Maidenhair syrup, or syrup of Maidenhair, also called capillaire, is a syrup made with the leaves of the maidenhair fern (Adiantum capillus Veneris).
In the 17th century, it was recommended by herbalist Nicholas Culpepper to treat coughs, jaundice, and kidney problems. The herb infusion was flavored with orange-flower water and honey, and eventually the fern become optional and capellaire referred simply to an orange-flower water syrup.
The syrup was called for in the original recipe for bavaroise, an 18th-century sweet tea drink, and was called for in some 19th century gin punch recipes. The syrup is still recognizable in the composition of the modern Portuguese syrup called Capilè.
Note that many species of ferns are poisonous or carcinogenic. Readers are cautioned that home preparation of capillaire syrup should be approached with great care.