You can wear a trefoil on St. Paddy’s Day, but do you know which plant it comes from? In the 1890s, a naturalist found “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.”