St Patrick's Day: Why is the US president given a bowl of shamrock?

There are few things as enduringly Irish as the simple shamrock.

Derived from the Gaelic seamróg (young clover), the three-leaf plant has had a symbolic purpose since the 18th Century, decorating coins, medals and coat lapels of the great and good at home and abroad.

Traditionally the plant was said to have been used by St Patrick to illustrate the Christian Holy Trinity.

But how did the US president ending up being presented with a bowl of them?

Every year the taoiseach (Irish prime minister) makes a trip to the US and has an audience with the president at the White House.

The plant first made its way into the White House in 1952 with the Irish Ambassador to the US John Hearne delivering a box for President Harry Truman.

Irish-American relations are said to have soured over the Republic of Ireland's neutrality in World War Two and the move was seen as Mr Hearne attempting to warm the relationship.

As small as the gesture was it appeared to work, with Mr Truman later sending a message to say he hoped "relations between the two countries will continue to be on a good and effective level for generations".

The ceremony grew the next year with the introduction of a cut-glass bowl supplied by Waterford Crystal, presented to the new President Dwight D Eisenhower.

The tradition has continued in one form or another ever since, giving the Republic of Ireland enviable access to the US president every March.

Of course the US president is not the only person who will be wearing a shamrock on St Patrick's Day.

Among the many others are soldiers in the Irish regiments of the British Army, including the Irish Guards and the Royal Irish Regiment.

The tradition began more than 100 years ago, when Queen Victoria decreed that Irish regiments wear a sprig of shamrock in honour of the soldiers who fought in South Africa during the Second Boer War.

In a telegram dated 28 February 1900, she said: "I have heard with the deepest concern of the heavy losses sustained by my brave Irish soldiers.

"I desire to express my sympathy and my admiration of the splendid fighting qualities which they have exhibited throughout these trying operations. R.V.I."

Queen Alexandra, the consort of King Edward VII, began the custom of giving out the shamrocks in 1901.

All of the shamrock sent abroad from Ireland comes from a single glasshouse, Irish Plants International at Cools, near Baile an Sceilg in County Kerry.

Father-and-son team Joe and Cian Sugrue told Irish national broadcaster RTÉ they would be flying shamrock this year to Irish Defence Forces personnel in Belgium, Austria, the Balkans, the Middle East and Africa.

High-profile clients will include the Princess of Wales, who will be presenting the plants to the Irish Guards.

"We also have the contract to make a special brooch of shamrock for her - those are the ones you would be very proud of," said Mr Sugrue.