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.