Pelargonium species are evergreen perennials indigenous to Southern Africa, and are drought and heat tolerant, but can tolerate only minor frosts. They are extremely popular garden plants, grown as bedding plants in temperate regions.
The first species of Pelargonium known to be cultivated was P. triste, a native of South Africa. It was probably brought to the Botanical Garden in Leiden before 1600 on ships which stopped at the Cape of Good Hope. In 1631, the English gardener John Tradescant the elder bought seeds from Rene Morin in Paris and introduced the plant to England. The name Pelargonium was introduced by Johannes Burman in 1738, from the Greek πελαργός, pelargós (stork), because the seed head looks like a stork's beak. The chemist, John Dalton, first realized that he was color blind when he heard others describe the color of the flowers of the pink (Pelargonium zonale), as pink or red, when to him it looked either pink or blue, having no relationship to red at all.
Pelargoniumleaves are usually alternate, and palmately lobed or pinnate, often on long stalks, and sometimes with light or dark patterns. The erect stems bear five-petaled flowers in umbel-like clusters called pseudoumbels. The flower has a single symmetry plane (zygomorphic), which distinguishes it from the Geranium flower, which has radial symmetry (actinomorphic). The leaves of Pelargonium peltatum, Ivy-leaved Geranium, have a thick cuticle better adapting them for drought tolerance.
Pelargonium species are native to southern Africa and Australia, and the north of New Zealand. Others are native to southern Madagascar, eastern Africa, Yemen, Asia Minor and two very isolated islands in the south Atlantic Ocean (Saint Helena and Tristan da Cunha). Most of the Pelargonium plants cultivated in Europe and North America have their origins in South Africa. 
Despite not being frost-hardy, pelargoniums are extremely easy to grow and to propagate. As they are evergreenperennials, they can be kept in flower throughout the winter months in a sufficiently favoured spot (or indoors). There are even one or two fully hardy species from Turkey. Their main requirement is a warm, sunny, sheltered location. Many varieties will tolerate drought conditions for short periods. They are commonly seen in bedding schemes in parks and gardens, but can also be grown indoors as houseplants if given enough light. More compact erect and trailing varieties are ideal for window boxes and hanging baskets, in association with other half-hardy plants like lobelias, petunias and begonias. Thousands of pelargonium cultivars are available from garden centres or specialist suppliers during the spring and summer months. They are regular participants in flower shows and competitive events, with numerous societies devoted exclusively to their cultivation. They are easy to propagate from cuttings.
There is considerable confusion as to which Pelargonium are true species, and which are cultivars or hybrids. The nomenclature has changed considerably since the first plants were introduced to Europe in the 17th century.
(1) Zonal Pelargoniums – mostly bush type plants grown for the beauty of their flower and often the one most confused by being wrongly named ‘Geraniums’ particularly in summer bedding arrangements etc. Often made worse by the fact that Americans still widely use this incorrect naming. Zonals include the following groups of plants: ‘Basic’, ‘Dwarf’, ‘Miniature’, ‘Micro-Miniature’ along with Genetic Hybrids such as the ‘Stellars’ section and Hybrid Ivy leaved varieties that display little or no ivy leaf characteristics. i.e. the Deacons group.
(i) Basic Plants – Mature plants with foliage normally exceeding 7” (180mm) in height above the rim of the pot. For exhibition these should be grown in a pot exceeding 4.75” (120mm) in diameter but not normally exceeding 6½” (165mm).
(ii) Dwarf Plants – Mature plants with foliage more than 5” (125mm) above the rim of the pot, but not normally more than 7” (180mm). For exhibition should be grown in a pot exceeding 3½” (90mm) but not exceeding 4¾” (120mm).
(iii) Miniature Plants – Mature plants with foliage normally less than 5” (125mm) above the rim of the pot. For exhibition should be grown in a pot not exceeding 3½” (90mm).
