A basil leaf is summer in a glass: one large, glossy leaf slapped and floated on a Gin Basil Smash doubles the aroma of the muddled leaves beneath it and turns a green gin sour into something that smells of a Genoese kitchen. It bruises black if you look at it wrong, so the rule is gentle hands and the freshest leaf on the plant.

What is a basil leaf, as a garnish?

A single fresh leaf of sweet (Genovese) basil, the large-leaved Italian kind, laid on the surface of a drink or tucked against the ice, where its oils (linalool, estragole, eugenol) give a sweet, peppery, faintly anise aroma to every sip. Basil's bar career began in 2008, when Jörg Meyer at Le Lion in Hamburg muddled a handful into a gin sour and called it the Gin Basil Smash; within a few years it was on menus everywhere and basil had joined mint as a cocktail herb. It suits gin above all (the two share floral and peppery notes), vodka, white rum, tequila, lemon and lime, strawberries, watermelon, peach, cucumber and tomato, and it belongs in summer. Thai basil is more anise-scented and suits rum and tropical fruit; purple basil is beautiful and milder; lemon basil is what it sounds like. Basil oxidises faster than any other herb: a bruised or cut edge turns black within minutes, and a leaf left in a drink too long goes limp, so it is a garnish to place at the last second. The leaf's job is aroma; the muddled leaves in the drink do the flavouring.

How to use it

Pick the largest, most perfect leaf from the top of a bunch, rinse it gently and pat it dry. Just before serving, lay it on the palm of one hand and slap it once with the other, which releases the oil without tearing the leaf. Float it on the surface of the drink, shiny side up, or stand it against the inside of the glass on the ice so it shows through. One leaf, or three small ones in a fan. For a Smash, the garnish leaf goes on after the muddled drink is strained over fresh ice. Do not cut or tear basil for garnish; the edges blacken. Keep a bunch like flowers, stems in water at room temperature (the fridge blackens it), and use it within two or three days. A basil leaf frozen into an ice cube is a pretty, if short-lived, variation.

Substitutes

A mint sprig, which is the obvious cousin and keeps better. Thai basil, for a rum or tropical drink. A sage leaf, earthier. A shiso leaf, which is basil's Japanese relative and dramatic on a clear drink. A cucumber ribbon, for a different kind of green freshness in a gin drink.

Cocktails with a basil leaf

The modern classic: the Gin Basil Smash. Every drink that takes a basil leaf is listed below, and any strawberry, watermelon or cucumber drink on this site would take one.

Questions

Why does my basil garnish go black? Bruising and cold. Slap it once, do not tear it, place it last, and keep the bunch out of the fridge.

Genovese or Thai basil? Genovese for gin and citrus; Thai for rum and tropical fruit, where the anise note suits.

How do I keep basil fresh? Stems in a glass of water at room temperature, like flowers, for two or three days.

Can I muddle the garnish leaf? Muddle separate leaves for the drink; the garnish leaf is slapped and floated, not muddled.

Does basil go with vodka? Yes, especially with strawberry, watermelon or lemon; a Vodka Basil Smash is a gentler version of the gin one.