Pelargonium

Pelargonium x hortorum

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At a Glance

Overview

Pelargonium × hortorum, commonly known as the zonal pelargonium or zonal geranium, is a hybrid pelargonium grown as a summer bedding and container plant throughout the United Kingdom. It was developed in the nineteenth century from crosses involving Pelargonium zonale and Pelargonium inquinans, both South African species in the family Geraniaceae, and is treated as a distinct hybrid group rather than a true botanical species. The "zonal" name refers to the darker band, or zone, of pigment that runs across the upper surface of the rounded leaves in most cultivars.

In British gardens pelargoniums occupy a niche between hardy perennial and tender exotic. They flower continuously from late spring until the first frosts, tolerate dry conditions far better than most bedding plants, and bridge the gap between spring bulbs and autumn chrysanthemums on patios, window ledges and balcony planters. They are also a mainstay of municipal parks bedding schemes and are widely sold as plug plants, young plants and finished specimens from late April onwards.

The taxonomy is often a source of confusion: in horticultural trade the name "geranium" is routinely applied to pelargoniums, although true hardy geraniums (the cranesbills) belong to a different genus. For clarity, this article refers throughout to pelargoniums, with geranium used only where this is the common cultivar-group name.

Appearance

Pelargonium × hortorum forms a bushy, branching sub-shrub with thick, semi-succulent stems that become woody at the base on older specimens. The stems are green when young, often flushed with red, and carry alternate leaves on long petioles. Stems and petioles are typically covered in fine, short hairs.

The leaves are the defining ornamental feature. They are roughly circular to kidney-shaped, with a palmate venation, shallowly lobed margin and a soft, slightly velvety surface from the dense indumentum. In most cultivars a pronounced horseshoe-shaped band of dark bronze, purple or near-black pigment develops on the upper surface as the leaf matures; this is the "zone" that gives the group its name. Leaf diameter is usually 5–10 cm. Cultivars are also sold as "silver" or "white" zonals where the band is replaced by a paler margin, and as "deacon" types with smaller, more numerous flowers and a more compact habit. A separate foliage group, the so-called "stellar" or "fancy-leaf" zonals, has narrower, almost star-shaped leaves and is grown principally for its leaf colouring.

Flowers are carried in rounded umbels, technically pseudoumbels, on stiff peduncles held above the foliage. Each umbel contains typically 20–40 individual flowers, opening in succession over several weeks. Individual flowers are 2–4 cm across, with five petals, the upper two usually broader than the lower three. The wild-type colour is red, but modern cultivars span scarlet, crimson, pink, salmon, white, lilac, magenta, orange and bicolours. The "basic" zonal group has single flowers of one colour per umbel; the "semi-double" and "double" groups have extra whorls of petals and tend to be less weather-resistant in heavy summer rain. "Fancy" or "Madame Salleron" types have variegated leaves and comparatively few, smaller flowers and are grown primarily as foliage plants.

Unlike the related ivy-leaved pelargoniums (Pelargonium peltatum) and the scented-leaved group, zonals are upright rather than trailing, with a more vigorous, somewhat stiffer habit that suits pots, tubs and formal bedding.

Growing Conditions

Pelargoniums are tender. The Royal Horticultural Society assigns zonal pelargoniums to hardiness rating H1C (heated glass, above 5 °C) and H2 (half-hardy, tolerating 1–5 °C for short periods). In most of the UK they will not survive a typical winter outdoors. They are best regarded as half-hardy perennials kept under glass or on a bright windowsill, or as annuals replaced each year.

For flowering, zonal pelargoniums need as much direct sun as possible. A minimum of six hours of sun in midsummer is the practical baseline; in the south of England, sunnier east-coast sites and most London gardens they can take full sun all day. In cooler, cloudier regions such as the West Country, parts of Wales, and the Scottish lowlands they still flower but the umbels tend to be smaller and growth more open unless given the brightest outdoor position available. Light midday shade is acceptable but reduces flower count.

Soil or compost should be free-draining and moderately fertile. A peat-free multipurpose potting compost amended with about 20 per cent by volume of sharp sand, perlite or grit gives the open structure that pelargoniums prefer, particularly in containers where waterlogging in winter is a common cause of loss. In the open ground, a light, loamy soil improved with well-rotted garden compost is ideal. Heavy clay is unsuitable unless raised beds or large containers lift the root zone above the surrounding ground. The preferred pH range is approximately 6.0–7.5, mildly acidic to neutral; pelargoniums are reasonably tolerant of slightly alkaline conditions.

