ON ST Patrick’s day, millions of people around the world will wear a sprig of “shamrock”. The tradition dates back many centuries, and the small, three-leafed (or trefoil) plant is famously a symbol of Irishness. Yet to scientists, it’s all a bit of a sham because – whisper it – botanically speaking, there is no such species as shamrock.

This conundrum led to something of a detective hunt in the 1890s, and prompted one Dublin naturalist to chase after specimens around the country in a bid to uncover the plant’s identity. His hunt would reveal a surprising answer.

But our shamrock trail properly begins in 1681, when an English traveller, Thomas Dinely, published an account of his travels in Ireland. Dinely wrote that, on March 17th “ . . . the vulgar superstitiously wear shamroges . . . [A] three-leaved grass which they likewise eat to cause a sweet breath”.

It’s the earliest historical reference we have to the wearing of shamrock; all the older references are descriptions of shamrock as a green leaf that, like watercress, the “wild Irish” liked to eat.

For an explanation as to why people should wear shamrock on St Patrick’s Day, we have to wait nearly 50 years, and an account by another English man, with the wonderful name of Caleb Threlkeld.

Threlkeld was a dissenting minister and medical doctor from Cumberland, who had come to Dublin in 1713 to a parish in the Liberties. Today he is best remembered for compiling the first account of Irish native plants. His Synopsis Stirpium Hibernicarum,or A Short Treatise of Native Plants, was published in 1726, and helpfully includes each plant's English, Latin and Irish name, as well as any medicinal uses and interesting anecdotes.

And for white clover, he writes: “This plant is worn by the people in their hats on the 17th day of March yearly which is called St Patrick’s day. It being the current tradition that by this 3-leafed grass [Patrick] emblematically set forth the mystery to them of the Holy Trinity.

“However that be, when they wet their Seamar-oge, they often commit Excess in Liquor, which is not a right keeping of a Day to the Lord . . . generally leading to debauchery.”

According to Dr Matthew Jebb, the director of the National Botanic Gardens, this revealing note is the earliest written reference that explains St Patrick’s connection with shamrock and the Holy Trinity.

If shamrock was popular on hats in 1726, that was nothing to how popular it would become over the next century, sprouting up everywhere in art and sculpture, decorations and architecture, even on coins.

The harp may be Ireland’s official emblem, but the shamrock soon became the one most popularly associated with the country. By the early 1800s, however, scientists and linguists had begun to debate the very nature of shamrock.

There were, for example, fanciful linguistic suggestions that the name was originally Persian, but the word "shamrock" simply means "young clover" (seamair óg). However, three very different types of clover grow in Ireland, so which one is it? The small yellow-flowered Trifolium dubium? The larger, white-flowered T repens? Or the even bigger red-flowered T pratense?

Or perhaps, something else entirely? Seamsóg is the similar-sounding Irish name for wood sorrel ( Oxalis acetosella), sometimes known as sourgrass and – here's a clue – "false shamrock".

So, in the 1890s a Dublin naturalist set out to answer the question once and for all. Nathaniel Colgan began his detective work by writing to clergymen around the country, and asking for people to send him rooted samples of “shamrock” around the time of St Patrick’s Day.

According to Dr Jebb, Colgan received dozens of samples, all trefoil plants that looked much the same. The rooted specimens were planted out and Colgan waited patiently until they flowered in early summer, at which point they could be formally identified.

What Colgan found was that he had five very different species of plant, which were being used around the country as shamrock: the yellow, white and red clovers (in that order of popularity); also wood sorrel; and, a small herb called black medic ( Medicago lupulina), that resembles a cross between a clover and a small creeping buttercup. Intriguingly, there were regional differences: yellow clover was most common in the southeast, and white clover in the northwest.

According to Dr Jebb, the timing of St Patrick’s Day in early spring is a crucial factor in this botanical mix-up, as none of the five species will be in flower this early.

“With just their leaves they all look more or less the same. Actually, when people see the small yellow clover later in the year, they think it’s such an insignificant flower.” He believes that this explains why Ireland has no national flower, although we do have an (unofficial) national plant.

In 1988, nearly a century after Colgan’s detective work, another naturalist repeated the shamrock experiment. Dr Charles Nelson, who was then curator at the National Botanic Gardens, again asked people around the country to send in shamrock specimens.

Amazingly the same five species turned up – even though few people get their shamrock in the wild now, as they would have done in Colgan’s day – and the most popular was still yellow clover.

While there is no single "scientific" shamrock species, the Department of Agriculture had at some point to nominate an "official" one for commercial licences to companies that export shamrock. It chose the most popular species, the yellow clover ( T dubium) – something you can easily check, if you plant out one of the commercial living shamrock specimens and check the flower colour in summer.

This botanical puzzle also has implications for the timing of St Patrick’s Day. Back in the 1960s, the then government considered moving the national holiday to summer, in hope of better weather. Just as well they didn’t: there’s lots of clover in July, but you won’t find any “shamrock” then.

St Patrick’s poser: shamrock gels with growth medium

Question: What is the connection between disposable nappies, Barack Obama, and shamrock? Answer: a highly absorbent hydrogel that can absorb 500 times its weight in water. And the man responsible for making the connection is a UCD horticultural technician, Ray O'Haire.

Nearly 20 years ago, O’Haire was working on a research project with Prof Joe Morgan, looking for a way to grow shamrock for export – but without soil, yet in a way that would keep the plants alive.

The solution he hit on was to take a hydrogel, similar to the one used to make disposable nappies absorbent, and modify it so it could support a growing plant. The challenge, he told The Irish Times, was to add a full range of nutrients and find a way to convert the gel into a suitable growth medium for yellow clover. His “trade secret” solution was licensed to a small Athlone company, IPI Teoranta, which now sells and exports under the Living Shamrock brand. A bowl of its shamrock will be given to Barack Obama this year.

The company employs 16 people at its Kerry production site each year during the “shamrock season”, which starts when the seeds are planted in the autumn. This year the company expects to sell more than 200,000 units, according to marketing manager Peter Martin. Thanks to the gel, the plants can survive for up to a month in cold storage. Afterwards, they can be planted into soil or compost, and grown on like any conventional plant.

Mary Mulvihill is a science writer and broadcaster. Her MP3 guided tours of Dublin have a science twist and are available at ingeniousireland.ie