I've never used costmary, but given its characteristics I shall look out for it.
Costmary has now become naturalised in many parts of Southern Europe. It became popular in more Northern latitudes in the Middle Ages, when it was grown in monasteries and Imperial gardens in accordance with Charlemagne's Capitulare de Villis. The plant was introduced into Britain in the 16th century and was soon found in almost every garden, but it has now gone so completely out of favour as to have become a rarity.
Asteraceae (daisy family).
The whole of this plant emits a soft balsamic odour, reminiscent of tansy but pleasanter and more aromatic.
The common English name "costmary"(Middle English costmarie) derives from the Old English cost, from Latin costum and Greek kostos "costusroot" (a root used as a spice and preserve) and Marie "the Virgin Mary", in biblical reference. In the Middle Ages, the plant was widely associated with the name Mary and was known in France as herbe Sainte-Marie.
Genus name tanacetum refers to the family relationship with tansy and species balsamita derives from Latin balsamum "balsam tree", originating from Old Hebrew bōshem denoting the balsam tree, but also meaning "fragrance" or "spice" in general. The plant has sometimes been erroneously called "mace" in English, this term being reserved for a spice derived from the nutmeg tree.
On account of its aroma and the taste of its leaves, costmary was much used to give a spicy flavouring to ale (before being superseded by hops), a practice which gave rise to the English alternative name "alecost". The fresh leaves were also used in salads and in pottage and dried leaves were often included in pot-pourri, as they retain their aroma well. In an earlier age bundles of costmary were tied up with lavender and used as a domestic air freshener.
Costmary was at one time employed medicinally, having somewhat astringent and antiseptic properties and use in treating dysentery. An ointment made by boiling the herb in olive oil with the woodland flower erythronium albidum "trout lily" and thickening the strained liquid with wax, resin and turpentine was considered to be very valuable for application to sores and ulcers.
Costmary is closely related to tansy.