Clover is the Trifolium genus of plant that includes around 300 annual and perennial species in the pea family (Fabaceae). Clover is found in most temperate and subtropical regions, except Southeast Asia and Australia. Clover is used as livestock feed, as a cover crop, and as green manure due to beneficial effects to the soil and subsequent crops. Flowers of clover are attractive to bees, and clover honey is a common secondary product of clover cultivation.
Clover was first domesticated around the year 1000 and spread throughout Europe as the chief provider of atmospheric nitrogen for cereals grown for an expanding population. As a feed for cattle, clover played a part in increasing cattle populations. Clover was a major contributor to the Agricultural Revolution, which began in Europe in the 17th century, which is thought to have been a nitrogen revolution. Clover solved the problem of nitrogen depletion. Domesticated clover, a plant that was better than other nitrogen-fixing plants at the time, expanded over Europe. Clover is thought to have first been domesticated in Moorish Andalusia, an important center of agricultural innovation.
Clover and other nitrogen-fixing plants have a symbiotic relationship with rhizobia, soil bacteria. Rhizobium infects the roots of legumes and form root nodules where nitrogen fixation occurs. The Rhizobium supplies the enzyme nitrogenase, a catalyst for the reaction where nitrogen from the air (N2) is split yielding ammonia (NH3), energy and electrons. NH3 is used by plants to produce the nitrogenous biomolecules they need to survive and grow.
According to legend, St. Patrick, the patron saint of Ireland, used the three-leaf clover to explain the Holy Trinity— the Father, Son and Holy Ghost—in the conversion of the Irish to Christianity in the fourth century A.D. During the Middle Ages, shamrocks began to be used as floral emblems and worn on hats, and by the 1820s shamrocks were used to indicate an Irish connection.
Shamrock is assumed to be a type of clover. The term shamrock is derived from the Gaelic word seamrog, or “little clover.” The term was first used in plays and poetry in the 1500s and was first connected to a recognizable plant, the meadow trefoil known as clover, by English herbalist John Gerard in 1596. Trifolium dubium is the species sold most often as shamrock by commercial growers in Ireland or in seed packets labeled as true shamrock.