Clover


Clovers, also called trefoils, are plants of the genus Trifolium, consisting of about 300 species of flowering plants in the legume family Fabaceae originating in Europe. The genus has a cosmopolitan distribution with the highest diversity in the temperate Northern Hemisphere, but many species also occur in South America and Africa, including at high altitudes on mountains in the tropics.
They are small annual, biennial, or short-lived perennial herbaceous plants, typically growing up to tall. The leaves are trifoliate, with stipules adnate 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 the calyx. Other closely related genera often called clovers include Melilotus and Medicago.
As legumes, clovers fix nitrogen using symbiotic bacteria in their root nodules, and are used as an alternative or supplement to synthetic nitrogen fertilizers. They are also valuable food source for grazing livestock and bees. The domestication of clover caused substantial increases in agricultural productivity.

Cultivation history

Clover was first domesticated in Spain in around the year 1000. During European urbanization, crop rotations involving clover became essential for replacing the fixed nitrogen exported to cities as food. 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. Fields of clover, used as forage and newly-invented silage, became an important part of the rural landscape; 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 by European colonists, and some clover species became invasive in some areas.
Imports of guano and the development of the Haber-Bosch process in the 20th century substantially displaced clover as a crop, with negative effects on pollinators, 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 has also seen interest in clover as a countermeasure to fight the global pollinator decline.

Cultivation

Several species of clover are extensively cultivated as fodder plants. The most widely cultivated clovers are white clover, Trifolium repens, and red clover, Trifolium pratense. Clover, either sown alone or in mixture with ryegrass, 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 for livestock; it fixes nitrogen using symbiotic bacteria in its root nodules, reducing the need for synthetic fertilizers; it grows in a great range of soils and climates; and it is appropriate for either pasturage or green composting.
In many areas, particularly on acidic soil, clover is short-lived because of a combination of insect pests, diseases and nutrient balance; this is known as "clover sickness". When crop rotations are managed so that clover does not recur at intervals shorter than eight years, it grows with much of its pristine vigor.
Clovers are most efficiently pollinated by bumblebees, which have declined as a result of agricultural intensification. Honeybees can also pollinate clover, and beekeepers 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 main nectar sources for honeybees.
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 of Trifolium repens. Trifolium medium, meadow or zigzag clover, a perennial with straggling flexuous stems and rose-purple flowers, has potential for interbreeding with T. pratense to produce perennial crop plants.
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 similar Trifolium dubium, common in pastures and roadsides, with smaller heads and small yellow flowers turning dark brown.

Uses

Bears, game animals, ruminants and birds forage for and eat clover, and it is edible by humans, although red clover in particular contains phytoestrogens that may carry risks during pregnancy. The plant is a traditional Native American food, which is eaten both raw and after drying and smoking the roots. The seeds from the blossoms are used to make bread. It is also possible to make tea from the blossoms.

Symbolism

Shamrock, the traditional Irish symbol, which was coined by Saint Patrick for the Holy Trinity, is commonly associated with clover, although alternatively sometimes with the various species within the genus Oxalis, which are also trifoliate.
Clovers occasionally have four leaflets, instead of the usual three. These four-leaf clovers, like other rarities, are considered lucky. 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. The previous record holder, Shigeo Obara, had discovered an 18-leaf clover in 2002, a 21-leaf clover in 2008 and a 56-leaf clover in 2009, also in Japan.
  • A common idiom is "to be in clover", meaning to live a carefree life of ease, comfort, or prosperity.
  • A common saying in surgery is "If it looks like clover, the trouble is over; if it looks like dahlia, it’s surely a failure."
  • A cloverleaf interchange is named for the resemblance to the leaflets of a clover when viewed from the air.

Phylogeny

The first extensive classification of Trifolium had been done by Michael 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, and Involucrarium, with Lotoidea placed most basally. Within this classification system, Trifolium repens falls within section Lotoidea, 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 of Lotoidea, 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 and Trifolium. This recent reclassification further divides subgenus Trifolium into eight sections. The molecular data supports the monophyletic nature of three sections proposed by Zohary and Heller, but not of Lotoidea. Other molecular studies, although smaller, support the need to reorganize ''Lotoidea.''

