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Flowers from The Cape

Diana Miller (Keeper of the Herbarium at RHS Garden Wisley)

Pelargonium Species

The genus Pelargonium is large and diverse and a very important constituent of the Cape Flora. It contains almost 250 species, perhaps 80% of which are native to the Republic of South Africa. Of these approximately 80% grow wild in the Cape region with its Mediterranean climate of cool winters, when most of the rainfall occurs, and hot summers. Pelargoniums have an extraordinary range of habits: succulents, annuals, herbaceous perennials, tuberous plants, shrubs or scramblers. Some are deciduous, some are evergreen and some are spiny. All these characters are adaptations to the habitat in which they grow but are also significant in the division of the genus into sections.

Although Linnaeus in 1753 classified the three genera Pelargonium, Erodium and Geranium as one, they are in fact very distinct and with a couple of exceptions, almost no geraniums or erodiums are found in South Africa. Pelargoniums are not frost hardy and are distinguished by irregular flowers and a nectar spur which is formed from an extension of the upper sepal of the flower. This spur is adapted to the insects which pollinate the species and varies from a shallow depression to a structure 3cm or more long. It is therefore influential in determining the flower shape. Being such a large and diverse genus, several taxonomists since Robert Sweet in 1820 have divided it up into a number of sections mostly distinguished by morphological characteristics. Results of recent molecular work carried out at Reading University, using techniques, inconceivable to botanists of the 1800s, follow to a large extent the older classifications although a few sections have been further split and match more closely to geographical locations. Some sections are not represented in the Cape region. The first pelargonium to be introduced into Europe was P. triste, possibly to Leiden before 1600. It is first recorded in England by John Tradescant who received seeds in 1631. A few species are still being discovered and named.

Cultivation of Pelargonium species

Pelargoniums come from regions with strong light, low rainfall (compared to the United Kingdom) which falls mainly in one season, more or less frost-free situations and rarely, if ever, water-logged or wet ground. Therefore, as with all plants, the aim of the gardener is to imitate these conditions as closely as possible within the restraints of the garden situation. The majority are easy to cultivate and need far less attention than the highly bred zonal and regal cultivars grown for exhibition on the show bench.

Growing conditions

Plants may be grown in a soil-based medium with added sharp grit, or in a soil-less compost. They need frost-free conditions, a well-ventilated atmosphere and plenty of light to thrive. During the growing season, the plants are best watered well and allowed to almost dry out before re-watering, never allowing the compost to become water-logged. The plants should be repotted as they outgrow their containers, either in autumn when excess growth may be cut back, or in spring as they start to regrow. The pots may be put outside in a sunny position in summer after all danger of frost is past. Many, especially the species which form sub-shrubs such as those of the section Ciconium (e.g. P. inquinans) and section Pelargonium (e.g. P. cucullatum, P. quercifolium, P. crispum), may be planted in the ground in well-drained soil, where they will form large attractive plants in a border. Some of those with an untidy or scrambling habit (e.g. P. peltatum, P. longicaule, P. trifidum) make attractive hanging baskets.

Overwintering

Most species, if kept dry and well ventilated, will survive surprisingly low temperatures. As a general rule, aim to keep the temperature above 5C but the occasional lower temperature, as long as it is not below freezing, will not do too much damage. Remove any dead leaves and allow as much air as possible to circulate around the plants to avoid excess humidity.

Feeding

Pelargonium species do not require the amount of extra feed given to the highly bred show plants. Overfeeding will produce lush growth but less flower, but for plants grown in a soil-less medium, a tomato feed with a high potash content given during the growing season can encourage flowering.

Pruning

Pelargonium species may be trimmed to keep them tidy or to avoid the very large species becoming too big for the space available. They may be cut back hard in autumn before overwintering to reduce the excess young growth and foliage which are more susceptible to fungal disease in the cooler, more humid conditions of winter.

Pests & diseases

In a collection of mixed species, pests and diseases appear to be less of a problem than in a collection consisting of one type of plant. Whitefly can be treated with proprietary brands of insecticide or by biological control. Sufficient ventilation and light should avoid fungal diseases although these can be a problem in cool, humid conditions. Certain leaf-rolling caterpillars can easily be picked off as soon as they appear. Vine weevils may be a problem as with many other greenhouse plants and can be controlled with insecticides containing imidacloprid.

Propagation

The majority of plants can be propagated by softwood cuttings taken during the growing season and rooted in a suitable compost or in vermiculite or sand. Bottom heat will speed the rate of rooting and hormone rooting powder is rarely necessary. Keep the cuttings well ventilated and in full light. Cuttings taken in autumn are less successful, probably due to the lower light intensity. Plants may be propagated by seed, especially when fresh, but germination can be erratic taking from a few days to several months. Even after a few seeds have germinated, it is worth retaining the rest as others may germinate a few weeks later. Proprietary products involving smoke may speed up germination.

Unusual groups that will benefit from extra care

Section Otidia (e.g. P. carnosum, P. paniculatum) contains plants with swollen succulent stems and often fleshy foliage, that need a very well drained compost with extra grit. It is best to avoid taking cuttings as the plants are slow growing and their shape may be lost for some time. Simply remove dead leaves and propagate by seed if possible. Succulent plants of other sections such as P. echinatum of section Cortusina may be treated in a similar way.

Sections Hoarea (e.g. P. appendiculatum, P. auritum, P. pinnatum) and Polyactium (e.g. P. triste, P. lobatum) contain tuberous plants which need well-drained compost containing extra grit. The flowers and foliage are produced during the winter months and will die down completely after flowering. As the growth dies back, reduce the water for the summer and start watering again when the first leaves or shoots begin to appear the following autumn. Propagate by seed or by separating the tubers when repotting.

Section Campylia (e.g. P. tricolor, P. ovale) consists of low-growing rosette plants that are not always easy to keep going. They need a very free-draining compost, space for an extensive root system and very bright light.

Sections Peristera (e.g. P. grossularioides) and Myrrhidium (e.g. P. longicaule, P. suburbanum) are annuals or short-lived perennials. Although not fussy in their cultivation requirements, it is a sensible precaution to save their seed to avoid unexpected loss.

Specialist society

Geraniaceae Group (www.geocities.com/RainForest/Canopy/3139/): interests include species of Pelargonium, Erodium and Geranium. Produces quarterly newsletter, excellent seed list, book lists and organises irregular meetings. Membership Secretary: P Starling, 22 Northfields, Girton, Cambridge CB3 0QG