The leek is the national symbol of Wales, with Shakespeare even nodding to the ancient custom when Henry V says, wearing a leek, “for I am Welsh, you know, good countryman.” The leek is found throughout Welsh lore, including in battle—14th-century soldiers adopted green and white uniforms, and a key fight against Saxon invaders supposedly took place in a leek field.
Per Wales Online, Queen Elizabeth II likely passed the brooch to King Charles when, as Prince of Wales, he succeeded his father, Prince Philip, as Colonel of the Welsh Guards in 1975. Princess Diana was also seen wearing the brooch; it is now a favorite of Queen Camilla for Welsh events.