Species

291 species of Trifolium are accepted:Trifolium absconditum Trifolium acaule Steud. ex A.Rich.Trifolium affine C.PreslTrifolium acutiflorum Trifolium × adulterinum Trifolium africanum Ser.Trifolium aintabense Boiss. & Hausskn.Trifolium albopurpureum Torr. & A. Gray – rancheria cloverTrifolium alexandrinum L. – Egyptian clover, berseem cloverTrifolium alpestre L. – owl-head cloverTrifolium alpinum L. – alpine cloverTrifolium alsadami Trifolium amabile KunthTrifolium ambiguum M. Bieb.Trifolium amoenum Greene – showy Indian cloverTrifolium amphianthum Trifolium andersonii A. Gray – Anderson's clover or fiveleaf cloverTrifolium andinum Nutt. – Intermountain cloverTrifolium andricum LassenTrifolium angulatum Waldst. & Kit.Trifolium angustifolium L.Trifolium ankaratrense Trifolium apertum BobrovTrifolium appendiculatum Trifolium argutum Banks & Sol.Trifolium arvense L. – hare's-foot cloverTrifolium attenuatum GreeneTrifolium aureum Pollich – large hop trefoilTrifolium baccarinii Chiov.Trifolium badium Schreb.Trifolium barbigerum Torr. – bearded cloverTrifolium barbulatum Trifolium barnebyi Dorn & LichvarTrifolium batmanicum Katzn.Trifolium beckwithii W.H.Brewer ex S.Watson – Beckwith's cloverTrifolium bejariense Moric.Trifolium × bertrandii Trifolium berytheum Boiss. & C.I.BlancheTrifolium biebersteinii Trifolium bifidum A.Gray – notchleaf cloverTrifolium bilineatum Fresen.Trifolium billardierei Spreng.Trifolium bithynicum Trifolium bivonae Guss.Trifolium blancheanum Boiss.Trifolium bobrovii Trifolium bocconei SaviTrifolium boissieri Guss.Trifolium bolanderi A.GrayTrifolium bordsilovskyi Trifolium brandegeei S.WatsonTrifolium breweri S. Watson – forest cloverTrifolium brutium Ten.Trifolium buckwestiorum Isely – Santa Cruz cloverTrifolium bullatum Boiss. & Hausskn.Trifolium burchellianum Ser.Trifolium calcaricum J.L.Collins & WieboldtTrifolium calocephalum Fresen.Trifolium campestre Schreb. – hop trefoilTrifolium canescens Willd.Trifolium carolinianum Michx.Trifolium caudatum Boiss.Trifolium cernuum Brot.Trifolium cheranganiense J.B.GillettTrifolium cherleri L.Trifolium chilaloense ThulinTrifolium chilense Hook. & Arn.Trifolium chlorotrichum Boiss. & BalansaTrifolium ciliolatum Benth. – foothill cloverTrifolium circumdatum KunzeTrifolium clusii Godr.Trifolium clypeatum L.Trifolium congestum Guss.Trifolium constantinopolitanum Ser.Trifolium cryptopodium Steud. ex A. Rich.Trifolium cyathiferum Lindl. – cup cloverTrifolium dalmaticum Vis.Trifolium dasyphyllum Torr. & A.GrayTrifolium dasyurum C.PreslTrifolium davisii E.HossainTrifolium decorum Chiov.Trifolium dedeckerae Trifolium depauperatum Desv. – cowbag clover, balloon sack clover, or poverty cloverTrifolium dichotomum Hook. & Arn.Trifolium dichroanthoides Rech.f.Trifolium dichroanthum Boiss.Trifolium diffusum Ehrh.Trifolium dolopium Heldr. & Hochst. ex Gibelli & BelliTrifolium douglasii HouseTrifolium dubium Sibth. – lesser hop trefoilTrifolium echinatum M.Bieb.Trifolium egrissicum Trifolium elgonense J.B.GillettTrifolium elizabethiae Trifolium eriocephalum Nutt. – woollyhead cloverTrifolium eriosphaerum Boiss.Trifolium erubescens FenzlTrifolium euxinum ZoharyTrifolium eximium Stephan ex Ser.Trifolium farayense Trifolium fergan-karaeri Trifolium fontanum Trifolium fragiferum L. – strawberry cloverTrifolium friscanum S.L.WelshTrifolium fucatum Lindl. – bull clover or sour cloverTrifolium gemellum Pourr. ex Willd.Trifolium gillettianum Jacq.