(iv) Micro-Miniature Plants – Mature plants with foliage normally less than 4” (100mm) above the rim of the pot. Usually no separate classes for these in exhibition and will therefore normally be shown as Miniature Zonals.
(v) Deacon Varieties –Genetic hybrid similar to a large Dwarf. For exhibition (when shown in a separate class), usually grown in a pot not exceeding 5” (125mm), otherwise as for Dwarf Zonals.
(vi) Stellar Varieties – A relatively modern genetic hybrid originating from the work done by the Australian hybridiser Ted Both in the late 1950s and 1960s from crosses between Australian species and Zonal types. Easily identifiable by their half star-shaped leaves and slim petalled blooms. Single varieties tend to have larger elongated triangular petals whereas doubles tend to have thin feathered petals that are tightly packed together. For exhibition purposes there is a separate class for Stellar varieties, but being Zonals could be shown in an open class for Basic, Dwarf or Miniature Zonals (unless otherwise stated)
(2) Fancy Leaf Zonal Pelargoniums – besides having green leaves with or without zoning, this group also have variable coloured foliage that is sometimes used in classifying for exhibition purposes, i.e. ‘Bicolour’, ‘Tricolour’, ‘Bronze’ or ‘Gold’. Other foliage types are: ‘Black’ or ‘Butterfly’. There are an increasing number of these plants with stunning blooms to add to their beauty.
(a) Bicolour – includes those with white or cream veined leaves or those with two distinct colours with clearly defined edges, other than the basic zone.
(b) Tricolour – (May be Silver Tricolour (usually called a Silver Leaf) or a Gold Tricolour).
(i) Gold Tricolour – Leaves of many colours including red and gold, but usually with clearly defined edges of golden yellow and having a leaf zone, usually red or bronze, that overlays two or more of the other distinct leaf colours, so that the zone itself appears as two or more distinct colours.
(ii) Silver Tricolour or Silver Leaf – These tend to resemble a normal bi-colour leaf plant with two distinct colours usually of green and pale cream or white; the third colour is usually made up of bronze zoning. When this zoning overlays the green part of the leaf it is deemed to represent a silver colour.
(c) Bronze Leaved – Leaves of Green or Golden/Green with a heavy bronze or chestnut coloured centre zone which is known as a medallion. For exhibition purposes, when exhibited in specific ‘Bronze’ Leaf class – Must have over 50% of leaf surface bronze coloured. The dwarf plant ‘Overchurch’ which has a heavy bronze medallion.
(d) Gold Leaved – Leaves coloured golden/yellow or green/yellow but not showing a tendency to green. For exhibition purposes, when exhibited in specific ‘Gold’ Leaf class – Must have over 50% of leaf surface gold coloured.
(e) Black Leaved – Leaves coloured black, purple-black or with distinct large dark zones or centre markings on green.
(f) Butterfly Leaved – Leaves with a butterfly marking of distinct tone or hue in centre of leaf. This can be encompassed in many of the coloured leaf varieties.
(3) Zonal Pelargoniums have many flower types as follows:
(a) Single Flowered – each flower pip normally having no more than five petals. This is the standard flower set for all Pelargoniums.
(b) Semi-Double Flowered – each flower pip normally having between six and nine petals.
(c) Double Flowered – each flower pip composed of more than nine petals (i.e. double the standard flower set) but not ‘hearted’ like the bud of a rose. Below: the dwarf ‘Dovepoint’ which has full double blooms.
(d) Rosebud or Noisette Group – each bloom fully double and ‘hearted’. The middle petals are so numerous that they remain unopened like the bud of a rose.
(e) Tulip Flowered – having semi-double blooms that never fully open. The large cup shaped petals open just sufficiently to resemble a miniature tulip.
(f) Bird’s-Egg Group – having blooms with petals that have spots in a darker shade than the base colour, like many birds eggs.
(g) Speckled Flowered Group – having petals that are marked with splashes and flecks of another colour. See the picture of ‘Vectis Embers’ above.
