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The name geranium can be applied either as a scientific term or as a common name to identify two separate types of flowers. Gardeners, especially in the United States, may use the word to refer to a group of plants scientifically known as pelargoniums, whereas geraniums are a separate scientific group. Superficially similar, the two sets of plants differ in ways such as environmental tolerances and original native areas of growth.
Gardeners originally mixed up the pelargoniums and geraniums in a group in the 17th century. Europe already had some native species of geraniums when new species of a similar plant were imported into the continent from South Africa. According to the Pelargonium Society and Geranium Society in the United Kingdom, some more pelargoniums originally grew in East Africa, Australasia, and Iraq.
All of these new plants looked like the geranium species to European gardeners, so they were included in the geranium group, and people called them geraniums for centuries. Gardening enthusiasts in certain countries, such as the U.S., still call all pelargoniums geraniums. In the U.K., most pelargoniums go by their scientific name, but some species with a dark zone on each leaf still retain the common name of geranium. Both pelargoniums and geraniums are part of the same Geraniaceae genus.
Although both pelargoniums and geraniums are grown as houseplants or garden plants, they do differ in environmental tolerances. Generally, pelargonium species are killed by freezing temperatures, whereas geraniums die down to the ground and recover in the spring. This hardiness extends to the geraniums' tolerance to partial shade. Pelargoniums typically need full sun exposure for growth.
Most pelargoniums are evergreen, which means they don't lose their leaves at some point during the year. Geraniums can be evergreen, or they can be herbaceous, which means they die back in winter and lie dormant until the spring. The pelargonium group contains many different species of plant with a wide variation in color, shape, and size. Geraniums usually have round flowers with five petals, leaves with lobes, or leaves with fingerlike divisions.
Pelargoniums and geraniums are both suitable as houseplants, but in the garden, they suit slightly different applications. Geraniums tend to spread in clumps along the ground at a low level and so are useful for a gardener who wants some ground coverage. They are also suitable for flower beds, as are pelargonium species. Geraniums are tolerant of all soil pHs, whereas pelargoniums prefer neutral to alkaline soil.