Plants are unhappy in cold, wet soil, which leads to blackleg and basal rot. In containers this is controlled by crocking the base, using a free-draining compost and avoiding standing the pot in a saucer of water for more than a few minutes. In the open ground, raised beds and gritty soil amelioration produce a substantial improvement in survival during a wet July or August.

Indoor overwintering is best at 5–10 °C with good daylight, a regime often described as "frost-free but cool". Warmer windowsill conditions at 18–21 °C keep plants alive but encourage etiolated, leggy growth that is vulnerable to fungal disease.

Planting and Care

Watering should reflect the pelargonium's drought tolerance. During the active growing season, pots should be allowed to dry out almost to the point of wilt between waterings; a useful test is to lift the pot and water only when it feels noticeably light. In a typical British summer this means watering once a day for a 20 cm pot in full sun, every two to three days for the same pot in light shade, and less in cooler weather. Overwatering, particularly in shade or in compost that holds too much water, is the single most common cause of failure. Plants in open ground, once established, usually require no routine watering.

Feeding starts about six weeks after potting, when the initial nutrients in the compost begin to leach. A high-potash liquid feed such as a tomato fertiliser, applied at half the label strength at every other watering, produces strong, free-flowering plants. Avoid nitrogen-rich lawn or general fertilisers, which produce lush leaf growth at the expense of flowers. A single pinch of sulphate of potash, roughly 5 g per 10-litre pot, is a useful mid-summer top-up.

Pruning is minimal during the season. Dead-heading individual spent flowers and removing whole faded umbels by snapping the peduncle at its base, taking care not to tear the stem, keeps the plant tidy and encourages a fresh flush of buds. From late July, a harder prune can be carried out if plants have become leggy: cut back to within two or three leaf nodes of the main stem. Indoor specimens benefit from a more substantial prune in late winter or early spring, before the new flush of growth.

Propagation is straightforward from softwood cuttings taken in late summer, which is also a practical way to maintain named cultivars year on year. Cuttings of 7–10 cm are taken from non-flowering shoots, the lower leaves removed, and the cut surface left to dry for a few hours before insertion into a 50:50 mix of peat-free multipurpose compost and sharp sand or perlite. Rooting typically takes two to three weeks at around 18 °C. Cuttings taken in August or early September and overwintered frost-free on a bright windowsill make sturdy young plants for setting out after the last frost, generally late May in southern England and early to mid-June further north and at altitude.

Pests to watch for include the common greenfly, whitefly under glass, and vine weevil larvae in peat-based composts. The pelargonium-specific Cacyreus marshalli, the geranium bronze butterfly, has been recorded in the UK since the late 1990s and is occasionally reported on indoor stock in the south, although it is not yet a widespread outdoor problem; its caterpillars feed inside stems and can be managed by cutting back affected growth and disposing of it.

Seasonal care in the UK follows a fairly fixed calendar. Young plants or overwintered stock are potted up in March, hardened off in a cold frame or by placing pots outside on mild days through April, and planted out once the risk of frost has passed. Late frosts are not uncommon in northern England and Scotland even in mid-May, and a covering of horticultural fleece is a useful insurance. In autumn, before the first hard frost, container plants are lifted, trimmed back by about a third, and brought into a frost-free greenhouse, conservatory or bright windowsill. Plants left out typically collapse at temperatures below about −2 °C, and even brief exposure to −5 °C kills the stems outright.

Common Problems

The most frequent complaint in UK cultivation is the failure to flower, which usually traces to one of three causes. Insufficient sun, often because pelargoniums are used as a quick fix for a dry, shaded corner where little else will grow, suppresses bud formation. Over-feeding with nitrogen, or over-potting into a container far larger than the root system, has the same effect, producing leafy plants with few or no umbels. Finally, plants raised from seed and sold as "mixed" or "f2" are not always uniform; a small percentage of any batch will be genetically slow to flower and will sit as leafy rosettes well into July.

