A Confusing Quintet

Yellow clover (1a)
Scrambling yellow clover on wall

St. Patrick arrived to spread Christianity throughout the island in 432 A.D. and it is said that he used the shamrock as a metaphor for the holy Trinity. But which one?  There are five contenders for the title.
The word “Shamrock” itself is derived from two Irish words, Seamair Óg, which mean young clover. If you are wearing the shamrock today you might be sporting Yellow clover or Lesser trefoil (Trifolium dubium) or Seamair bhuí in Irish. You’re probably on the right track with this one because this is believed by about half the population to be the true shamrock. (A trefoil has leaves divided into three leaflets). It is common on roadsides, old walls and pastures. It has a creeping habit and is sold commercially as “shamrock” (1).

White clover

White clover (Trifolium repens) or Seamair bhán in Irish also lays claim to the title with many botanists and about one third of the population thinking this is the true shamrock.  This creeping plant is common in grassland and lawns. It has curved white v shapes on each leaflet.  It is an important agricultural plant (2).

Red clover
Red clover

 Red clover (Trifolium pratense) or Seamair dhearg in Irish is used as shamrock too.  This is common in grassland.  It was once used extensively in agriculture because it fixes nitrogen in the soil (3a; 3b).

Black medick
Close up of Black medick

Black medick (Medicap lupulina) or Dúmheidic in Irish is also used as shamrock.  This is common on dry grassy areas and waste ground. It is easily distinguished from similar looking species by examining a leaflet. In each one there is a notch at the end with a tiny tooth (4a; 4b) 

Wood sorrel (Oxalis Acetosella) or Seamsóg in Irish is also a contender. It grows in woodlands and shady places and produces white flowers.  The leaves were eaten and used in salads.  However, they should be eaten with caution because they do contain small amounts of a poison (5).