On St Patrick’s Day, 17 March, Irish people and their overseas descendents traditionally wear bunches of shamrock, a practice first recorded in 1681. In 1726 Caleb Threlkeld noted the St Patrick used the three leaflets of shamrock ’emblematically to set forth to them [the Irish] the Mystery of the Holy Trinity’. Today Patrick is commonly depicted holding a shamrock in his right hand and trampling a serpent underfoot (he is reputed to have banished snakes from Ireland).
The name shamrock is thought to be deived from the Irish seamroge – ‘little clover’ – and first appeared in print, as shamrote, in 1571. This was a herb eaten by the Irish. It may have been wood sorrel (Oxalis acetosella), or perhaps watercress (Nasturtium officinale), but John Gerard, writing in 1597, identified it as a species of clover (Trifolium).
The potted shamrocks which are commonly sold in North America are usually species of Oxalis, often exotic species which are unknown in Ireland, and in 1992 a St Patrick’s Day card on sale at Dublin Airport depicted wood sorrel.
Although Matthias del L’Obel in 1570 mentioned that the Irish ate a cake made from ‘meadow trefoil’, there are very few records of clovers (trefoils) being eaten. Possibly the original shamrock was wood sorrel, which was (and by children still is) widely eaten, but it was too delicate for wearing on St Patrick’s Day, so more robust clovers were used as a substitute.
Towards the end of the nineteenth century Nathaniel Colgan in Ireland and James Britten in London investigated which species was then known as shamrock. They found that lesser trefoil (Trifolium dubium) was the plant most usually identified so, although some people people identified white (T. repens) or red (T. pratense) as such. About a century later Charles Nelson conducted a similar investigation, found that 46% of his correspondents considered lesser trefoil to be shamrock, and concluded that ‘little significant change has taken place during almost one century in the folk concept of shamrock’. In recent years the plant sold and worn on St Patrick’s Day in London has invariably been lesser trefoil.
It is said that shamrock never flowers. This belief can be explained by the fact that while lesser trefoil is compact and bright green in springtime, as the year progresses it tends to sprawl and become yellowish green, thus the springtime plants bear little resemblance to the summer ones.

For further information see:
E.C. Nelson, Shamrock: Botany and History of an Irish Myth, Aberystwyth, 1991.
R. Vickery, A Dictionary of Plant-lore, Oxford, 1995: 344-51.
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Images: main,postcard, posted Kentish Town, London, 17 March 1913; uppermost inset, Failte Ireland, National Tourism Development Authority, symbol, Dublin, November 2014; middle inset, Green Man pub, Euston Road, London Borough of Camden, February 2014; lowermost inset, shamrock (lesser trefoil) planted at supposed site of St Patrick’s Well, St Patrick’s Park, Dublin, April 2017.

Updated 17 April 2017.