-Fél.Trifolium glanduliferum Boiss.Trifolium globosum L.Trifolium glomeratum L. – clustered clover or bush cloverTrifolium gordeievii Z.WeiTrifolium gracilentum Torr. & A.Gray – pinpoint cloverTrifolium grandiflorum Schreb.Trifolium gymnocarpon Nutt. – hollyleaf cloverTrifolium hatschbachii Trifolium haussknechtii Boiss.Trifolium haydenii PorterTrifolium heldreichianum Hausskn.Trifolium hickeyi Trifolium hirtum All. – rose cloverTrifolium howellii S.Watson – canyon clover or Howell's cloverTrifolium humile Trifolium hybridum L. – Alsike cloverTrifolium hydrophilum Trifolium incarnatum L. – crimson cloverTrifolium infamia-ponertii Trifolium israeliticum Zohary & Katzn.Trifolium isthmocarpum Brot.Trifolium jokerstii Vincent & Rand.MorganTrifolium juliani Batt.Trifolium kentuckiense Chapel & VincentTrifolium kingii S.WatsonTrifolium lanceolatum J.B.GillettTrifolium lappaceum L.Trifolium latifolium GreeneTrifolium latinum Sebast.Trifolium leibergii A.Nelson & J.F.Macbr. – Leiberg's cloverTrifolium lemmonii S.Watson – Lemmon's cloverTrifolium leucanthum M.Bieb.Trifolium ligusticum Balb. ex Loisel.Trifolium longidentatum NábelekTrifolium longipes Nutt. – longstalk cloverTrifolium lucanicum Gasp.Trifolium lugardii BullockTrifolium lupinaster L.Trifolium macilentum GreeneTrifolium macraei Hook. & Arn. – Chilean clover, double-head clover, or MacRae's cloverTrifolium macrocephalum Poir. – largehead cloverTrifolium masaiense J.B.GillettTrifolium mattirolianum Chiov.Trifolium mazanderanicum Rech.f.Trifolium medium L. – zigzag cloverTrifolium meduseum C.I.Blanche ex Boiss.Trifolium meironense Zohary & LernerTrifolium mesogitanum Trifolium michaelis Trifolium michelianum SaviTrifolium micranthum Viv.Trifolium microcephalum Pursh – smallhead cloverTrifolium microdon Hook. & Arn. – thimble cloverTrifolium miegeanum MaireTrifolium minutissimum Trifolium modestum Trifolium monanthum A.Gray – mountain carpet cloverTrifolium montanum L.Trifolium multinerve A. Rich.Trifolium multistriatum W.D.J.KochTrifolium mutabile Port.Trifolium nanum Torr.Trifolium nerimaniae M.KeskinTrifolium × neyrautii Trifolium nigrescens Viv.Trifolium noricum WulfenTrifolium obscurum SaviTrifolium obtusiflorum Hook. – clammy cloverTrifolium occidentale CoombeTrifolium ochroleucon Huds. - sulphur cloverTrifolium oliganthum Steud. – fewflower cloverTrifolium olivaceum Trifolium orbelicum Trifolium ornithopodioides L.Trifolium owyheense GilkeyTrifolium pachycalyx ZoharyTrifolium palaestinum Boiss.Trifolium pallescens Schreb.Trifolium pallidum Waldst. & Kit.Trifolium palmeri Trifolium pamphylicum Trifolium pannonicum Jacq. – Hungarian cloverTrifolium parnassi Boiss. & SprunerTrifolium