Traits: Sun, Shade, Feature, Groundcover, Rockery

Most of the plants commonly called geraniums are actually pelargoniums, and while both genera are members of the geranium (Geraniaceae) family, the true geraniums are a very different group of over 300 species of perennials and subshrubs that are at times evergreen, and are widespread in the temperate zones. These plants are popular with gardeners for their ease of cultivation and attractive flowersthey make excellent subjects for rockeries and flower borders. The genus name, from the Greek word geranos, meaning crane, and the common name of cranesbill both refer to the long narrow fruits, which somewhat resemble a cranes long beak. Geraniums have a long history of use in herbal medicines and some species yield aromatic oils used in fragrances.

Flowering Season: Summer, Spring

Appearance
Many Geranium species are low spreading plants with often finely hairy, usually hand-shaped leaves, with toothed lobes. They bloom in spring and summer and have simple, flat, 5-petalled flowers in pink or purple-blue shades, less often white or darker purple-black. Garden forms widen the flower color range to include white and pink. A few species are larger-growing plants and have semi-woody stems and large sprays of blooms.

Cultivation
Most species are hardy and will grow in a wide range of conditions, preferring a position in sun or partshade and moist humus-rich soil. Many are drought tolerant but some prefer damp ground. The rhizomes can be invasive. Propagate by division or from cuttings or seed; these plants may self-sow.

Source: ABC Gardening Australia

Confusion with pelargoniums

Showing the "bill" and seed dispersal mechanism of Geranium pratense
Confusingly, "geranium" is also the common name of members of the genus Pelargonium (sometimes known as 'storksbill'), which are also in the Geraniaceae family. These are generally half-hardy plants which are offered as bedding in spring, and discarded after flowering. Linnaeus originally included all the species in one genus, Geranium, but they were later separated into two genera by Charles L’Héritier in 1789. Other former members of the genus are now classified in genus Erodium, including the plants known as filarees in North America.
The term "hardy geranium" is often applied to geraniums to distinguish them from the half-hardy pelargoniums. However, not all geranium species are winter-hardy (see below).
[edit]Structure
The shape of the flowers offers one way of distinguishing between the two genera Geranium and Pelargonium. Geranium flowers have five very similar petals, and are thus radially symmetrical (actinomorphic), whereas pelargonium flowers have two upper petals which are different from the three lower petals, so that the flowers have a single plane of symmetry (zygomorphic).

Source: Wikipedia