
Elderflower syrup is a sweet, non-alcoholic infusion of elderflowers in sugar and water, sweeter than a cordial and with little or no acid.
In a spritz or a lemonade it brings both the sugar and the flower, one bottle instead of two. Past a certain amount elderflower turns soapy, so add it gradually and taste. It goes with cucumber, gin and citrus, and a Hugo is built on it.
You will need more, since it is less sweet. The exact amount depends on the brand: a little more for Bottlegreen, nearly twice as much for Belvoir.
St-Germain and other elderflower liqueurs
They carry about half the sweetness of the syrup, so even out the balance with plain sugar syrup. The liqueur also adds alcohol: keep that in mind if the drink is meant to be alcohol-free.
Use the same amount. The drink keeps its balance of sweetness but loses the floral aroma entirely.
Elderberry syrup
Will not do. Elderflower syrup is made from the blossoms, this one is boiled from the berries. It gives the drink a dense, jammy flavor with none of the floral lightness.
A sealed bottle keeps for months; once opened it belongs in the fridge. Homemade elderflower syrup lasts a couple of weeks refrigerated. If you are making it, strip the blossom off the green stalks first. The stalks are bitter, and the green parts of the plant are not for infusing.