parryi A.GrayTrifolium patens Schreb.Trifolium patulum TauschTrifolium pauciflorum d'Urv.Trifolium × permixtum Trifolium peruvianum VogelTrifolium philistaeum ZoharyTrifolium phitosianum N.Böhling, Greuter & RausTrifolium phleoides Pourr. ex Willd.Trifolium physanthum Hook. & Arn.Trifolium physodes Steven ex M. Bieb.Trifolium pichisermollii J.B.GillettTrifolium pignantii Fauché. & Chaub.Trifolium pilczii AdamovićTrifolium pilulare Boiss.Trifolium piorkowskii Rand.Morgan & A.L.BarberTrifolium plebeium Boiss.Trifolium plumosum Douglas ex Hook.Trifolium polymorphum Poir.Trifolium polyodon GreeneTrifolium polyphyllum C.A.Mey.Trifolium polystachyum Fresen.Trifolium praetermissum Greuter, Pleger & Raus.Trifolium pratense L. – red cloverTrifolium productum Trifolium prophetarum M. HossainTrifolium pseudomedium Trifolium pseudostriatum Baker f.Trifolium pulchellum Trifolium purpureum Loisel.Trifolium purseglovei J. B. GillettTrifolium quartinianum A. Rich.Trifolium radicosum Boiss. & Hohen.Trifolium rechingeri Trifolium reflexum L. – buffalo cloverTrifolium repens L. – shamrock Trifolium resupinatum L. – Persian clover, shaftalTrifolium retusum L.Trifolium × retyezaticum Trifolium rhizomatosum Trifolium rhombeum Trifolium riograndense BurkartTrifolium rollinsii Trifolium roussaeanum Boiss.Trifolium rubens L.Trifolium rueppellianum Fresen.Trifolium salmoneum MouterdeTrifolium sannineum Trifolium sarosiense Trifolium saxatile All.Trifolium scabrum L.Trifolium schimperi A.Rich.Trifolium schneideri Trifolium × schwarzii Trifolium scutatum Boiss.Trifolium sebastiani SaviTrifolium semipilosum Fresen.Trifolium setiferum Boiss.Trifolium simense Fresen.Trifolium sintenisii FreynTrifolium siskiyouense J.M.GillettTrifolium somalense Taub. ex HarmsTrifolium sonorense Trifolium spadiceum L.Trifolium spananthum ThulinTrifolium spumosum L.Trifolium squamosum L. – sea cloverTrifolium squarrosum L.Trifolium stellatum L.Trifolium steudneri Schweinf.Trifolium stipulaceum Thunb.Trifolium stoloniferum Muhl. ex A. Eaton – running buffalo cloverTrifolium stolzii HarmsTrifolium striatum L. – knotted cloverTrifolium strictum L.Trifolium subterraneum L. – subterranean cloverTrifolium suffocatum L.Trifolium sylvaticum GérardTrifolium tembense Fresen.Trifolium tenuifolium Trifolium thalii Vill.Trifolium thompsonii C.V.Morton – Thompson's cloverTrifolium tomentosum L.Trifolium × traplii Trifolium triaristatum Bertero ex CollaTrifolium trichocalyx A.Heller – Monterey cloverTrifolium trichocephalum M. Bieb.Trifolium trichopterum PančićTrifolium tumens Steven ex M.Bieb.Trifolium ukingense HarmsTrifolium uniflorum L.Trifolium usambarense Taub.Trifolium variegatum Nutt. – whitetip cloverTrifolium vavilovii EigTrifolium velebiticum DegenTrifolium velenovskyi VandasTrifolium vernum Phil.Trifolium vesiculosum SaviTrifolium vestitum D.Heller & ZoharyTrifolium virginicum SmallTrifolium wentzelianum HarmsTrifolium wettsteinii Dörfl. & HayekTrifolium wigginsii J. M. GillettTrifolium willdenovii Spreng. − tomcat cloverTrifolium wormskioldii Lehm. – cow clover