Catmint

Catmint is the ultimate supporting act. Like a really brilliant bassist — you might not notice them first, but take them away and everything falls apart. 'Six Hills Giant' is the one I grow, and it's the backbone of every border I've ever planted that actually works.

The colour is a soft, hazy lavender-blue that sits beautifully with almost anything. Roses, obviously — the combination of catmint and roses is a cliche for good reason, because it genuinely looks like a painting. But it's equally good with alliums, with poppies, with grasses, with practically anything you put next to it. It's the most accommodating plant I know.

Brush against it and your fingers smell of something halfway between lavender and mint, with a warm, slightly medicinal undertone. It's not a conventional perfume-counter scent but I love it — it smells like a herb garden and a flower border had a very successful meeting. The grey-green foliage is lovely in its own right, and the whole plant has a soft, billowy quality that makes hard edges disappear.

Cut it hard after the first flush of flowers in July and it'll come back with a second wave by September. For a plant that costs about four pounds from a garden centre, that's extraordinary value. I've got a long drift of it along the front path, and from June onwards it turns my slightly scrappy garden into something that looks almost romantic.