During medieval times knights of the realm would wear their lady’s colours upon their left lapel to show their everlasting love and commitment, just as buttonholes reflecting the bride’s bouquet are worn by the groom today. The best man and ushers and page boys may wear buttonholes to complement the groom.
Groom’s buttonhole in image shows:
peony (white) – happy marriage
cineraria – you are my delight
astilbe – I’ll still be waiting