(h) Cactus Flowered Group – having petals twisted and furled like a quill. In the USA are known as the Poinsettia Flowering.
B. Ivy-Leaved Pelargoniums (Derived from P. peltatum).
(1) Ivy Leaved Pelargoniums – Usually of lax growth, mainly due to the long thin stems, with thick, waxy ivy shaped leaves developed by the species P. Peltatum to retain moisture during periods of drought. Much used for hanging pots, tubs and basket cultivation. In the UK we tend to prefer the bulbous double headed types whilst on the European continant they tend to prefer the balcon single types for large scale hanging floral displays. Ivy Leaved Pelargoniums embrace all such growth size types including small leaved varieties and genetic hybrid crosses, which display little or no zonal characteristics. May have bicolour leaves and may have flowers that are single, double or rosette.
C. Regal Pelargoniums (Pelargonium x domesticum)
Large bush type floriferous pelargoniums grown for the richness and beauty of their large flower heads. The majority of those grown today have been hybridised in the last 50 years and tend to be very short jointed and compact thus requiring little work to achieve a floriforous well rounded plant. They have serrated leaves, unlike the Zonal groups, without any type of zoning. The Regal Pelargonium group includes “Decorative” types which are the descendants of older, less compact, smaller flowered varieties that are more suited to outdoor conditions; and “Oriental” types, which are the result of crosses between Regals and members of the Angel group (see below). Some have bicolour foliage. In the USA they are often known as the ‘Martha Washington’ or ‘Lady Washington’ Pelargoniums.
D. Angel Pelargoniums
Cultivars, the majority of which, originate from a cross between P. Crispum and a Regal variety in the early part of the 20th Century. Angels have grown in popularity in the last 30 years or so due mainly to an explosion of new varieties being released by specialist nurseries resulting from the work done by dedicated amateur hybridisers. These hybrisers have managed to obtain many new flower colour breaks and tighter growth habits resulting in smashing plants suitable for all sorts of situation. ‘Angels’ basically have the appearance of a small Regal with small serrated leaves and much smaller flowers. The group extends to include similar small leaved and flowered types but usually with P. Crispum in their parentage. They are mostly upright bush type plants but there are some lax varieties that can be used for basket or hanging pot cultivation. Often called ‘Pansy-Faced’ in the US. Some varieties have bicolour foliage.
E. Unique Pelargoniums
Bushy cultivars originating in the mid 19th century, all having P. fulgidum in their pedigree. May have bicolour foliage. Some types, popularly known in the hobby as ‘Hybrid Uniques’, have been crossed with Regal pelargoniums and, as a result of this cross, are much more floriferous.
F. Scented Leaved Pelargoniums
Grown chiefly for their fragrance, may be species or cultivars but all must have a clear and distinct scented foliage. Scent is emitted when the leaves are touched or bruised with some scents aromatic, others pungent and in a few cases, quite unpleasant. Several of the scented leaved pelargoniums are grown for the oil ‘geranol’, which is extracted from the leaves and is an essential oil much used commercially in perfumery. The scent of some species growing in their natural habitat, acts as a deterrent to grazing animals who appear to dislike the emitted scent. Conversely, it also attracts other insect life to visit the bloom and pollinate the plant. The scented leaves can be used for potpourri and they also have a use as flavourings in cooking. Occasionally scented types can be found in the some of the other groups mentioned i.e. the Angels having P. Crispum in their genes can often have a strong citrus scent.
G. Species Pelargoniums
The species are the forfathers of all the cultivar groups listed above and are, of course, a member of the genus Geraniaceae family (see Group page). In general, the definition of a species is that it breeds true, and is to be found doing this in the ‘wild’.
The majority of the pelargonium species are to be found in the Republic of S. Africa, mostly in the south western corner. There are also about 20 species in Eastern Africa and a few others to be found in Australia, New Zealand, Tasmania, Turkey, Iraq, and the South Atlantic islands of St. Helena and Tristan da Cunha.
