They are smallannual,biennial, or short-livedperennialherbaceous plants, typically growing up to 30 cm (12 in) tall. Theleaves aretrifoliate (rarely, they have more or fewer than three leaflets; the more (or fewer) leaflets the leaf has, the rarer it is; seefour-leaf clover), withstipulesadnate to the leaf-stalk, and heads or dense spikes of small red, purple, white, or yellow flowers; the small, few-seeded pods are enclosed in thecalyx.[3] Other closely related genera often called clovers includeMelilotus (sweet clover) andMedicago (alfalfa orCalvary clover).
Aslegumes, cloversfix nitrogen using symbiotic bacteria in theirroot nodules, and are used as an alternative or supplement to synthetic nitrogenfertilizers. They are also valuable food source for grazing livestock and bees. The domestication of clover caused substantial increases in agricultural productivity.
A clover with a dewdrop in the middleColorful flowers of clovers besideZarivar Lake inIranLesser hop trefoilOwl-head cloverWhite clover
Clover was first domesticated in Spain in around the year 1000. During European urbanization,crop rotations involving clover became essential for replacing thefixed nitrogen exported to cities as food.[4] Increased soil nitrogen levels from the spreading use of clover were one of the main reasons why European agricultural production in 1880 was about 275% of the production in 1750.[5] Fields of clover, used asforage and newly-inventedsilage, became an important part of the rural landscape;[4] adding clover made livestock feed more nutritious. Honey production also rose drastically, and clover remained the main nectar source for bees until the mid-twentieth century. Clover was carried around the world as a crop byEuropean colonists, and some clover species becameinvasive in some areas.[5]
Imports ofguano[5] and the development of theHaber-Bosch process in the 20th century substantially displaced clover as a crop, with negative effects on pollinators,[4] but in the 1990s and 2010s, the cost of industrially-fixed nitrogen rose substantially, approximately doubling, and reviving interest in forage mixes that include clover. As the fixation process is energy-intensive, prices are closely tied to energy prices. The 21st century also saw interest in clover as a countermeasure to fight the globalpollinator decline.[5]
Several species of clover are extensively cultivated asfodder plants. The most widely cultivated clovers arewhite clover,Trifolium repens, andred clover,Trifolium pratense. Clover, either sown alone or in mixture withryegrass, has for a long time formed a staple crop for silaging, for several reasons: it grows freely, shooting up again after repeated mowings; it produces an abundant crop; it is palatable to and nutritious forlivestock; itfixes nitrogen using symbiotic bacteria in itsroot nodules, reducing the need for syntheticfertilizers; it grows in a great range ofsoils andclimates; and it is appropriate for either pasturage orgreen composting.[3]
In many areas, particularly onacidic soil, clover is short-lived because of a combination of insect pests, diseases and nutrient balance; this is known as "clover sickness". Whencrop rotations are managed so that clover does not recur at intervals shorter than eight years, it grows with much of its pristine vigor.[3]
Clovers are most efficiently pollinated bybumblebees, which have declined as a result of agricultural intensification.[6]Honeybees can also pollinate clover, andbeekeepers are often in heavy demand from farmers with clover pastures. Farmers reap the benefits of increased reseeding that occurs with increased bee activity, which means that future clover yields remain abundant. Beekeepers benefit from the clover bloom, as clover is one of the mainnectar sources for honeybees.[7]
Trifolium repens, white or Dutch clover, is a perennial abundant in meadows and good pastures. The flowers are white or pinkish, becoming brown and deflexed as the corolla fades.Trifolium hybridum, alsike or Swedish clover, is a perennial which was introduced early in the 19th century and has now become naturalized in Britain. The flowers are white or rosy, and resemble those ofTrifolium repens.Trifolium medium, meadow or zigzag clover, a perennial with stragglingflexuous stems and rose-purple flowers,[3] has potential for interbreeding withT. pratense to produce perennial crop plants.[8]
Other species are:Trifolium arvense, hare's-foot trefoil; found in fields and dry pastures, a soft hairy plant with minute white or pale pink flowers and feathery sepals;Trifolium fragiferum, strawberry clover, with globose, rose-purple heads and swollen calyxes;Trifolium campestre, hop trefoil, on dry pastures and roadsides, the heads of pale yellow flowers suggesting miniature hops; and the somewhat similarTrifolium dubium, common in pastures and roadsides, with smaller heads and small yellow flowers turning dark brown.[3]
Clover is foraged for by wildlife such as bears, game animals, and birds. Clover is edible by humans,[9] although red clover should be avoided by pregnant women.[10][medical citation needed] The plant is a traditional Native American food,[11] which is eaten both raw and after drying and smoking the roots. The seeds from the blossoms are used to make bread.[12] It is also possible to make tea from the blossoms.[12]
Shamrock, the traditional Irish symbol, which according to legend was coined bySaint Patrick for theHoly Trinity, is commonly associated with clover, although alternatively sometimes with the various species within the genusOxalis, which are also trifoliate.[13]
Clovers occasionally have four leaflets, instead of the usual three. Thesefour-leaf clovers, like other rarities, are considered lucky.[3] Clovers can also have five, six, or more leaflets, but these are rarer still. The clover's outer leaf structure varies in physical orientation.
