What we usually call geraniums are properly known as Pelargonium, Zonal Hybrids. The name comes from the leaf zones that are apparent on many of these plants. Though we do get some foliage disease problems with them here in Sydney, because of our summer humidity, they do grow pretty well here on the whole. Their decorative, rounded, fleshy leaves and brilliantly coloured flowers happily at home with other warm-climate plants. For hot-coloured gardens, red and orange-red flowered cultivars are perfect in pots or forming a wide shrubby mound (up to 60 cm if unpruned) amongst taller plants or an informal row along the front of a low fence.
There are also pretty pastels of pinks or white that fit in with cooler colour themes. There is even a soft pale yellow cultivar, called 'Creamery', although it is not a strong grower. There are single, semi-double and double flower forms, as well as rosebud and star-shaped types. There are literally hundreds of named cultivars, though I have rarely known the names of any of mine, and in my experience, many of the newfangled cultivars don't have the toughness of the old varieties (which are often obtainable these days only as cuttings from the gardens of friends). 'Big Red' (pictured at the end of the plant entry) is a recent release that has proved the exception to this: it has rich red flowers through much of the year and I have had good success with it. It is actually a 'Calliope' Pelargonium: a cross between a zonal Pelargonium and an ivy-leaved Pelargonium, and is more resistant to disease and robust than the regular zonal types, with a semi-trailing habit. Other 'Big' varieties of different colours, however, don't seem to be as good as 'Big Red', unfortunately. Fancy-leaved types with gold or blackish leaves, or dark-banded foliage are a subgroup of the zonal type.
They are South African in origin, and flower almost all year round if trimmed back every so often through the year they will flower almost perpetually in our mild climate. Early to mid-spring is probably their absolute peak flowering time, especially if they have been pruned by around two-thirds in early autumn to allow the development of a good plant structure. It seems that painting or squirting the tops of pruned stems with methylated spirits helps to quickly seal the cut. They should be tip pruned after the main pruning to develop a compact shape. These plants want sun and light, alkaline, well-drained soil on the dry side, with good air circulation around them. I do, however, have a bright pink one called 'Shady Lady' that grows well in shade.
In general, I have had better success growing them in the garden than in pots, as I found they got too leggy in containers. Also, they don't do well in heavy soil like mine, as there isn't enough oxygen for the roots to respire; in a pot they can be given a very well-drained growing medium. Some growers recommend a mix of untreated sawdust with hydrated coir fibre, or even just the coir fibre on its own. If rust or other diseases strike in humid summers, I cut them back and they usually get over it, but they could be treated with an organic fungicide. The 'Deacon' cultivars are said to be disease-resistant. In general, healthier plants are less likely to be attacked by rust. Tiny caterpillars can ruin the flowers: spray with a low-toxic spray (such as Success) - or try to catch the tiny moths when they hang around the plants to lay their eggs!
The best time for taking cuttings of any of these Pelargonium is in early autumn, when the plants are pruned. If cuttings are dipped in methylated spirits, this seems to expedite the propagating process. Alternatively, let the cuttings dry out for a day before inserting them into the propagating mix. Use a free-draining medium for this. I find that it is best to replace my specimens after a few years, as they can get woody with age. Don't overwater them, especially in January and February when it is humid. Try to avoid water splashing on the foliage as this can cause fungal problems, Mulching around them with cane mulch is said to help reduce the occurrence of diseases on the leaves. Fertilising with something like Osmocote every four months will promote more flowering.
The flowers can be picked for vases.