flowers are arranged on tall, branching spikes, and they have a Gothic, architectural quality that nothing else in the autumn border matches. They look like they belong in a medieval illuminated manuscript.

I grow them at the back of a border with Japanese anemones and thalictrum, and in late September that combination — the dark indigo spires behind the pale pink anemone cups — is one of the best things my garden does all year. The colour deepens in overcast light, becoming almost black-blue, which gives the whole border a moody, atmospheric quality.

A word of caution: always wear gloves when handling monkshood. The toxins can be absorbed through the skin, and this isn't one of those plants where the danger is theoretical. It's real. I find that edge of danger oddly compelling — the beauty and the threat existing in the same plant, inseparable. The garden isn't always safe, and I think that's part of what makes it interesting.