The record for most leaflets is 63, set on August 2, 2023, by Yoshiharu Watanabe in Japan.[14] The previous record holder, Shigeo Obara, had discovered an 18-leaf clover in 2002,[15] a 21-leaf clover in 2008[16] and a 56-leaf clover in 2009,[17] also in Japan.
A commonidiom is "to be (or to live) in clover", meaning to live a carefree life of ease, comfort, or prosperity.[18]
A common saying in surgery [regarding the appearance of wound after hemorrhoidectomy] is "If it looks like clover, the trouble is over; if it looks like dahlia, it’s surely a failure."
Acloverleaf interchange is named for the resemblance to the leaflets of a (four-leaf) clover when viewed from the air.[19]
The first extensive classification ofTrifolium had been done byMichael Zohary and David Heller, and it was subsequently released in 1984. They divided the genus into eight sections:Lotoidea, Paramesus, Mistyllus, Vesicamridula, Chronosemium, Trifolium, Trichoecephalum, andInvolucrarium, withLotoidea placed mostbasally.[20] Within this classification system,Trifolium repens falls within sectionLotoidea, the largest and least heterogeneous section.Lotoidea contains species from America, Africa, and Eurasia, considered a clade because of their inflorescence shape, floral structure, and legume that protrudes from the calyx. However, these traits are not unique to the section, and are shared with many other species in other sections. Zohary and Heller argued that the presence of these traits in other sections proved the basal position ofLotoidea, because they were ancestral. Aside from considering this section basal, they did not propose relationships between other sections.Since then, molecular data has both questioned and confirmed the proposed phylogeny from Zohary and Heller. A genus-wide molecular study has since proposed a new classification system, made up of two subgenera,Chronosemium andTrifolium.[21] This recent reclassification further divides subgenusTrifolium into eight sections. The molecular data supports the monophyletic nature of three sections proposed by Zohary and Heller (Tripholium, Paramesus, andTrichoecepalum), but not ofLotoidea (members of this section have since been reclassified into five other sections). Other molecular studies, although smaller, support the need to reorganizeLotoidea.[22][23]
^"Cambridge Dictionary".Cambridge Dictionary. June 26, 2021.Archived from the original on 2015-10-04. RetrievedJune 26, 2021.
^Pollard, Michael (1986).Travel by Road and Rail. Independence, Ohio: Schoolhouse Press. p. 31.ISBN9780808610403.
^Zohary, Michael (1984).The genus Trifolium. Heller, D. Jerusalem: Israel Academy of Sciences and Humanities.ISBN978-9652080561.OCLC11057949.
^Ellison, Nick W.; Liston, Aaron; Steiner, Jeffrey J.; Williams, Warren M.; Taylor, Norman L. (2006). "Molecular phylogenetics of the clover genus (Trifolium—Leguminosae)".Molecular Phylogenetics and Evolution.39 (3):688–705.Bibcode:2006MolPE..39..688E.doi:10.1016/j.ympev.2006.01.004.PMID16483799.
^Vižintin, Liliana; Javornik, Branka; Bohanec, Borut (2006). "Genetic characterization of selected Trifolium species as revealed by nuclear DNA content and ITS rDNA region analysis".Plant Science.170 (4):859–866.Bibcode:2006PlnSc.170..859V.doi:10.1016/j.plantsci.2005.12.007.
