The origin of the idea:
In ancient times and mythology, the olive tree was considered sacred (in Greece) to the goddess Athena – a symbol of peace, wisdom and constancy. A wish made on an olive tree was considered a wish made to time itself.
In Italy (especially Rome), plants were placed with offerings at new beginnings (marriages, births, alliances) and the wishes were handed over to the earth to be nourished by Tellus, the earth goddess.
The Celts regarded trees as portals between worlds and buried their wishes during the new moon. The growth of the tree was seen as a sign of whether the wish had been granted.
In rural areas of Central Europe, notes with requests were placed under fruit trees. This was particularly common for weddings or wishes for children, as the tree ‘carries’ the wish with its fruit.