The four-leaf clover is an uncommon variation of the common, three-leaved clover. According to tradition, such leaves bring good luck to their finders, especially if found accidentally. In addition, each leaf is believed to represent something: the first is for faith, the second is for hope, the third is for love, and the fourth is for luck.
It has been estimated that there are approximately 10,000 three-leaf clovers for every four-leaf clover; however, this probability has not deterred collectors who have reached records as high as 160,000 four-leaf clovers.
Clovers can have more than four leaves: The most leaves ever found on a single clover stem (Trifolium repens L.) is 56 and was discovered by Shigeo Obara of Hanamaki City, Iwate, Japan, on 10 May 2009. Five-leaf clovers are less commonly found naturally than four-leaf clovers; however, they, too, have been successfully cultivated. Traditionally children have been told that a five-leaved clover is even luckier than a four-leaved one. Some four-leaf clover collectors, particularly in Ireland, regard the five-leaf clover, known as a rose clover, as a particular prize.
It is debated whether the fourth leaflet is caused genetically or environmentally. Its relative rarity suggests a possible recessive gene appearing at a low frequency. Alternatively, four-leaf clovers could be caused by somatic mutation or a developmental error of environmental causes. They could also be caused by the interaction of several genes that happen to segregate in the individual plant. It is possible all four explanations could apply to individual cases.
Researchers from the University of Georgia have reported finding the gene that turns ordinary three-leaf clovers into the coveted four-leaf types. Masked by the three-leaf gene and strongly influenced by environmental condition, molecular markers now make it possible to detect the presence of the gene for four-leaves and for breeders to work with it. The results of the study, which also located two other leaf traits in the white-clover genome, were reported in the July/August 2010 edition of Crop Science, published by the Crop Science Society of America.
The other leaf traits, the red fleck mark and red midrib, a herringbone pattern that runs down the center of each leaflet in a bold red color, were mapped to nearby locations, resolving a century-old question as to whether these leaf traits were controlled by one gene or two separate genes.
White clover has many genes that affect leaf color and shape, and the three in the study were very rare. These traits can be strikingly beautiful, particularly if combined with others, and can turn clover into an ornamental plant for use in flower beds.
There are reports of farms in the US which specialize in four-leaf clovers, producing as many as 10,000 a day (to be sealed in plastic as "lucky charms") by feeding a secret, genetically engineered ingredient to the plants to encourage the aberration (there are, however, widely available cultivars that regularly produce leaves with multiple leaflets – see below). 
There are some cultivars of white clover (Trifolium repens) which regularly produce more than three leaflets, including purple-leaved T. repens 'Purpurascens Quadrifolium' and green-leaved T. repens 'Quadrifolium'.
Trifolium repens 'Good Luck' is a cultivar which has three, four, or five green, dark-centered leaflets per leaf.
Other plants may be mistaken for, or misleadingly sold as, "four-leaf clovers"; for example, Oxalis tetraphylla is a species of wood sorrel with leaves resembling a four-leaf clover. Other species that have been sold as "four-leaf clovers" include Marsilea quadrifolia.
- "I'm Looking Over a Four Leaf Clover"
- Rabbit's foot
- Raskovnik, a fictional plant in Bulgarian mythology with similar properties and appearance
- Child, Madge (letter); and Dodge, Mary Mapes (magazine editor) (1877), Letter entitled "Four-Leaved Clovers", St. Nicholas (Scribner's Illustrated Magazine for Girls and Boys), Volume 4 (November 1876- November 1877), Scribner & Co., New York, New York, U.S.A., pp. 634-5, in the subsection of letters called "Jack-in-the-Pulpit", July 1877.
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- Mabey, Richard, Flora Britannica, p. 225 (citing Edward and Helene Wenis of Leonia, New Jersey, U.S., writing in BSBI News, 56, 1990)
- Tashiro, R.M., et al. Leaf Trait Coloration in White Clover and Molecular Mapping of the Red Midrib and Leaflet Number Traits. Crop Science 7 June 2010.
- Mabey, Richard, Ibid, p. 225
- Lord, Tony (ed), RHS Plant Finder 2006–2007, (20th edition), Dorling Kindersley, London, 2006, p. 743. ISBN 1-4053-1455-9
- Killerplants.com (photo)
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- Good Luck Plant Kit. FourLeafClover.com. Retrieved 7 December 2008.
- All About Shamrocks Four-Leaf Clovers. LollySmith.com. Retrieved 7 December 2008.
- Keenan, Susan M. The Four Leaf Clover. HotFact.com. 11 March 2008.