When King Henry says, in Shakespeare’s “Henry V,” that he will wear a leek in his cap, he is identifying himself with his Welsh subjects. The reason this created a connection is that, when King Cadwallader and the Welsh faced the Saxons in battle in AD 640, the Welsh soldiers identified themselves by wearing leeks in their caps. Leeks were so closely associated with the Welsh, that a leek appears on some Welsh coins. It is to the Welsh what the thistle is to the Scottish.
Leeks have been consumed for so long in Wales and other Celtic countries that some hypothesize that the British Isles are a possible point of origin. Other scholars say the Mediterranean is where they emerged. They were popular in Egypt and ancient Rome (Nero’s nickname was “leekeater”), the Chinese were praising leeks by 1500 BC, and leeks were being written about in Mesopotamia as early as 2100 BC. So wherever they started, they clearly were popular and on the move pretty much from the get-go.
If leeks did start in the Mediterranean, rather than colder climes, as some hypothesize, then the Romans would have brought them to Britain when they invaded in 43 AD. So Britain has had them for a couple millennia, even if leeks didn’t actually start out there.
Though the Welsh are most commonly associated with leeks, Ireland being a Celtic country, has also long been a hotbed of leek consumption. In fact, there is even a story about St. Patrick and leeks (he is said to have created them out of rushes). However, it is thought that they may not have made it into the Scottish culinary repertoire until the Scots and the French became allies in 1295 — because the Scots succeeded in keeping Rome out of Scotland. When the Auld Alliance was signed (thanks in part to William Wallace, for those of you who remember him from the movie Braveheart), the French and Scots began exchanging ideas, and leeks got included in the catalog of things Scotland adopted from France.
Europe has long valued leeks — though, ironically, leeks have also long been associated with poverty. Leeks appear in a large number of local and regional specialties, and are insulted with almost the same frequency as they appear. They are sometimes called “poor man’s asparagus.” The French word for leek, poireau, is also slang for “simpleton.” And there is a saying in Italian that something is “not worth a leek leaf.” And yet, they keep showing up on the menu.
Actually, comparing the leek to asparagus is not as far-fetched as it may sound. Both are members of the lily family. And leeks are grown rather like white asparagus is grown — and for the same reason. Leeks are kept white by mounding dirt up around them as they grow. This is also why leeks have to be washed carefully, as leaves have to push up through mounds of dirt and sand, and they tend to accumulate grit (and the more white you see, the more likely there is to be soil of some sort—if there’s only an inch of white, you may not see much dirt between the layers).