When I was younger I worked (sometimes) in the garden of our posh near neighbours. They had a walled garden, and a flower garden, and a garden with a swimming pool, and a orchard with three grumpy geese and a big beech hedge (they used to pay me to cut that, with hand shears, which might have been what got me into hand-cutting hedges). They also had a peach house, against one of their walls, and although I only seldom went in it (just to pick up or return tools) I can remember the smell with exact precision. It didn't smell of peaches. It smelt of geraniums*.
Their geraniums were spidery things; small red flowers and deeply cut leaves, with a sharp musky fragrance that seemed more animal than vegetable. As if set by that first experience, any greenhouse, growhouse, garden shelter or cold frame I am using, big or small, temporary or permanent, doesn't seem finished without a geranium. I buy them from the bedding tables at the garden centre, picking the brightest and brashest plants. In the greenhouse they last a few years, and as the plants get older and weaker, their colour seems to intensify, until they glow against the green.
*More correctly and usually nowadays called Pelargoniums, to differentiate them from perennial border Geraniums.