Species pelargoniums have an incredible diversity of characteristics in habit, shape, size and colour, which probably accounts for them having retained their popularity for more than 300 years.
H. Primary Hybrids
A Primary Hybrid is recognised as being the resultant plant from a first time cross between two different known species. Examples are P x ‘ardens’ – from P. lobatum x P. fulgidum (1810). P x ‘glauciifolium’ – from P. gibbosum x P. lobatum (1822). Usually, but not always, primary hybrids are sterile.
Cultivars are derived from a great number of species, amongst others P. graveolens. These include: Species
Other than being grown for their beauty, species such as P. graveolens are important in the perfume industry and are cultivated and distilled for their scents. Although scented pelargoniums exist which have smells of citrus, mint, pine, spices or various fruits, the varieties with rose scents are most commercially important. Pelargonium distillates and absolutes, commonly known as "scented geranium oil" are sometimes used to supplement or adulterate expensive rose oils. The edible leaves and flowers are also used as a flavouring in desserts, cakes, jellies and teas. Scented-leafed pelargoniums can be used to flavor jellies, cakes, butters, ice cream, iced tea and other dishes, The rose-, lemon- and peppermint-scents are most commonly used. Also used are those with hints of peach, cinnamon and orange. Commonly used lemon-scented culinary species include P. crispum and P. citronellum. Rose-scenteds include P. graveolens and members of the P. ‘Graveolens’ cultivar group. Other species and cultivars with culinary use include the lime-scented P. ‘Lime,’ the lemon balm-scented P. ‘Lemon Balm,’ the strawberry-lemon-scented P. ‘Lady Scarborough’ and the peppermint-scented P. tomentosum.
The Japanese beetle, an important agricultural insect pest, becomes rapidly paralyzed after consuming flower petals of the garden hybrids known as "zonal geraniums" (P. × hortorum). The phenomenon was first described in 1920, and subsequently confirmed. Research conducted by Dr. Christopher Ranger with the USDAAgricultural Research Service and other collaborating scientists have demonstrated the excitatory amino acid called quisqualic acid present within the flower petals is responsible for causing paralysis of the Japanese beetle. Quisqualic acid is thought to mimic L-glutamic acid, which is a neurotransmitter in the insect neuromuscular junction and mammalian central nervous system.
A study by the Laboratory of Apiculture & Social Insects group at the University of Sussex on the attractiveness of common garden plants to pollinators found that a cultivar of Pelargonium × hortorum was unattractive to pollinators in comparison to other selected garden plants such as Lavandula (lavender) and Origanum.
In herbal medicine, Pelargonium has been used for intestinal problems, wounds and respiratory ailments, but Pelargonium species have also been used for fevers, kidney complaints and other conditions. Geranium (Pelargonium) oil is considered a relaxant in aromatherapy, and in recent years, respiratory/cold remedies made from P. sidoides and P. reniforme have been sold in Europe and the United States.P. sidoides along with Echinacea is used for bronchitis.P. odoratissimum is used for its astringent, tonic and antiseptic effects. It is used internally for debility, gastroenteritis, and hemorrhage and externally for skin complaints, injuries, and neuralgia and throat infections. The essential oil is used in aromatherapy. It is also used to balance the hormonal system, menstrual flow, and clean the body of toxins.
^Davis, J.J. 1920. The green Japanese beetle. New Jersey Department of Agriculture Circular. 30: 33.
^Ballou, C. H. 1929. Effects of geranium on the Japanese beetle. Journal of Economic Entomology. 22: 289-293.
^Potter, D. A. and Held, D. W. 1999. Absence of food-aversion learning by a polyphagous scarab, Popillia japonica, following intoxication by geranium, Pelargonium x hortorum. Entomologia Experimentalis et Applicata. 91: 83-88.
^Held, D. W. and Potter, D. A. 2003. Characterizing toxicity of Pelargonium spp. and two other reputedly toxic plant species to Japanese beetles (Coleoptera: Scarabaeidae). Environmental Entomology. 32: 873-880.