Botrytis, or grey mould, is the most damaging fungal disease and is favoured by the cool, damp conditions of a typical British August. Infected flowers turn brown and become covered in a fuzzy grey sporulation, and stems may develop a sunken, water-soaked lesion. Improving ventilation, dead-heading promptly and avoiding late-evening watering on already damp foliage reduce the risk. Heavily affected plants should be cut back hard and the prunings disposed of in household waste rather than the compost bin.

Blackleg is a bacterial soft rot of the stem base, usually initiated by wounds and waterlogging. Affected stems turn black and mushy at soil level and the plant collapses. There is no cure, and affected stock should be destroyed; the lesson is to take cuttings only from healthy plants and to use a free-draining cutting compost.

Edema, or oedema, appears as small corky brown spots on the undersides of leaves and is caused by the roots taking up water faster than the leaves can transpire it, typically after a cool, dull, damp spell. It is physiological rather than infectious and clears up as the weather improves.

Leaf yellowing is most often a watering issue, either too much or, less commonly, too little. Magnesium deficiency, manifesting as interveinal yellowing on older leaves while the veins remain green, responds to a foliar feed of Epsom salts at about 5 g per litre.

Popular Varieties

Real, currently available UK cultivars of Pelargonium × hortorum include the following. Cultivar names are stable in the trade, but availability varies year to year, particularly in smaller garden centres.

'The Alde' is a vivid scarlet, single-flowered zonal, raised by the late Brian West, that has been a popular choice for bedding and container work for several decades. It reaches roughly 45 cm in height and is reliably free-flowering even in cooler UK summers.

'Horizon' is a series, sold as a mixture and in individual colours including 'Horizon Rose', 'Horizon Scarlet' and 'Horizon White'. It is widely sold as plug plants by mail order in spring, typically for 9 cm pot production. Plants are compact at 25–35 cm, with a uniform flowering time that suits formal bedding.

'Apache' produces bright scarlet semi-double flowers on compact, 25 cm plants and is a popular choice for hanging baskets and smaller containers.

'Black Velvet' is one of a small group of cultivars, also including 'Black Knight', selected for their very dark, almost black leaves with a narrow green margin. Flower colour is usually red or pink, and the combination is striking in a sunny spot. Height is around 30–40 cm.

'Bredon' is a soft pink semi-double raised by Derek Clifford and sold particularly through specialist pelargonium nurseries; it is grown for the unusual warm-pink shade rather than for compact habit.

'Mr Henry Cox' is one of the best-known variegated zonals, sometimes sold as 'Mrs Henry Cox'. The leaves carry a vivid combination of green, bronze and red, with a single salmon-pink flower. It reaches around 35–45 cm and is grown primarily for its foliage.

'Frank Headley', named after the founder of the British National Pelargonium Collection, is a silver-zonal cultivar with white-edged leaves and single salmon-pink flowers. It is reliably hardy down to about 0 °C in a sheltered, well-drained position, which is unusual for the group, and is therefore sometimes used in milder coastal or town gardens as a permanent outdoor planting.

'Caliente' is a more recent interspecific hybrid, sold as a heat- and rain-tolerant zonal, useful for south-facing patios and balconies. Colours include 'Caliente Coral', 'Caliente Deep Red' and 'Caliente Pink'. Compact at around 30 cm, it has a slightly trailing habit that bridges zonal and ivy-leaved types.

'Stellar' or "stellata" cultivars such as 'Vectis Sparkler' and 'Arctic Star' have narrow, deeply lobed leaves and a more open flower. They are widely available from specialist nurseries and are grown for the architectural quality of the foliage as much as for the flowers.

Some widely sold seed-raised strains, such as the 'Multibloom' and 'Pulsar' series, are technically mixed F1 hybrids rather than clonal cultivars. They are excellent for uniform bedding and are considerably cheaper, but individual plants are usually discarded at the end of the season rather than overwintered.

Toxicity: there is no widely accepted clinical evidence that Pelargonium × hortorum is significantly toxic to humans, dogs or cats, although some dogs and cats do show mild gastrointestinal upset if they chew the leaves. The plant should not be confused with the unrelated Geranium species, several of which have documented toxicity. Gardeners handling large quantities of pelargonium foliage occasionally report contact dermatitis, but this is uncommon.

Pests and Diseases

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